A Francis Barker Yