A Russian Naval Dip Meter

24 06 2012

This post was preceded by  “An Improvised Dip Meter”

In the preceding post, I wrote a little about the Blish prism and the Gavrisheff dip meter and pointed out that if the angle between two horizons opposite to each other could be measured, the local dip could be deduced. Through the kindness of Alex Eremenko, I have recently been able to examine in detail an N 5 Russian dip meter which has several interesting design features. It is probably easier to appreciate these if an idea is had of the optical path of the instrument (Figure 1). I explained a little about dip and its importance in celestial navigation in the preceding post.

Figure 1 : Light path of Russian N5 dip meter.

The horizon is viewed simultaneously to the left and right of the observer. Taking the rays from the left side first, shown in green,  they pass through an adjustable slit, used to make the brightness of the two horizons equal, through a watertight window in the side of the instrument and thence to a roof prism, where they are deflected downwards at 90 degrees. They then pass through a semi-reflecting junction, and are again deflected by a second prism through 90 degrees in a plane at right angles to the first, into the objective of a x 4 Keplerian (inverting) telescope.

The rays from the right hand horizon also pass through a window and then through a weak positive (convex) achromatic lens of 1.5 dioptres power. They then enter a negative (concave) lens whose power is such that the two lenses together have no net power, so that they behave like a piece of plane glass. However, the positive lens can be moved up or down from a central position, so that the light path also moves up or down – but not by very much. In fact, the total travel of the lens is 12 mm and the total deflection of the rays is 15 minutes of an arc each way. The right rays continue on through the right angled prism and are reflected off the common, semi-reflective face, off the opposite face and thence into the telescope objective, where they are combined to be viewed through the eyepiece. A yellow filter can be attached to the latter to reduce glare. Figure 2 shows the practical realisation of Figure 1.

Figure 2 : Practical light path

Projecting from the bottom of the slide that carries the sliding lens is a boss which carries an adjustable cam follower. This cam follower is held by two anti-backlash springs against a large cam (Figure 3). The cam is rotated by means of the adjusting ring, which carries a scale graduated plus or minus 15 minutes from zero, each minute being subdivided to 0.2 minutes (Figure 4).

Figure 3 : Adjusting cam.

Figure 4 : 3/4 Plan view.

Figure 5 shows the 4-power telescope. The images of the two horizons are seen to be vertical and, by rotating the adjusting ring they are brought into coincidence, when the dip can be read off the scale. The zero is set using two autocollimators, aligned as described in the preceding post An Improvised Dip Meter.

Figure 5 : The telescope.

The instrument is well-sealed against the ingress of moisture by greased felt rings where there are rotating parts and by heavy wax at metal-to-metal and glass-to-metal joints. It is provided with a stout leather carrying case. Figure 6, modified from a drawing in the original Russian handbook, shows the general arrangement of the parts. It can be seen at a larger scale by clicking on the picture. Use the back arrow to return to the text.

Figure 6 : General arrangement drawing.

To use, the window with the slit is directed to the brighter horizon and the slit adjusted to make its brightness equal with the opposite horizon. The adjusting ring is rotated to bring the horizons (which will be seen to be vertical) into coincidence and the reading noted.. The observer then rotates through 180 degrees (about a vertical axis, of course) and also rotates the instrument through 180 degrees on a horizontal axis. A second reading is taken and the dip taken as the mean of the two.

I have provided this post to support my book The Nautical Sextant, which covers solely the nautical sextant. If you have enjoyed reading this and others of my posts, I am sure you will enjoy reading the book, available from the publishers, from Amazon and from good booksellers.

Fleuriais’ Marine Distance Meter

4 11 2012

This post was preceded by  ” A Stuart Distance Meter”;“A Russian Naval Dip Meter”; and  “An Improvised Dip Meter”

In 1890, Admiral Georges Ernest Fleuirais (1840 to 1895) published a description of his “Micrometre à réflexion”. At the time, he was Director of the Cartographic Department of the French Navy and he had had a distinguished scientific career. He had led an expedition to Santa Cruz in Patagonia, as part of an international effort to observe the transit of Venus on December 6th, 1882. The international cooperation led to the solar parallax being established as 8.847 ± 0.012 seconds, allowing the distance of the Sun to be calculated as 92,384,000 miles (148,677,000 km). He also invented a sextant provided with a gyroscopic artificial horizon.

His micrometre was in fact a marine distance meter, in which the angle subtended by an object of known height is used to calculate its distance. For example, if the mast head of a distant ship is known to be 30 metres above the water line and the angle between the masthead and the waterline is 2 degrees, the ship lies  approximately 30/ tan 2 º = 860 metres away. I recently acquired a French Navy Fleuriais Micrometer, as it is a doubly reflecting instrument of the sextant type, and was able to examine it in detail.

Figure 1: View of front (left-hand-side).

It is immediately obvious that it is a sextant-type instrument. It has a radius of about 85 mm, a plate brass frame about 4 mm thick and two mirrors which, for want of better terms have to be called the index and horizon mirror, even though the horizon is not usually viewed through either. Instead of the usual arc, the instrument has a micrometer which  looks much more like an early engineer’s micrometer than the typical sextant micrometer which, in any case, had yet to be invented by C Plath in about 1907. The periphery of the drum is divided into 100 minutes and 12 turns of the screw, as denoted on the index, cover an observed angle of 1200 minutes, or 20 degrees (Peter Ifland in his Taking the Stars incorrectly writes that the drum is divided to read 1/100th of a degree). As the drum is rotated, the micrometer screw advances through an adjustable nut and presses on the capstan head of a screw attached to the end of the index arm, thus rotating the index mirror. A spring takes up any backlash between the head of the capstan screw , the micrometer screw and the nut . The nut can also be closed up to adjust the clearance between it and the screw (Figure 2). Legs on the face of the instrument allow it to be put down without changing hands. The Galilean telescope is x 3 power with an objective lens of 24 mm diameter.

Figure 2: Details of adjustments

The capstan headed screw is used to adjust out index error, while another screw acts on the base of the horizon mirror to adjust out side error if required, though in such instruments a little side error is helpful while having a negligible effect on the accuracy of the observations. The base of the mirror bracket is slotted and a captive screw is used to close or open the slot in order to tilt the mirror (Figure 3). No provision is made to adjust the index mirror for perpendicularity.

Figure 3: Side error adjustment.

Figure 4 shows the almost featureless rear or right hand side of the instrument. The traditionally shaped handle can be held comfortably either way up, so that observer who wishes to hold the instrument in his left hand may do so at the cost of some slight discomfort while operating the micrometer.

Figure 4: Rear (right hand side) of instrument.

Early instruments were provided with a drum fitting around the telescope to convert the angle reading to a distance, but later ones came with a circular slide rule devised by  Commander Émile Guyou,( 1843 – 1915) shown in Figure 5. The index arrow was set against the height of the object on the outer circle and its distance in metres read off the inner scale opposite the angle in minutes on the outer scale. In the figure, the index is set at about 91.8 and if the micrometer had read 60 minutes (1 degree), then the distance could be read off as about 5,260 metres.mile

Figure 5: Guyou’s circular slide rule.

While I continue to acquire, restore and describe sextants, I also have a small collection of chronometers, and have recently completed a book The Mariner’s Chronometer which will be available via amazon.com from 10 November 2012.

A Stuart Distance Meter

13 07 2012

 This post was preceded by  “A Russian Naval Dip Meter”; and  “An Improvised Dip Meter”

When sailing in company with other ships, as for example, in a convoy, or when maintaining a safe distance when rounding a danger, it was useful to know the distance of one’s ship from the other objects. Until the advent of radar, a variety of distance meters was used. The most obvious is perhaps the sextant, likely to be found aboard every ship of any size. If some dimension of the object is known, like height of the mast above the waterline, and the vertical angle subtended by the dimension is measured, its distance can be calculated, but every self-respecting set of nautical tables had a table of “Distance by vertical sextant angle” to obviate calculation. A variety of distance meters was invented in the late nineteenth century to eliminate even the incovenience of looking up tables by giving a direct reading of the distance once the mast height had been set.

Most, like that of Fiske, in effect used a modified sextant that read through a relatively small angle while having a scale that gave the distance directly in yards. Recently I came into the possession of a Stuart distance meter that uses a different measurement principle, somewhat similar to that of the N5 dip meter described in the preceding post. The instrument was very dirty and the ivorine scales had shrunk and torn away from their screws, but happily no parts were missing and I anyway paid very little for it. Figure 1 shows the meter after cleaning and restoration.

Figure 1 : General view of distance meter.

The height of the object, say, a ship, up to 200 feet, is first set against the left edge of the transverse height scale. This need not necessarily be mast head to waterline. The note pad on the other side of the meter has provision for noting also the distance from the mast head to the “lower top” and “Upper speed(?) to stern lt.” The ship is then viewed through the telescope, when a field split vertically is seen. The image of the head of the mast in one half is brought alongside the image of the waterline in the other half by rotating the knob, when the distance in cables (a cable is one tenth of a nautical mile) can be read against the index on the distance scale. In Figure 1, the height is set to 60 feet and the distance is one cable.

Figure 2 shows the somewhat shrunken note pad on the front of the instrument and Figure 3, showing it with the telescope removed, begins to reveal some of its workings. In front of the left half of the telescope objective is a fixed slice of a negative lens of about -2.3 dioptres (about -440 mm) and a similar but longer slice is in front of the right half of the field. This latter lens is attached to a slide that carries the scale and as the slide moves through a usable distance of about 60 mm, the images separate as shown in Figure 3. Note that in Figure 2 “Patt 498” is probably a naval designation and cetainly not a reference to a patent. The telescope is about x 3 power and has an interrupted thread that allows it to be fitted in its bracket with just one sixth of a turn

Figure 2 : Front of distance meter

Figure 3 : To show split image.

Figure 4 shows the effect of time and sunshine on ivorine. I replaced the scale with a sheet of brass 1.6 mm thick and glued to it a paper scale copied from the original scale. It does not allow for shrinkage and the meter is probably no longer accurate, but it does allow the principle of the meter to be illustrated.

Figure 4: New scale for old.

Figure 5 shows the relative complexity of a Stuart distance meter’s competitor in the form of a Fiske-type distance meter or “stadimeter”, invented at  the same time in about 1895. While the Stuart instrument has a single slide machined in an aluminium casting requiring no great precision of manufacture, in the Fiske instrument the distance screw and scale are carried in a close fitting bronze carriage running in a precisely machined bronze frame. There are two mirrors, each needing means of adjustment, two lead screws and a bearing for the height scale, which corresponds to the index arm or alidade of a nautical sextant. It may well be that the Fiske is capable of greater accuracy of measurement, but no great accuracy is required in station keeping in a convoy, while one would err on the side of caution in rounding a danger. It may be that the instruments were originally envisaged as a range-finder for gunnnery or as a rangefinder during a “creeping attack” by two ships hunting U-boats. In this, one attacking ship remained at 1000 yards  astern of the submarine, where the latter was in its asdic cone and guided another ship moving from astern at slow speed so that its approach was masked by the submarine’s own propellor noise, until the distance of the sub by asdic and the distance of the ship by rangefinder coincide.

Figure 5: Fiske-type stadimeter or distance meter (Mark II, Mod 0).

A reproduction MHR1 Position Line Slide Rule

10 06 2021

A guest post by Arthur Leung

This post was preceded by  “Hughes Marine Bubble Sextant”, “An improvised sun compass”, ” C Plath Sun Compass”; “A Fleuriais’ Marine Distance Meter” A Stuart Distance Meter”;“A Russian Naval Dip Meter”; and  “An Improvised Dip Meter”

A little while ago I was idly reading and came across something called the Bygrave Position Line Slide Rule.  Being already interested with things navigational, I started looking for more information about what this instrument was and how to use it.  One thing led to another and before long I desired to make my own reproduction.  To that end, I contacted Bill Morris with a request that he publish a blog post about his reproduction of the German version of the Bygrave manufactured by Dennert and Pape called the MHR1. Bill has added remarks in parentheses in italics.

(I had made the rule several years previously as a construction project and, having made it and tested it once or twice, placed it in storage.)

At the time I mostly hoped that he’d read my request and eventually write a post that covered the details of the methods he used to make his.  Much to my surprise, he responded almost immediately and offered to send it to me to inspect.  (For the most part, construction consisted of simple turning, sawing and filing)

I, of course, jumped on the offer.  A few weeks later, this instrument arrived, very carefully packed.

Figure 1 Bill Morris’ fine work

Here are my musings that contains a little history, a little description, and a bit about using one.

Prior to the widespread introduction of modern sight reduction tables, a navigator would reduce sights by referring to books of trigonometric tables, reading them out, then adding/subtracting/multiplying them together, and then going back into the tables to extract an angle.  For a 3-star fix, the amount of effort required would of course by multiplied by three.  This can take a bit of time as it is a painstaking process. (When I started to take an interest in navigation more than forty years ago, in the (forlorn) hope that someone would invite me aboard a yacht for an ocean voyage, this is how I reduced my sights, using Burton’s Nautical Tables. Then sight reduction tables followed and finally I used a scientific calculator.)

The 1920s and 1930s saw aircraft capabilities in both speed and range increasing rapidly and, since aircraft can move considerably faster than a ship, a fix determined 30 minutes in the past is less helpful than a fix determined 5 minutes in the past.  Thus, considerable effort by many people and institutions went into fixing the position of an aircraft more quickly.

Captain Leonard Charles Bygrave (RAF) at the Air Ministry Laboratories developed what would become the Position Line Slide Rule in the early 1920s.  The motivation was to provide an air navigator a compact, lightweight, accurate enough, and fast means of solving the celestial triangle so the navigator could determine the altitude of the body (Hc) and the true bearing to the body (Zn).

The equations to solve the celestial triangle use trigonometric functions in combination.  The equations themselves are shown below and were found on the following web site: Formulas (thenauticalalmanac.com).

Hc = sin-1[sin(declination) x sin(Latitude) + (cos(Latitude) x cos(declination) x cos(LHA)]

Z  = cos-1[(sin(declination) – sin(Latitude) x sin(Hc)) / (cos(Latitude) x cos(Hc))]

Note that Z requires some additional, minor, manipulation to form Zn depending on the magnitude of Z and whether the assumed position is in the Northern or Southern Hemisphere but that is beyond the scope of this post.

In theory, a standard slide rule with the trigonometric functions on the face could be used to solve these equations.  This method is more compact than the traditional method of trigonometric values taken from a large book of tables but still quite painstaking as each step needs to be recorded on paper, sums and products taken, additional sums taken, and then inverse trig values extracted.

Taking the slide rule idea slightly further, however, the scales of a slide rule could be created to enter assumed position and declination and then directly extract Hc and Z which would greatly simplify and speed up the process.

The problem with a linear slide rule is that, to get the required accuracy, the slide rule might be many meters long.  The solution to this problem is to wrap the scales in a helix around a cylinder of modest diameter.  In this way, the required precision could be attained while also having a reasonably compact instrument.

The Bygrave solution wraps a log-cosine scale around a middle cylinder and a log-cotangent scale around a slightly smaller, coaxial cylinder.  The smaller cylinder has an outer diameter such that it forms a light friction fit with the inner diameter of the middle cylinder.

A third coaxial outer cylinder contains the two inner cylinders and holds the cursors to align the two cylinder’s scales, along with instructions to the user.

Figure 2 Bygrave Position Line Slide Rule

The helical log-cotangent scale length, when unwound, is 758.4cm (7.58 metres) while the helical log-cosine scale length is 414.3cm (4.14 metres).  The instrument, when fully collapsed, is 23.1cm long and 6.6cm in diameter.  Aluminum is the primary material used in its construction, so the mass is reasonably modest.

The Bygraves were made by Henry Hughes & Son and appear to have been made thru the 1940’s but production records appear to have been lost.  Very few original Bygraves remain, mostly in museums.

The German and Japanese saw the benefits of the Bygrave and each made their own versions with some minor differences.  The German versions added a locking mechanism to lock the two inner cylinders together rather than relying on simple friction – having the scales rotate out of alignment while doing the manipulations was a user complaint when using a worn out Bygrave.  The Japanese version appears to be, other than the instructions, direct copies of the Bygrave.

The German units were made by Dennert and Pape and called the “Höhenrechenschieber” (altitude slide rule).  The HR1 and the marine-ized (or perhaps “modified”) MHR1 had similar characteristics to the Bygrave.

Figure 3 Dennert and Pape MHR1 fully collapsed for storage and transport

A considerably larger, and thereby likely more accurate, variant was the HR2.  It is believed that this unit was meant to be bolted to a table so likely was a naval instrument.

Dennert and Pape production records show the MHR1s were made thru the waning days of WW-II and many were likely never issued.  After the war, a number of units were manufactured under the Aristo brand until about 1958 and may have been sold until the 1970s from remaining stock on hand.  Compared to Bygrave slide rules, a larger number of HR1/MHR1 units still exist but they are still quite rare.  One recently came up for auction and sold for more than US$1700.

Bill Morris’ 2010 reproduction was of an MHR1, so I’ll mention a few specifics about the historical MHR1.

Figure 4 MHR1 showing full extent of scales

The inner tube is 5.4cm in diameter and approximately 23cm long and carries a log-cotangent scale in a helix.  The scale length is 758.4cm over 45 turns and a pitch of 0.45cm.  The scale runs from 0⁰20’ to 89⁰40’ and then back from 90⁰20’ to 179⁰40’.  Because of the nature of cotangents, the scale is condensed near 45⁰ but the marks become further and further apart near the extremes.

The middle tube is 5.8cm in diameter and 22.5cm in length and carries a log-cosine scale in a helix.  The scale length is 407.3cm over 22 turns and a pitch of 0.45cm.  The scale runs from 0⁰ to 89⁰40’ and then back from 90⁰20’ to 180⁰.  Because of the nature of cosines, the scale is condensed near 0⁰ but the marks become further and further apart as we near 90⁰.

The tube is 6.2cm in diameter and 22.2cm in length.  It holds two Perspex windows with red index pointers to align the two scales.

Instructions are printed on the outer tube along with a scratch pad for pencil scribbling by the user.

Bill’s reproduction is almost identical in dimensions and other outward details including the Perspex windows with red index pointers, and end caps.  I am unaware of the details regarding the locking mechanism, but Bill’s reproduction does have a working locking mechanism which I find very handy. (Rotating the locking knob causes a close-fitting  O ring to be squeezed by a circular plate. This increases its diameter slightly so that friction can be increased to the point of locking.) Purists may note that a “real” MHR1 locks the cylinders by turning the knob counter-clockwise, whereas the reproduction locks by turning clockwise, a detail which takes nothing away from the reproduction.

And, it should be added, no one seems to have yet found the courage to take an actual MHR1 apart to inspect the details of the locking mechanism internals. (The original appeared to pull a cone into a split tube to open it out.)

Unlike the originals, which used aluminum tubes, this instrument uses laminated pressed paper tubes with the scales printed on paper and then glued to the tubes.  I understand Bill sourced the tubes from Wolfgang Hasper, a member of the NavList discussion board and the tubes appear to still be available for about 40 Euros despite the passage of a number of years (See References at end of this post).

The cotangent scale runs 1⁰30’ to 88⁰30’ and then back from 91⁰30’ to 178⁰30’ while the cosine scale runs from 0⁰ to 89⁰30’ and then back down from 90⁰30’ to 180⁰.  This is slightly different than the original scales but the areas where they differ are only in the expanded region of cotangent and cosine where adding the additional scale length provides very little added range while increasing the length of the helix by a non-linear amount.  As mentioned below, the scales were generated by some very smart people at NavList and this may well be an optimization over the original scales.

The instrument has a bit of heft to it (I lack a proper scale but it feels to be around half a kilogram) and has a solid feel.  However, as the tubes are not metal, I take care with it so as not to drop it.

I’m not sure where Bill found the images for the original German language instructions but they are also faithfully reproduced on the outer cylinder. (I think I may have simply copied them using a near-identical typeface.)

Figure 5 Instructions

Figure 6 More instructions, scribble pad, and stop

The outer cylinder is also made of laminated pressed paper and the upper part is painted black to match the historical instruments.  Two windows are cut out to show the scales.  Attached to each window is a piece of Perspex molded to fit the different diameters of the two scale cylinders.  Each Perspex piece has a red index mark.  Visually, this is very similar to a historical MHR1.

Figure 7 Cosine scale index

The lower end cap appears to be plastic – I do not know how he made this piece but it appears to follow the form of the original very closely. (I think I used a scrap of the cursor tube and milled grooves in it.)

The upper end cap and tightening knob are turned and milled from aluminum stock.  Bill says that the knob is a bit smaller than the historical instruments but I find no loss in functionality because of it.

Figure 8 Cotangent scale index, upper endcap, and locking knob

Bill got the scales from the clever and skilled people on NavList as PDFs which are easily printed at home.  I believe the scales taken from NavList were scaled to fit the outer diameters of the MHR1 cylinders exactly – attempting to fit the scales to tubes of different diameters would necessitate a bit of experimentation with the printer magnification settings.

Care must be taken to align the scales on the tubes so that the edges butt against each other without gap and without overlap.  Bill did an outstanding job, of course, putting the scales on.  He then added a bit of shellac (French polish) to protect the scales.  He tells me, “Purists say that French polish is a process, not a substance.”

The tubes rotate with just enough friction to make the scales easy to adjust both in rotation and in length.  They will stay put without the locking mechanism being set and the outer tube mostly protects the middle tube from unintended inputs when manipulating the inner tube.

I feel that Bill has created a very fine instrument faithful to the lines and function of the original Dennert and Pape MHR1s.

So, how well does it work?

I shall present the answer to the question in two parts by answering two more directed questions.  First, how easy it is to manipulate?  Second, how accurate are the results taken from it?

I have never held an actual Bygrave or MHR1, so I cannot speak for those instruments.  However, I cannot imagine they would be significantly different to manipulate than the reproduction.

The manipulations are relatively straightforward, but some practice is required before competency is attained.  As mentioned above, the scales for log-cotangent are very compressed around 45⁰ and become widely spaced around 0⁰ and 90⁰ – and likewise for the log-cosine scale being compressed near 0⁰ and more widely spaced around 90⁰.  For those of us with aged eyes, a pair of reading glasses and good lighting will be helpful to read the scales.  The excellent index line that Bill constructed helps greatly in the alignment department.

Bygrave’s original patent stated that the inner cylinder carried a log-tangent scale, but it is believed that this was switched to a log-cotangent scale for production instruments because it allowed the scales to advance in the same rotational direction.  This is a great help in practice when the scales become widely separated as it is quite easy to misread the value on the scale.  For example, is this 87⁰5’ or 87⁰55’?

Figure 9 Labels are far apart at the extremes

Since both scales advance to the right, a potential source of errors is eliminated.

That said, it is still easy to mis-read the scales, especially when aligning from one scale to another – trying to read an unlabeled mark on the cotangent scale while aligning to another unlabeled mark on the cosine scale does require a bit of back-and-forth twisting.  I dare not put a pencil mark on the paper/shellac reproduction, but I have a sneaking suspicion that is precisely what navigators did on historical instruments.

Locking the two scale-carrying cylinders together is quite easy.  Historical Bygraves did not have this feature and a worn-out instrument could have sufficiently reduced friction to allow them to slip relative to each other which would result in errors.  Care must be taken to align the index pointers carefully before locking them, however, as the two cylinders will be rotated and slid along the central axis – by carefully aligning the scales at the start, the index pointers will be aligned to the correct part of the helical scale after sliding them to the next step in the process rather than being ambiguously in between.

The reproduction is still quite pristine so there is a reasonable amount of friction between the cylinders from a liner layer of baize between the tubes so a locking mechanism isn’t important, yet.

Now, we tackle using it.

One of the nice things about the Bygrave is that one is able to find Hc and Zn from a Dead Reckoning (DR) position which can have non-integer LHA, latitude, and longitude.  This is in contrast to typical HO229 and HO249 reductions where one uses an assumed position to get integer LHA and latitude.  In theory this saves a plotting step since the navigator can plot directly from the DR position.  Of course, it would still be possible to work the fix using an assumed position different from the DR position that would provide integer inputs – an advantage with using integer inputs would be that the scales would be easier to read as you could move the pointers to the integer values on the scales for some, but not all, of the steps.

To make it more interesting I will show an example using an “interesting” DR position as the input to the computation, which was, incidentally, my first try with the instrument.  Here are the parameters:

  • May 19, 2021 – 01:52:00 UTC
  • Object: Spica
  • DR position: 34⁰1’N, 77⁰2’W

From the Nautical Almanac:

  • GHA-Aries: 251⁰55.5’
  • GHA-ms increment: 13⁰2.1’
  • SHA-Spica: 158⁰25.2’
  • Computed GHA-Spica: 63⁰22.8’
  • Declination Spica: S 11⁰16.4’

In the Western Hemisphere, I subtract the DR longitude from GHA-Spica to get LHA-Spica: 346⁰20.8’.

Now we have the information that allows us to enter into a special worksheet.  This worksheet is from Gary LaPook’s “flat Bygrave” work and is applicable here.

We must first calculate “H” which is a function of LHA – since LHA is greater than 270 and less than 360, we enter LHA into the corresponding column and, as instructed by the form, subtract it from 360⁰ to get H of 13⁰39.2’.  Because DR latitude and declination are contrary name we circle “-W” – then enter these values onto the worksheet along with a “-“ on the W line and the declination in D.

Figure 10 Initial pre-computation

Now we pick up the Höhenrechenschieber and begin to follow the zig-zag instructions on the worksheet or those on the side of the instrument.  The first zig-zag solves for W, the second for Az, and the last for Hc.

Figure 11 The three manipulations we will do on the instrument

Set 0 under the index line on the cosine scale:

Figure 12 Set 0 on the cosine scale

Then, while ensuring that the index remains on 0 on the cosine scale, set declination on the index line on the cotangent scale:

Figure 13 Set 11-16.4 on the cotangent scale – note scale expansion so care must be taken to set the pointer

Now we lock the tubes to they do not shift relative to each other.  Then shift the cosine scale so that H is under the index line:

Figure 14 As close to 13-39.2 as I can photograph

Now read W from under the index line on the cotangent scale:

Figure 15 Trust me, that says 11-36

We write the extracted value of W (11⁰36’) on the form reserved for it – remember in this example, we previously wrote “-“ on the line so W is negative.  Enter the DR latitude and compute the “co-latitude” as 90⁰ – DR latitude:

Figure 16 Enter W read from cotangent scale, enter latitude and calculate co-latitude

Summing the resulting co-latitude with W, we compute X.  As X is < 90, we set Y equal to X.

Figure 17

Now, with W, H, and Y computed, we can pick up the instrument again.  First, we unlock the scales from the previous step, then we set W on the cosine scale:

Figure 18 W = 11-36

Then set H on the cotangent scale:

Figure 19 Set H=13-39.2 (the “16” to the right is showing “166” (the angle complement of 14), not “16”

Then lock the cylinders together, then slide the scales so that Y is on the index on the cosine scale:

Figure 20 Rotate to set Y = 44-23

And finally we read Az from the index on the cotangent scale.

Figure 21 Read Az as 18-24.5

Then enter Az on the worksheet space.

Figure 22 Insert the Az read from the cotangent scale onto the worksheet

The worksheet lists rules for computed Zn for North and South Latitudes, LHA, and DEC/W vs. DR latitude. 

Figure 23 Rules for computing Zn from Az – LHA = 346deg 20.8′, Contrary name

In this case, declination and DR latitude are contrary name and LHA is between 180 and 360 so we use Zn = 180 – Az and enter it onto the worksheet.

One more step on the device is required to extract Hc.  We start by unlocking the scales, then setting Az on the cosine scale:

Figure 24 Set Az on cosine scale

Then setting Y on the cotangent scale and re-locking the scales:

Figure 25 Set Y on cotangent scale

Then we rotate the cosine scale to set 0 on the index:

Figure 26

And we then read Hc from under the index line of the cotangent scale:

Figure 27 Done!

Figure 28 Extract Hc from the cotangent scale and enter it on the worksheet

For this exercise we ignore the worksheet entries for Ho and INT – these are the standard sextant observed altitude and the intercept distance (Zn-aligned To/Away) from the DR position, which is in common to standard methods of plotting lines of position.

I have programmed a spreadsheet with the equations to compute Hc and Zn directly from the trigonometric formulas.  This spreadsheet takes the same inputs as the MHR1 – namely, DR latitude and longitude, declination of the object, and LHA of the object.  The results for both the MHR1 computations and the spreadsheet are:

Hc161⁰ 35.5’161⁰ 36.0’
Zn42⁰ 53.0’42⁰ 53.1’

I’d say this is a pretty good result and certainly not bad for a first try!  Bill’s excellent workmanship made the manipulations easy and at no point did I have to expend very much of my limited brain power on fighting the instrument.

I must admit that this wasn’t my “first try” with a Bygrave-like approach.  As part of my research, I found LaPook’s “flat Bygrave” and so printed the scales onto paper and transparency to make one.  While not to say that the “flat Bygrave” cannot be just as accurate, I could not get the same level of accuracy as with Bill’s excellent reproduction.  The relatively small size of the flat scales and the lack of color saturation of my ink-jet printer conspired to make my printed “flat Bygrave” very difficult to read, but it does have one advantage over the cylindrical instruments in that you cannot get lost on the scales since you can easily see the degree marks on either side of the “index.”

I managed a 3-star series using only my sextant, the MHR1, and the Nautical Almanac and came up with a plot that was quite good.

Figure 29 Look, Ma!  No HO229!

Sadly, there will never be a market for a mass-produced Position Line Slide Rule.  Time and technology have moved past.  As it was, it took me all of 5 minutes to program a spreadsheet to solve for Hc and Zn, and then milliseconds for the computer to spit the answers out after I enter the last parameter.  However, there is no sense of accomplishment from tapping at a keyboard – unlike twisting, turning, sliding, scribbling, and sweating to get the required answers.  And, maybe for a moment, I am a young navigator on a trans-Atlantic crossing standing under an astrodome and swaying sextant trying to guide my aircraft and crew to a safe landing on the other side.

Thank you, Bill, for entrusting his beautiful instrument to me and letting me play with it.  Your kindness is greatly appreciated. 

Arthur Leung

North Carolina, USA


A better history than mine of the position line slide rules can be found here in Ronald W.M. van Riet’s excellent paper, a link to which is here:

PositionLineSlideRules.pdf (rechenschieber.org)

A Bygrave manual may be found here as part of a post by Gary LaPook:

NavList: Bygrave slide rule (106329) (fer3.com)

Gary LaPook’s writeup on using the Flat Bygrave and PDF of his workform, both pertinent to working a “not-flat” MHR1, can be found here:

NavList: New compact backup CELNAV system (changed for archive) (107414) (fer3.com)

Modern Bygrave Slide Rule – Fredienoonan (google.com)

Virtual Bygrave emulators also exist:

SlideRule (friendsofthevigilance.org.uk) – historical Bygrave emulator

SlideRule (friendsofthevigilance.org.uk) – flat Bygrave emulator

For those wishing to try the “Flat Bygrave” or build a cylindrical Bygrave, PDFs of the scales may be found here in posts by Robin Stuart:

NavList: Flat Bygrave alternative configuration (127166) (fer3.com)

NavList: Postscript code for making Bygrave Scales (127398) (fer3.com)

The two scales must be printed separately but care needs to be taken so that the magnification is correct so the scales will align properly between the two cylinders. 

Wolfgang Hasper had additional tubes, as of 2019, for those wishing to make their own:

NavList: Re: Buy or build a modern Bygrave slide rule (146016) (fer3.com)

Hughes Marine Bubble Sextant

26 11 2018

This post was preceded by  “An improvised sun compass”, ” C Plath Sun Compass”; “A Fleuriais’ Marine Distance Meter” A Stuart Distance Meter”;“A Russian Naval Dip Meter”; and  “An Improvised Dip Meter”

Jaap Brinkert has kindly provided the following post . With his agreement, I have added an occasional comment in blue.

Recently, I won the bidding on a ‘Vintage Marine Sextant’ which I soon discovered to be rather unusual. At first sight, it resembles the Hughes Mk IX bubble sextant as used by the RAF (and others) during WWII (Figure 1 and 2).  However, this sextant is intended for marine use. It appears that the German navy started using bubble sextants on board submarines, so that they could take sights when surfaced at night The Hughes Marine Bubble Sextant (HMBS) was the English answer, developed after tests using the Mk IX on board a submarine. {1} “Highly accurate results” seem to me to have been unlikely. One to three nautical miles would be counted good using a normal  nautical sextant and the natural horizon.


Figure 1: Rear view of Mk IX A and HMBS sextants

A good general description of the device is given in “Specification of instruments exhibited at the seat of the international hydrographic bureau during the Vth international hydrographic Conference, Monaco, April 1947,” in which exhibit number. 8 of Marine Instruments Ltd, entitled “Marine Super Integrating Sextant” is described as follows. “The instrument consists of a sextant, the mechanism of which is totally enclosed with the usual fixed horizon mirror and adjustable index mirror. The mechanism is arranged to give arbitrary increments of altitude of 10 degrees, -10 to 90. Attached to the main body of the sextant by two screws is the bubble complete with eyepiece through which the observer sees the bubble and the object observed. A single instantaneous observation is made by setting the next lowest whole tens of degrees and then using the slow motion to obtain coincidence between the centre of the bubble and the object, reading the altitudes on the tens of degrees scale and the degrees and minutes scale (instantaneous reading. 

An averaging observation is made by maintaining the coincidence as nearly as possible during the one-minute period of observation between starting the clock drive and the automatic raising of the cut-off shutter, the altitude being read on the tens of degrees scale and the degrees and minutes scales (averaging). 

A second bubble unit is provided, interchangeable with that on the instrument. This unit is exactly the same as the first, except that it carries a 2X Galilean telescopes mounted in the unit itself, which, when sea conditions permit, gives brighter star images than would otherwise be obtained.  

Two dry batteries and two spare lamps are supplied.”

LH side

Figure 2: LHS of Mk IX A and HMBS sextants.

The Marine Bubble sextant has an entirely different mechanism for averaging, which is contained within the main body, where it is protected from salt spray. It is a continuous integrating mechanism, which runs for one minute. The adjustment of the index mirror also sets the transmission ratio between a slender cone, driven at constant speed by a spring mechanism, and a cylinder connected to the totaliser (Figure 3).


Figure 3: Integrator mechanism.

This is in effect the reverse of the mechanism used in the German SOLD and Kreisel (gyro) sextants, where the inclination of a roller that bears  on a shaft moving longitudinally, variably rotates the shaft, on the end of which is attached the read-out. Full details may be found in the post for  4 November 2013 . After the one minute run, the average position of the index mirror is read from a dedicated dial, and added to the setting of the index mirror, for instance 70 +4 35′.

The averaging device for the Mark IX A aeronautical sextant sampled one sixtieth of the reading every two seconds for two minutes, in effect integrating the reading over sixty intervals. The potential disadvantage of this is that if the sampling interval happens to coincide with the approximate frequency of rolling of the vessel, large errors may be introduced. The HMBS, like the SOLD and Kreiselsextant, continuously integrates the reading.

 https://www.thejot.net/article-preview/?show_article_preview=85&jot_download_article=85 has a submarine rolling at 2.4 to 3.9 seconds for a closed casing submarine. The implication of this is that it might be best to avoid sampling periods in this range. (Thanks to Murray Peake for this information)


Figure 4: Calibration record.

The calibration record (Figure 4) is interesting as it illustrates the consequence of a difference in timing mechanism between the SOLD and the HMBS. The former contains a “proper” clockwork with balance wheel and escapement. The latter, on the other hand, contains a regulator mechanism which uses centrifugal force and friction (if the regulator turns too fast, it ‘expands’ against a stationary drum, resulting in deceleration). This mechanism needs to be calibrated. The certificate shows  a deviation of up to 1%, depending on the set angle). A small error in counter reading follows automatically. The calibration also shows the separate extra corrections for the two bubble units. Using this sextant requires quite a bit of bookkeeping!

As for a normal sextant, the HMBS has a conventional set up, except for the placing of the shades (Figure 5).

Front labelled

Figure 5: Front view, showing mirrors

The index mirror is rotated on a shaft which emerges from the main housing, and is operated by a mechanism described below. In operation, this mechanism is similar to that of the Mk IX series. There is a large step setting in tens of degrees (-10 (“D”) to +80 ) and a fine setting ( 0 0′ to 14 50′ in 1.5 turns of the adjusting wheel). The index mirror is quite large: 70 mm by 27 mm. Another difference between the Mk IX and the HMBS is the horizon mirror: it has no ‘5 degree increase’ facility, which also simplifies the read out mechanism for the averager. The horizon mirror of the HMBS is fixed; the whole nearly 15 degree range is set by the mirror fine-setting control. The Mk IX index mirror is the same length but only 24 mm wide. In both sextants, the length of the mirror is required because the axis of rotation of the mirror is quite far behind the mirror in order to accommodate a central helical spring and concentric shaft mounting (Figure 6).

Index mirror 3

Figure 6: Index mirror mounting.

The fixed mirror, on the other hand, is small, only 28 by 20 mm. It is fully silvered, so if  the natural horizon is used, it must be viewed past the horizon mirror. In my sextant, both mirrors have deteriorated over time, so I plan to replace them with the help of the local glazier and optician.

There are three small shades (18 mm diameter transparent area). Both the horizon and the index mirror are equipped with two adjustment screws on opposite corners, in the usual fashion. In order to use the natural horizon, it is necessary either to use no shades, or to remove the bubble unit, because the shades, small as they are, cover the whole view. A sun sight using the horizon is therefore not possible with the bubble unit in place, and in any case the instrument would not normally be used in daylight with the natural horizon available..

The aim seems to have been to produce a waterproof device, and this is clear is clear from the fact that to open the main housing, 11 screw must be undone. The separate bubble scope’s lid is fastened by no less than 17 screws.  Indeed, the internals looks as if they left the factory yesterday.


 As in the MkIX series, a shutter cuts out the view unless the integrating mechanism is fully wound or running,  to signal to the user that the one minute integrating run is over. The user could then immediately look at his watch on the inside of his left wrist or, more likely on a submarine, call out to an assistant to mark the time. In the Mk IX series, the left wrist was illuminated via a prism in the right hand side handle but this ingenious system which also projects a beam to each of the read-outs using a single bulb, is only partly used in the HMBS. Instead, the handle is attenuated and the prism and some holes  omitted. One of the several remaining holes for the lighting of the scales is visible in Figure  5.

RHS labelled

Figure 7: Controls and read-outs.

 A winding lever primes the integrator by ten strokes of the lever shown in Figure 7 and the integrator is started by operating the lever seen below the ten degrees adjustment knob. As in the Mark IX series this latter is pushed in against a spring load to rotate through ten degrees steps, governed by the three groups of holes seen in Figure 6. Further adjustment is by rotating the fine adjustment knob. Two windows give the instantaneous readings of the altitudes in tens and one degrees, and minutes are read from a further window. A fourth window behind the handle gives the integrator readout, which must be added to the tens of degrees shown in the top window.

The bubble unit

The unit, which is apparently taken directly from the Mk IX series,  is attached to the main body by two screws. It contains the bubble mechanism, a partially-reflecting mirror and a mirror/lens assembly (Figure 8). There is a slanted clear glass window on the front and a clear glass window at the rear for the eye.

Bubble unit labelled

Figure 8: Interior of bubble unit.

The principle of the bubble unit is shown in Figure  9. The bubble is lit by daylight or a bulb via a Perspex do-nut directly below the bubble chamber.  Light rays, shown in yellow, then pass through a partially reflecting glass (shown in white) and are reflected by a mirror-lens combination whose focal length is the same as the radius of curvature of the top of the bubble unit. The bubble is in the focal plane of the mirror-lens, so the reflected rays emerge as parallel rays and are reflected into the eye via the partially reflecting glass. Rays, shown in red, from the observed body via the fixed “horizon” mirror pass straight through the partial reflector.  Effectively, the body and the bubble then both appear together at infinity at the eye.

Light path 2

Figure 9: Light paths in bubble unit

Despite the complexity of the sextant, and thanks to the use of a light alloy for all housing parts, the weight is just over 2 kg (2045 g) (including batteries). The German WWII Kreiselsextant (Gyro sextant) weighs by contrast 3 kg.


The bubble unit contains a light bulb and a simple intensity regulating mechanism. A strip with a vee-shaped slit, which is placed between the light bulb and the bubble unit, is moved up or down using a knurled wheel, seen labelled in Figure 8.

The left handle of the sextant is a battery holder for two C size cells. The rotary switch is operated by the left thumb. Turned clockwise it activates the bubble lighting and turned the other way lights the readouts as noted above. The bulb socket for the latter ought to be in the right hand side handle,  but it is missing on my sextant.

Box (Figure  10)

The box is made of solid mahogany, and has a stout leather strap over the lid, which can be used to carry it. There are green felt covered blocks to immobilise the sextant. There is a similar arrangement for the spare bubble/scope, which is secured by two keepers (one of which was missing). There are two battery holders, which are obviously intended for the now obsolete Eveready No 8 (3 V). When two batteries of size C are stored in each holder, the lower ones can only be removed by holding the box upside down.

In case

Figure 10: HMBS in its case.


I do not know the recent history of this sextant. It was donated to a Sea Scout group and sold on eBay to raise money. Its production date could be close to 1949, judging from the serial number (123).The National Maritime Museum at Greenwich has a Marine Bubble Sextant with serial number 114 with a certificate by Henry Hughes & Son dated 3 March 1949.

The certificate of my sextant is dated 18 July 1978, issued by Fenns, Farnborough Ltd. The sextant is also marked FEN/R/7/75.  Fenns did the calibration of the instrument for the Air Ministry, so it appears the sextant was still in the care of the Air Ministry in 1975.  Whether it was also in use is impossible to say. It seems likely that the sextant was sold sometime after 1978 and perhaps used at sea, as some external parts have corroded. However, this could also have occurred in humid storage conditions.

Concluding remarks

My impression is that the Hughes Marine Bubble Sextant was a product that was developed just too late to play a role in WWII, and which was unsuccessfully marketed after the war. Online, there is evidence of fewer than ten individual examples, two of which are in museums, three in past auctions and two in unrelated accounts. Some of these could even be the same. The two serial numbers I know are 114 and 123, which doesn’t tell much. This sextant used parts from the Mk IX (bubble unit, handle, general lay-out), but contains a completely new integrating mechanism. This mechanism may have found use in later aircraft or submarines sextants of the periscope type. It is clear that a lot of effort was put into designing and producing this sextant, so it must have been a disappointment for the manufacturer. Nevertheless, as a nautical sextant, it deserves a place on this blog. I have found a number of reports and articles on the internet which mention this sextant, but I’d like to hear from anyone who has more information.

If you enter this in the “Comments” section (below), I will forward your information to Jaap.

End note: [1] “Another enterprise of Plaskett and Jenkins was entirely successful in itself- the demonstration that the bubble sextant could be employed to aid the fixing of position of a submarine surfacing only at night when the sea horizon is invisible. Pleskett obtained highly accurate results from observations made on board a submarine off Start Point. As a result the ‘Hughes Marine Bubble Sextant’ made its appearance in 1944 and underwent trials, but apparently it never actually went into service.” (Biographical Memoirs: Harry Hemley Plaskett (5 July 1893 – 26 January 1980), Biogr. Mems Fell. R. Soc. 1981 27, 444-478, published 1 November 1981)

Readers who own a Mark IX series sextant who would like to know more about its construction, operation and restoration could do worse than buying a copy of my restoration manual. . See “My Bubble Sextant Restoration Manuals” for details.





An Improvised Sun Compass

23 12 2014

This post was preceded by  ” C Plath Sun Compass”; “A Fleuriais’ Marine Distance Meter” A Stuart Distance Meter”;“A Russian Naval Dip Meter”; and  “An Improvised Dip Meter”

In October, I and Dan La Porte described a C Plath sun compass in which a 24 hour clock is used to keep the alidade aligned with the sun. Recently, a friend in Australia sent me an old Astro-compass Mark II which someone had attempted to convert to a dumpy level, with only limited success. However, it gave me the opportunity to attempt to make a sun compass of my own along the lines of the Plath instrument, itself a modification of the Bumstead sun compass.

An essential requirement was a 24 hour clock. I had a Hamilton Master Navigational watch which has a 24 hour face, but I was not about to use that valuable instrument. What I did have was a quartz clock with a 24 hour face and, as it cost only a handful of dollars, I was happy to use that. However, I live in the southern hemisphere and as there was no room to mount it upside-down on the Mark II, it would have to run anticlockwise on the top. For those who might wish to copy me, let me say at the outset that reversing the battery will not cause the clock to go backwards. No doubt there is a diode somewhere in the circuit that prevents damaging reverse current from flowing. This led me to explore the mechanical insides of the clock. There is quite a lot of variation between makes, so I will not attempt to illustrate it, but all have an electronic circuit that delivers pulses every second to a tiny stepping motor whose rotor rotates through 180 degrees with each pulse. As it does so, it transmits movement to a gear chain that drives the hands in the correct relationship. There is a coil of fine wire wound around the armature of the stepping motor and I thought that I could simply reverse the polarity of the coil attachments to make the rotor go backwards. To save others quite a lot of difficult de-soldering and re-soldering trouble, let me say now that it does not work. The armature has two loose pieces that embrace the rotor and if the left is exchanged for the right (and vice-versa), this will  make the rotor and the clock go backwards. If you Google something like “How to make a clock run backwards” you will find several videos that show exactly how this is done.

So I had a 24 hour clock movement that ran backwards and now needed a face numbered in reverse. This is relatively easy to make using a drafting program such as TurboCAD and Figure 1 shows my result. Northern hemisphere readers who wish to make the compass will not of course have to go to the trouble of making the clock go backwards or of making the anti-clockwise dial.

Figure 1: 24 hour reversed dial.

Figure 1: 24 hour reversed dial.

It was then necessary to glue it to a suitable piece of sheet steel or brass, taking care to ensure that the central holes coincided exactly. I am happy to send a pdf file of the dial to anyone who might want to follow in my footsteps and who has no drafting program to draw such a dial. The dial is then fixed to the clock using two hexagonal nuts on the central pillar.

To allow easy removal of the movement to change the battery, I made a rectangular clip out of thin and springy sheet steel (Figure 2). Fixing the movement in its clip to the top of the compass will vary according to the maker. Sperti’s version has a smooth top and after removal of the alidade and its bracket, the clip could be glued with contact cement to a spacer glued in its turn to the top of the instrument, taking care to get things well centred with 00 hrs/12 hrs  correctly aligned fore and aft. The spacer is necessary so that the bottom of the clip clears the trunnions. My version was made by Henry Hughes and Son and had the round heads of three 6 B.A. screws projecting from the top, so I exchanged them for longer, countersunk head screws and used them to attach the clip via three spacers. Other improvisations may occur to readers.

The knob on the left in Figure 1 is used to adjust the longitude , using the “True bearing” scale and a little mental arithmetic. For example, I live at 173 degrees East longitude so the True Bearing scale will have to be set 7 degrees anti-clockwise. At 00 hours GMT, the sun will still have 7 degrees to go before it bears true north at local noon.

Figure 2: General arrangement from above left.

Figure 2: General arrangement from above left.

Attention now has to turn to the alidade. This could be simply a vertical bar arising from the end of an hour hand, or even simply a long hour hand bent up at a right angle so that its shadow falls on the face, passing through its centre. I chose to imitate a little the Plath arrangement as I had some Perspex (Lucite) available to cut drill and glue into shape, as shown in Figure 3. I trimmed a minute hand to length and bent it to clear the alidade.

Figure 3: Face of clock.

Figure 3: Face of clock.

Figure 4 shows the latitude setting knob and scales, set at my latitude of 35 degrees south. While the declination of the sun as I write is just under 23 1/2 degrees, there is enough length in the shadow bar to make it unnecessary to allow for this, though the original Mark II alidade had a separate declination scale to use with its sighting arrangements.

Figure 4: View of instrument from right and above.

Figure 4: View of instrument from right and above.

In use, the instrument is levelled , the clock is set to read GMT (or UTC which amounts practically to the same thing) and placed in its clip, 00 hours upwards, the latitude set and the longitude (after a little careful thought) allowed for on the True Heading scale. If the north on the compass scale is aligned with the lubber line and directed at true north, you should find that the shadow of the shadow bar passes right down the middle of the clock face and between the two lines scribed on the face of the screen. While the shadow remains aligned, the North point on the compass card will remain pointed at true north and any other desired course can be read off against the lubber’s line. The equation of time reaches a maximum of 16 1/2 minutes on November 5. This amounts to just over a degree in direction, so if you do not need direction to this precision, it can be ignored. Otherwise it has to be applied and the clock offset from GMT as outlined in the preceding post.

As an aside, I found that bringing out the instrument on its tripod was an excellent way of causing the sun to disappear behind clouds. It needed only for me to take it back inside to make the sun re-appear…


C Plath Sun Compass

16 10 2014

Figure 1: Dan LaPorte's sun compass.

Frontispiece: Dan LaPorte’s sun compass.

This post was preceded by  “A Fleuriais’ Marine Distance Meter” A Stuart Distance Meter”;“A Russian Naval Dip Meter”; and  “An Improvised Dip Meter”

Sometimes, kind people lend me their precious instruments for me to deconstruct and examine so I can post details on this site. I invited Dan LaPorte to contribute a “guest blog post” and he has kindly obliged. Dan’s contribution is in blue, and my comments and additions are in black.

Last year I obtained a WWII Plath sun compass via an e-bay purchase.    At the time I really didn’t know exactly what I had bid on and won, but I did know it was something out of the ordinary.

First, a little about me and why I would be remotely interested in such an item.  I am a retired  US Merchant Mariner having sailed for some thirty-five years at sea, twenty of those as a Ship’s Master.  During that time, I acquired many skills and interests, one of them being magnetic compass correction.  For years I’ve used an Abrams and an Astro sun compass for such duties.  Both work on the same basic principal of local time or hour angle to obtain a true bearing of the sun or other celestial body.  Hence I was immediately interested in the Plath sun compass.

Upon delivery of the item I was saddened to find that the clock work no longer functioned (the Plath uses a Junghans 30 clock work  with optical sight for taking a bearing of the sun).  In fact the Junghans 30 movement was also used in the ME 109 fighter of the same era.   The idea is to set the correct time (more on this later), so the sun compass will track the sun’s path and hence a constant bearing using the sun can be obtained.  When functioning and set correctly a true bearing can be recorded of a landmark to obtain a position, or drive (or fly) from a known position to another by following the desired course.  Another use of the Plath would be to check and correct an aircraft’s magnetic compass while on the ground.  

After a bit of research I found that this model was used almost exclusively by  German troops deployed to the  North Africa corps during WWII.   Of course all this would have been unknown to me if not for the assistance of Mr Malcolm Barnfield.   By contacting Malcolm via his web site: http://www.sundials.co.za , I was able to obtain a wealth of information on the Plath.   Malcolm is without a doubt one of the most knowledgeable people in the world on the topic of sundials, sun compasses and their use.   Without Malcolm’s expertise on the topic I would have certainly been lost for much longer, and perhaps forever.  Malcolm was also good enough to put me in contact with other very talented men, highly regarded on the same topic, such as Mr Konrad Knirim who provided a manual for the Plath, and Mr Kuno Gross, who translated it from German to English for me.  These highly accomplished men in their fields were good enough to assist me in my search for information regarding the Plath. 

While history of the Plath was very interesting, it did nothing to solve the one large remaining problem – it simply didn’t keep time and thus was nothing more than an interesting item to marvel at and only ponder at its use.  

Enter Bill Morris.   Bill and I had communicated for months on various topics related to celestial navigation both air and sea.  Bill is regarded by all that know him as one of the most knowledgeable people in the world where navigation instruments and their structure are concerned.   Bill has written books on the topic and provides detailed manuals for repair of several  sextant types both aeronautical and nautical.   His manuals are truly works of art and allow the layman to repair and bring sextants back to working order.  Bill had in the past repaired an old A10A aircraft sextant for me that works perfectly to this day.  Given his talent for repairs, knowledge of machine tools and ability to work on intricate and complex antiques with a sure touch, I asked if he would be good enough to have a look at the workings of the Plath.   I should state at this point that I was, and remain very protective of the Plath and would not allow just anyone to begin repairs on it.  Bill was my first and only choice that I would trust enough to allow any attempt at repairs.  As luck would have it Bill was to travel to Katy in Texas, not all that far from my home.  Add to this I would be able to finally meet Bill face to face.  In short it was a perfect and fortunate turn of events. 

I was able to meet Bill and his lovely wife for a visit in August of this year.  We enjoyed a very nice chat and lunch, covering topics ranging from navigation to what should be seen while in Texas.   I left the Plath safely in Bill’s hands, with hopes he could repair it.  A mere week later I had the Plath in my hands and working perfectly.

At this point the Plath was repaired, and I had a basic knowledge of how to use it.   When the Plath arrived I at once set it to local standard  time adjusted with the EQT ( Equation of time) from the nautical almanac.  To my dismay it did not point to North or any other direction.  In fact is seemed to be some 15 degrees off to the East at best.  That is, I would have to be another time zone to the East for the Plath to be anywhere near correct in obtaining a true bearing. Adding to my frustration, I was not entirely clear on how to orientate it to obtain a true bearing (the manual giving scant information in the translation). I set both the Abrams and Astro Compass in a hope to clarify the situation, this only proved to entangle my thoughts even more, at least for the moment. 

A few words about the equation of time are perhaps appropriate. Our daily life is governed mainly by the sun and its passage across the sky is not perfectly regular. It slows at some times of the year and speeds up at other. This is partly because the Earth’s orbit is slightly elliptical, so that it speeds up when nearer the sun and partly because the Earth’s axis of rotation is inclined at about 23 1/2  degrees to the plane of its orbit, so that the component of the Sun’s apparent velocity parallel to the equator  varies with the seasons. It is very difficult to make clocks to follow these variations, so the concept of mean solar time was invented, the average time for the Sun’s apparent rotation around the Earth. The difference between the apparent time on a given day and the mean solar time is known as the equation of time, often shown as a graph as in Figure 2, and in the bottom right hand corner of the daily pages of the Nautical Almanac. In the sun compass, it has to be applied as a correction on a given date to the mean time so that the alidade will point correctly to the sun.

Figure 2: The equation of time.

Figure 2: The equation of time.

 After further study I found the problem. To outline what the problems was I first have to explain the use of the two sun compass types I was more familiar with.   As stated previously, my tools used for obtaining a bearing of the sun or other celestial body was the Abrams or Astro Compass.   The Abrams uses local standard time, adjusted east or west of the standard time meridian, the observer’s approximate latitude and an adjustment for EQT provided on the face of the sun compass.  When all these details are known and set the instrument will provide the desired bearing by using the shadow of the sun.  The instrument has to be updated by moving the time marker one mark on the scale every four minutes.  This of course is due to the movement of the sun covering one degree of longitude every four minutes.  Simple when you know how.  The Astro Compass uses the settings of:  Local Hour Angle (LHA), declination of the body and latitude of the observer.  The declination of the body is obtained from the Nautical Almanac, LHA is calculated from your known longitude and applying it to the GHA of the sun or other celestial body.  Latitude of the observer would also need to be known and set on the instrument.  As with the Abrams the Astro needs to be constantly updated by moving the LHA scale in keeping with the sun’s motion across the sky.  

Why am I boring the reader with these details?  Simply to drive home the use of the Plath and the ingenious setting of the unit.  Unlike the Abrams and Astro the Plath is set to GMT standard time (not DST).  The user would also need to apply EQT to the time setting in order to obtain solar time with the EQT sign ( -/+) reversed due to the correction from a local time to GMT.  Once set to GMT – Solar time (GMT with EQT applied) the user then simply sets his latitude and longitude on the Plath.  No further corrections  and no almanac entries are needed.  As long as the Plath keeps correct time, and the user updates the estimated position of latitude and longitude, the unit will continue to function.  My mistake was in setting the Plath to local time.  I had wrongly assumed local time would be used as with my other instruments.  The Plath’s use of GMT is a perfect solution when one has time to reflect on the subject.  

Needless to say when the details of correctly setting the Plath were known and understood another test was in order.   So, one afternoon with the sun high and bright in the sky I set the Plath.  It should be noted that as with all other sun compasses it needs to be mounted securely to a stable platform and levelled with the provided spirit level. I also set the Abrams and Astro compass at the same time, a kind of a sun compass smorgasbord if you will.    To my amazement the Plath indicated true north as checked by my Abrams and Astro Compass (any course could have been selected for the test).  Given that the ultimate test of the Plath was to maintain a true bearing for hours or even throughout the night a test for the rest of the evening continued. I allowed the Plath to run for a few hours checking it now and then.   As per the design, the Plath displayed a constant true bearing until sunset due to the clock works keeping time and following the transit of the sun across the sky.  The Abrams and Astro compass would have had to be manually corrected continually for the entire event.  The value of the Plath became even more clear when you imagine using it in a desert with no natural land marks.  Given the successful test I personally would not have a problem using it for land navigation across a desert to this day if I knew what course I needed to travel from my location to destination.  No need for GPS signals or the like.  Just simple old style navigation would serve the user very well indeed. In fact I’d prefer to use the Plath instead of the Abrams or Astro compass due to the Plath’s ability to constantly maintain the required bearing, thanks of course to the Junghans clock works. 

The final test was the following morning.  As the sun rose in the East the Plath tracked perfectly still displaying the true bearing of North as she was set the evening before.  Perfect!   After seventy plus years the Plath with the assistance of Bill Morris worked as she worked many years ago in the North African desert.

 I wish to thank Dr Bill Morris,  Mr Malcolm Barnfield, Mr  Kuno Gross, and  Mr Konrad Knirim with having similar interest, assisting me, answering my questions, being patient,  and at times commiserating with me on this project.   These men:  doctor, military historian, engineers, authors, experts in their fields, took the time to assist a retired sea Captain with his quest to restore an antique sun compass to operational status.  Simply put, without them and their assistance the Plath would have remained locked away in my study with other odd instruments, not used, not understood and in a non-functional condition.  It would have been an unfortunate end for such a fine instrument.  As it turns out she may well run another seventy plus years.

 Captain Dan LaPorte (ret)

Figure 3: General arrangement.

Figure 3: General arrangement.

Now for a few anatomical details. The compass is mounted on the vehicle or aircraft via a universal joint, which can be quickly locked or unlocked in order to level the base plate (which I have labelled “compass card” in Figure 3 above) using a circular level in the centre of the plate. The base plate index corresponds to the lubber’s line in a magnetic compass and the line can be correctly aligned with the fore and aft axis of the vehicle using a sort of iron sight. In Figure 3 this can be seen as a thin vertical rod in a gap in the trunnion above the W of the base plate. On the other side is a point, just visible in Figure 1. The two trunnions support the horizontal axis of the compass and this axis is provided with a latitude scale on one end and a knurled locking knob on the other. A moment’s thought shows that when the scale is set to the local latitude, the equatorial axis, which I have drawn as a red line, will be parallel to the Earth’s axis. The horizontal axis bears a clock with a twenty-four hour dial and the clock can be rotated inside an equatorial mounting ring provided with a longitude scale and locked at the local longitude.

Figure 4: Equatorial mounting ring and longitude scale.

Figure 4: Equatorial mounting ring and longitude scale.

The watch is provided with a rather unusual hour hand in the form of an alidade (Figure 5a) and also has a conventional minute hand.

Figure 5a: Structure of the alidade.

Figure 5a: Structure of the alidade.

The alidade is made of Perspex (Lucite). One end has a vertical cylindrical lens that projects an elongated image of the sun on to a ground screen at the other end. The screen has two vertical setting lines and a lower, transverse extension to help in initial setting. When the time is set to the correct Universal or Greenwich Mean time, adjusted for the equation of time,  and the latitude and longitude scales set to their local values the base plate is rotated so as to bring the elongated image of the sun between the setting lines. If the vehicle is pointing north, the base plate index will then indicate north. If the vehicle turns and the base plate re-adjusted to bring the sun back between the setting lines, the direction of travel will be indicated by the base plate reading. The clock will keep the alidade tracking the sun correctly as long as the direction of travel does not change and if the direction of travel  does change  it is necessary only to bring the alidade back into alignment to get a correct indication of the new direction of travel. Amateur astronomers will recognise this as an adaptation of the familiar equatorial telescope mounting. The whole is enclosed by a protective Perspex cover. Figure 5b gives another view

Figure 5b

Figure 5b: Further view of alidade and scales.

I do not propose to give many details of the clock mechanism except to point out that, somewhat unusually, it is wound up by rotating the spring barrel rather than by rotating the spring arbor. This allows some simplification of the internal power train and also allows setting of the hands by means of a co-axial arbor (Figure 6).

Figure 6: Winding and setting knobs.

Figure 6: Winding and setting knobs.

Figure 7 shows how power is transmitted to the train from the spring via the central winding gear.

Figure 7: Winding gear detail.

Figure 7: Winding gear detail.

Dan and I would be glad to know of any errors or significant omissions and to hear from other owners about their experience with this ingenious instrument.

Bill Morris


New Zealand






29 12 2013

This alphabetical list of posts may help you to find what you want. When you have found a post of interest, enter the part of interest as a search term in the search box.

A10 vapour pressure bubble chambers, Sealing

Admiralty pattern vernier sextant

Admiralty pattern micrometer sextant

AN 5851-1 bubble sextant averager., Gummed-up

AN 5851-1. Jammed shades carrousel

Battered Observator sextant, A

 Battery Handle Structure, C Plath

 Box Sextant, A

Broken legs

 Bubble Horizon Attachment, C Plath

Bubble illumination of Mk V and AN 5851 bubble sextants

Bubble sextant, land use conversion of Mark IX     May 2021

Bubble Sextant Restoration Manual, A12

 Bubble Sextant Restoration Manuals, A10 and Mark IX series

bubble sextants, Aircraft

Bubble sextant, Hughes Marine 

 Bubble Horizon, A Nautical Sextant

 Byrd Aircraft Sextant, Update on

Byrd Sextant Restored, A

Carl Plath’s Earliest Sextant

C Plath Sextant Lives Again

 C. Plath Drei Kreis sextant, Restoring a

 C. Plath Vernier Sextant, A Fine

C Plath Yachting Sextant

C18 sextant named J Watkins

C19 sextant restoration

C Plath Sun Compass

Carl Plath micrometer sextant

 Carl Plath Sextants, Eighty Years of

Cercle de reflection     October 2021

circular sextant mirrors, Making

Compass, a C Plath sun

Compass, an improvised sun

Damaged Rising Piece, A

 Dip Meter, A Russian Naval

 Dip Meter, An Improvised

Distance Meter, A Stuart

Distance Meter, Fleuriais’ Marine

Drowned Husun Three Circle Sextant, A

Ebony quadrant, restoring a

Errors, Backlash and Micrometer

Faking it., Is it a SNO-M or is it a C Plath?

 Filotecnica Salmoiraghi of Milan, A Fine Sextant by

Freiberger scale illuminator    May 2020

Freiberger Drum Sextant (Trommelsextant)

Freiberger Skalen Sextant

Freiberger Yacht sextant

A French Hydrographic Sextant

A French Sextant Restoration

Gyrosextant     January 2019

Half-size Sextant by Hughes and Son, A

Half-size Sextant by Lebvre-Poulin, A

Heath and Company’s best vernier sextant

Heath Curve-bar sextant compared with Plath

Heath Vernier Sextant Restored

Hughes Marine Bubble Sextant

Hungarian sextant via Bulgaria, An

Hydrographical sextant, a French

Ilon Industries Mark III sextant

Jesse Ramsden and his Dividing Engine

 Keystone Sextant Case, Making a

Kollsman bubblechamber, refilling

Kreisel-sextant    January 2019

Land use conversion of Mark IX bubble sextant     May 2021

 LEDs 1: miniature screw bases., Adapting to

LEDs 2: Plath bubble horizon unit, Adapting to

Lefebvre-Poulin, a Half sized sextant by

left-handed sextant, Unusual

Legs, broken

Mark V / AN5851 sextant bubble chambers, Refilling

 Markk V/ AN5851 sextant bubble chamber, Overhaul of

Mending a 1975 SNO-T sextant

MHR1 position line slide rule, A reproduction        June 2021

mirrors, How flat are sextant ?

Mercury amalgam mirrors     May 2021

 monocular mounting, Making a sextant

Observator Classic sextant, restoring a

Observator Mark 4 sextant

 Prismatic Monocular, Making a

 Quadrant Restored, An Early C19 Ebony

Quadrant Restored, an old wooden          June 2018

Ramsden sextant, a small                         October 21

Reflecting circle       October 2021

Reproduction MHR1position line slide rule, A       May 2021

Scale illuminator, a Freiberger             June 2020

 Scale Lighting Systems, Later Tamaya

Sextant ‘scopes for myopes

Sextant Mirrors, New  for Old

Sextant Calibrator, A

Sextant Frame, Evolution of the

Sextant, a small Ramsden              October 2021

sextant shades, Polarising

Sextant, 210 years on,

Shackman sextant and a link to Jesse Ramsden

a later.          March 2018

 Shades-adjusting Tool, Making a

Simex Sextant(s

Skalensextant, Inside the

SNO-T Mirror Bracket Repair

SNO-T sextant, Mending a,     October 2020

 SOLD KM2 Bubble Sextant

Sounding Sextants 1

Sounding sextants 2

Sounding sextants 3

Spanish Vernier Sextant, A Late

Spencer, Browning and Co sextant

Sun compass, a C Plath

Sun compass, an improvised

switch overhaul, Tamaya

Tamaya Collimation Blunder

The Case of the Broken Screw

Troughton and Simms Surveying Sextant

Turn-of-the-century French Sextant

US Maritime Commission Sextant, A

USN BuShips Mark II sextant: some design oddities

USSR SNO-M sextant, The

USSR SNO-T sextant, The

 Vernier Sextant, British Admiralty

Watkins, J, A C18 sextant named

Which lubricant?

Wooden quadrant, a late C18, restored.    Jan 2020

Worm with wrong thread angle?

Worm Turns, A


6 03 2010


 As the pages of the blog posts are not numbered, I have given the dates of the index entries as the easiest way of retrieving the information they contain. Don’t forget that WordPress also allows you to search this site specifically (see search box in top right-hand corner of the home page).

adhesives, industrial       27 Mar 11

Admiralty, British, vernier pattern of sextant     27 Mar 11,  24 June 11

Admiralty, British, micrometer version of sextant    15 Feb, 18

alcohol, filling of A10 A bubble chambers   13 Oct 09

angle, vertical sextant     13 Jul 12

Araldite, for cementing lenses       24 Jan 10

refractive index of                    24 Jan 10 (Comment)

arc, dangers of polishing                                    24 Jan 10

ivory                                           26 July 22

platinum                                                           10 June 10

silver, letting in to limb                               10 June 10

silver in brass on wood                               26 July 2022

astigmatiser shade             20 Mar 11

autocollimator, principle of                                6 July 09, 13 Feb 11

accuracy of                     12 Mar 09,  13 Feb 11

calibrating dip meter with             5 Apr 2012

setting wires of                13 Feb11

view through eyepiece of      13 Feb 11

backlash     12 Mar 09

beam splitter     13 Feb11


index arm           18 Feb 10, 13 Jul 12

adjusting     12 Mar 09

of US Maritime Commission sextant    15 Dec 10

swing arm

conical centres     12 Mar 09

plain     12 Mar 09

of US Maritime Commission sextant     15 Dec 10

Belleville, Julien                                  26 July

Belleville washer                                               18 Feb 10, 11 Aug 09, 26 July 22

Blish prism                                                      5 Apr 2012

Bochard de Saron                                  26 July 22

Bracket, mirror,

bent     20 Oct 16

slotted                1 Oct 14

shade                                14 June15

brass, hammered                                              10 Nov 09

bubble chamber

overhaul, of AN5851                            20 Dec 08

refilling, of AN5851     20 Dec 08

bubble sextant

A  Coutinho  Pattern                   6 May 16


A10 series                                            25 Feb 09, 13  Oct 10

screw closure of     13 Oct 10

Mk IX series                                        25 Feb 09

British Admiralty pattern sextant      24 June 11

Broken legs, mending  7 Oct 16

Byrd sextant                                                     30 May 09

Byrd, Commander Richard                               30 May 09

calibration, of C18 sextant                        10 June 10

calibrator, sextant

calibration of           13 Feb11

construction of        13 Feb 11

mounting sextant on    13 Feb 11

Carbonara, Victor             1 Oct 14

Carl Path’s earliest sextant (see also C Plath)       21 April 2017


latches                                                      18 Feb 10

keystone                                                  17 Dec 09

furniture, of US Maritime Commission sextant  15 Dec 10

plywood, delamination of                           14 June 15

repair of SNO-T                                        28 November 2020

casting, pressure die-                                     14 July10

C Plath (See also Carl Plath and Plath)

production numbers       14 July 10 (see also Plath)

sextants, eighty years of     13 Nov 12

yachting sextant                    14 June 15

centres, as bearings     12 Mar 09

chassis, swing arm     12 Mar 09

of La Filotecnica sextant 5 Oct 10

CHO-M sextant. See SNO-M sextant

Chronometer, marine    See www.chronometerbook.com

cleaning sextant parts             18 Feb 10, 23 Nov 08

clearance, axial of worm     12 Mar 09

collimating rising piece     2 September 11

collimation, faulty     2 September 11,  28 November 2020

collimator      13 Deb 11

horizontal     13 Feb 11

Commission, US Maritime   15 Dec 10

Cooke and Son                       28 Apr 10

Coutinho Pattern Bubble sextant   6 May 16

Dip                                        5 Apr 2012

abnormal                  5 Apr 2012

meters                       5 Apr 2012, 23 June 2012

calibration of             5 Apr 2012

Dollond, John                           10 June 10

Dollond, Peter                           10 June 10, 26 July 22

patented mirror mounting                            26 July 22

double sextant                                                  16 Jul

micrometer mechanism of                     16 Jul 09

handle of                                              16 Jul 09

mirrors of                                             16 Jul 09

optical paths of                         16 Jul 09

telescope of                                          16 Jul 09

distance meter,

Fiske     13 Nov 12

Fleuriais     4 Nov 12

Stuart      13 Nov 12, 13 July 12

drawings, of SNO-T mirror adjusting screw and bush 17 april 10

drill, slot, use of  17 April 10

error, index, adjustment            1 Oct 14, 26 July 22

side, adjustment            1 Oct 14, 26 July 22

perpendicularity, adjustment            1 Oct 14, 26 July 22

errors, of worm     12 Mar 09

Entandrophragma cylindricum, see sapele


micrometer                                           22 Mar 09

monocular, disassembly                        18 Nov 09

positioning                                18 Nov 09

of C Plath’s earliest sextant          April 21 2017

fake C Plath sextants                   14 July 10

La Filotecnica Salmoiraghi        5 Oct 10

Filby, David      13 Nov 12, 21 April 2017

float glasss                                                        11 Feb 09

Florez, Luis de                                      30 May 09

Francis Barker ,sextant, small craft precision     14 June 15


        aluminium alloy          29 Sept 10, 13 Nov 12, 14 June 15

        bent     20 Mar 11

braced                                                            10 June 10

bronze                    29 Sept 10, 13 Nov 12

         curve bar pattern      28 Jan 10

          Dreikreis                                             24 Jan 10

of C Plath’s earliest sextant    21 April 2017

evolution of                                        29 Sept 10

of Freiberger Skalensextant                                   22 Mar 09

of La Fil0tecnica               5 Oct 10

plastic                                                      29 Sept 10

pressure die cast                                  26 Oct 08,  29 Sept 10

stiffness of Heath sextant                      28 Jan 10

of Plath sextant                        28 Jan 10, 14 June 15

                      of  tulip pattern                                                10 Nov 09

of US Maritime Commission sextant  15 Dec 10

Freiberger , yacht sextant              14 June 15, 10 Aug 11,

skalen sextant 4 Apr 10

drum sextant   22 Mar 09

Lighting unit of                       10 June 2020

Gavrisheff dip meter                                         5 Apr 2012

Gilbert and Company                                      10 June 10

glass, float                                                        11 Feb 09, 27 Jan 09

graduations, sextant, refilling                              18 Nov 09

grinding paste, for circular mirror     27 Mar 11

Hall, Chester Moor                                                    10 June 10


detachable            1 Oct 14

kinematic mounting of       20 Mar 11

of double sextant     16 July 09

of Gilbert sextant   10 June 10

of La Filotecnica sextant 5 Oct 10

of Troughton and Simms sextant   28 April 10

of US Maritime commission sextant  15 Dec 10

of WWII C Plath sextant                     14 July 10

Heath curve bar sextant,

frame stiffness                          28 Jan 10

tangent screw                                      28 Jan 10


micrometer shaft bearings                      24 May 09, 5 May 09

rack                                                     24 May 09

swing arm bearings                               5 May 09

vernier sextant                                      23 Nov 08


Admiralty pattern sextant, vernier    27 Mar 11,  24 June 11

micrometer    14 Feb 18

lamp switch assembly                            18 Feb 10

micrometer mechanism              18 Feb 10, 18 Feb 18

rack                                                     3 Jan 09

telescope mounting                               18 Feb 10

index arm

Ilon Industries Mark III sextant            1 Oct  14           14 Jun 15

index arm, bearing                             18 Feb 10, 22 Nov 08, 14 June 15

C Plath                                          14 July 10, 13 Nov 12, 14 June 15

SNO-M                                           14 July 10

US Maritime Commission    15 Dec 10

index mirror turntable                                10 June 10

sealed     24 June11, 14 Jun 15

joint, corner rebate                                           11 Aug 09

Keeper, index arm                 14 June 15

, swing arm                14 June 15

lacquer, see paint

lamp assembly, making new for Husun  18 Feb 10

latches, of case                                     18 Feb 10, 24 June 11

left-handed sextant                                            2 Dec 08


broken, mending                           7 Oct 16

turning new                                           24 Jan 10

placement of                                         6 May 09, 26 Apr 09, 2 Dec 08

Lenoir, Etienne                                     26 July 22


auxiliary                                                30 May 09

re-cementing                                          24 Jan 10

swaging                                                24 Jan 10

achromatic, airspaced                   10 June 10

Leupold and Stevens see US Maritime Commission sextant, 15 Dec 10

level, spirit                                                        30 May 09

sensitivity of                                          30 May 09

light path, in Ilon Mark III sextant              1 Oct 14

lighting, of scale                                                30 May 09

limb, rivetted to frame                                  10 June 10

chromium plated                  5 Oct 10

logo, probable fake Plath                              14 July 10

of La Filotecnica Salmoirhagi       5  Oct 10

lost motion    See backlash

makers names                                                   10 Nov 09

mandrel, test     20 Mar 11

meter, dip, an improvised                 5 Apr 2012

Gavrisheff                                5 Apr 2012

Russian naval                        23 Jun 2012


eyepiece                                         22 Mar 09, 13 Feb 11


bearing, by C Plath     20 Mar 11

bent         20 Mar 11

calibration of                                         6 July 09

drum, making new, dividing       20 Mar 11

numbering       20 Mar 11

evolution of                                           3 Jan 09

examination of                          6 July 09

index, bent          20 Oct 16

mechanism                          18 Feb 10, 14 June 15

of  Husun                                   3 Jan 09

of Ilon Industries Mark III            1 Oct 14

by Carl Plath                            3 Jan 09, 13 Nov 12

shaft, bearings,  by Heath                        24 May 09, 5 May 09

bent, repair of     20 Oct 16

by La Filotecnica     5 Oct 10

by C Plath     20 Mar 11

setting errors of     13 Feb 1

of US Maritime Commission sextant 15 Dec 10

version of Admiralty pattern sextant   15 Feb 18

worm, axial play in       14 June 15

Micrometre a reflexion, Fleuriais’     4 Nov 12


adjusting screws, of SNO-T    17 April 10

auxiliary                                     30 May 09

backing of brass sheet,  15 Dec 10

blank, preparation of circular     27 Mar 11

bracket, Ilon Mark III sextant            1 Oct 14

bracket, SNO-T, repair of     17 April 10

Observator, repair of     20 Oct 16

bracket, C Plath casting numbers   14 July 10

circular, cutting       27 Mar 11

first surface            1 Oct 14

f latness of                                             27 Jan 09

grinding  edges                          11 Feb 09

horizon, C19 mounting assembly           10 Nov 09

Gilbert method of adjusting     10 June 10

Overcoated    13 Nov 12

Plath and SNO-M  compared  14 July 10

sealed  24 June11

index,              of Troughton and Simms sextant    28 April 10

,         , adjustment   14 June 15

adjustment by turntable      10 June 10

making new                                          11 Feb 09

removing silvering from             11 Feb 09

rounding corners                                   11 Feb 09

sealed                                                   6 May 09,  24 June 11

Mk IX A sextant restoration manual                  17 Nov 08

magnifier, scale, mounting in vernier sextant   24 June 11


eyepiece disassembly                           18 Nov 09

labyrinth seal of objective lens      20 Mar 11

La Filotecnica                     5 Oct 10

mounting,                                             18 Nov 09

Freiberger, boring                      18 Nov 09

marking out                  18 Nov

milling and shaping        18 Nov 09

squareness                                18 Nov 09

prismatic, in Ilon sextant            1 Oct 14

prismatic, making                                  18 Nov 09

removing objective                                18 Nov 09

removing prisms                                    18 Nov 09

separating halves                                  18 Nov 09

Tamaya etc                                           28 Jan 09

mounting, of Husun telescope                            18 Feb 10

of La Filotecnica telescopes     5 Oct 10

Navistar professional sextant     13 Nov 12

objective, removing from monocular                  18 Nov 09

octant, aeronautical, Mark I Mod 3                   2 Dec 08

O rings, Viton  13 Oct 10

Neoprene 13 Oct 10

Nut , half                                     26 July 22


defective    15 Dec 10

for sextant finishing                                   24 Jan 10,10 Nov 09

photoluminescent                                  9 Dec 08, 26 Oct 08

wrinkle finish                                         18 Feb 10

Parallax, solar     4 Nov 12

Perpendicularity, archaic method of adjustment.               26 July 22


Dreikreis sextant                          24 Jan 10, 13 Nov 12

Eighty years of

logo                                                      24 Jan 10, 14 July 10

optics of, post WWII     20 Mar 11

sextant frame stiffness                           28 Jan 10

sextant production, WWII                 14 July 10

shades adjusting tool                 6 Oct 13

tangent screw mechanism                      24 Jan 10,  24 June 11

telescope rise and fall mechanism          24 Jan 10, 24 June 11

polish, French                                                   24 Jan 10

Porro, Ignacio                              5 Oct 10

Poulin, Roger              29 June 14, 14 June15

pressure die casting                                           26 Oct 08,

working life of dies                                     14 July 10

prism, eyepiece, of AN5851                             9 Dec 08

Blish                             5 Apr 2012

Roof                            23 Apr 2012

removing from monocular                      18 Nov 09

protractor, three armed. See station pointer

quintant, vernier                                                30 May 09


Hezzanith                                              3 Jan 09

Hughes                                                 3 Jan 09

Heath                                                   24 May 09

Ilon Industries            1 Oct 14

C Plath Yachting sextant       14 June 15


paint, decay of                                       9 Dec 08

half life of                                             9 Dec 08

release catch

of La Filotecnica sextant     5 Oct 10

of US Maritime Commission sextant   15 Dec 10

of C Plath yachting sextant         14 June 15

Repsold      13 Nov 12

rising piece

bent, repair of    28 Apr 10,      20 Oct 16

collimating,    2 Sept 11

faulty construction of Tamaya    2 Sept 11

structure of, in Admiralty pattern sextant  24 June11

of Carl Plath’s earliest sextant        21 April 2017

in Ilon Industries sextant            1 Oct 14

in La Filotecnica sextant      5 Oct 10

in Troughton and Simms sextant    28 Apr 10

Salmoiraghi, Angelo       5 Oct 10

sapele                                                               24 Jan 10

Saron, Bochard de                               26 July 22


glass                                                     22 Mar 09

ivory                                                     10 Nov 09

screwcutting conical threads                              6 July 09


broken, removal of                    24 May 09, 17 April 10, 20 Oct 16

grub, replacement of                          4 Mar 10

pin    29 Apr 10

tangent, of  Troughton and Simms sextant    28 Apr 10

             of French sextant                               26 July 22

of C Plath’s earliest sextant      21 April 2017

Service, US Maritime    15 Dec 10


Admiralty pattern     27 Mar 11

calibration of         13 Feb 11

cleaning of                                            23 Nov 08, 18 Feb 10

double. See double sextant

fake                                             14 July 10

left handed                                            2 Dec 08

small craft            14 June 15

survey. See survey sextant

Yachting             14 June 15


assembly of horizon                              18 Feb 10

assembly of index                                 18 Feb 10

bracket, construction of                       28 Nov 2020

carrousel of AN5851                            14 May 09

remounting index                                   18 Feb 10

repainting                                              18 Feb 10

shellac, sealing bubble chambers with    20 Dec 08

secondary sealing bubble chambers with   13 Oct 10

Simms, William      29 Apr 10

Skalensextant, Freiberger                                  22 Mar 09

bearing of                                             22 Mar 09

dimensions of                                        22 Mar 09

optical path of                                       22 Mar 09

prism, cleaning of                                4 Mar 10


lighting                                         22 Mar 09

focus screw, access to            4 Mar 10

focus of                                        4 Mar 10

SNO-M sextant (Navigation sextant with illumination, marine)

compared to C Plath                             26 Oct 08

origins of                                              26 Oct 08

telescope of                                          26 Oct 08

SNO-T sextant (Navigation sextant with illumination – tropicalised)

index arm bearing of                             22 Nov 08

instrumental accuracy of                        22 Nov 08, 13 Feb 11

micrometer of                                       22 Nov 08

mirror adjusting screws            17 April 10

use of                                               17 April 10

repair of                                          17 April 10,   28 Nov 2020

drawing of                                       17 april 10

origins of                                              22 Nov 08

position of index arm                             22 Nov 08

telescopes of                                        22 Nov 08


of tangent clamp                                    10 Nov 09

material of                                    10 June 10

of swing arm chassis     12 Mar 09

pre-load, of worm shaft     12 Mar 09,  15 Dec 10

Stanley, W F and Co, merger with Heath          26 Apr 09

station pointer                                                   26 Apr 09

survey sextant                                                   6 May 09, 26 Apr 09, 28 Apr 10

frame of                                                6 May 09

handle of                                              6 May 09

placement of legs                                  6 May 09

telescope mounting of                           6 May 09

use of                                                  26 Apr 09

swaging, of lens                                                24 Jan 10

swing arm

of Observator sextant       20 Oct 16

bearings, Heath                                5 May 09

chassis     12 Mar 09

Ilon Industries            1 Oct 14

keeper       14 June 14

switch, lamp, of Husun                          18 Feb 10

Tamaya, faulty rising piece,   2 Sept 11

tangent screw,

Heath                                                  28 Jan 10

Hezzanith endless                                 3 Jan 09

Hughes Admiralty pattern   24 June 11

of C Plath’s earliest sextant    21 April 2017

taper pin,

removal                                               18 Dec 08

replacement                                         18 Dec 08

sealing bubble unit with      5 Oct 10


bracket, filing to correct collimation     28 Nov 2020

collimation of     2 Sept 11,     28 Nov 2020

making                                                23 Nov 08

Galilean, optics of                                 30 May 09

of C Plath’s earliest sextant     21 April 2017

of C Plath WW II sextant                  14 July 10

of C Plath yachting sextant           14 June 15

of Hughes Admiralty pattern sextant          24 June 11

of La Filotecnica Salmoiraghi sextant     5 Oct 10

of SNO-M sextant                               26 Oct 08

of SNO-T sextant                                 22 Nov 08

of US Martitime Commision sextant   15 Dec 10

Willson bubble                          2 Dec 08

telescope mounting

   of survey sextant                                   6 May 09

   Husun                                                   24 Jan 10

telescope rise and fall mechanism, Plath 24 Jan 10

   Admiralty pattern  24 June11

thread, interrupted of telescope mounting     5 Oct 10, 24 June 11

internal, measuring of                                         26 July 22

tool, shades adjusting, Plath           6 Oct 13

Troughton, Edward    28 Apr 10, 10 June 10

Troughton, John     28 Apr 10

US Maritime Commission  15 Dec 10

US Maritime Service   15 Dec 10

US Navy Bureau of Ships   Dec 10

Védy, of Paris                          26 July 22

vial, of spirit level                                              30 May 09

washer, Belleville                                              18 Feb 10, 26 July 22

    lead, for sealing filling hole in A10 bubble unit  13 Oct 10

   making                                                 11 Aug 09

Wild, Heinrich                                                  22 Mar 09

Willson bubble telescope                                  2 Dec 08

Wollaston, William                                10 June 10


bent shaft of                                         6 July 09

burrs on                                               6 July 09

cutting new                                          6 July 09

errors     12 Mar 09

Hughes                                                3 Jan 09

Ilon Industries           1 Oct 14

periodic error of             6 July 09

progressive error of                              6 July 09


effects on plastic                                   18 Dec 08

refilling bubble chamber with ,              18 Dec 08, 13 Oct 10

resistance of Viton to, 13 Oct 10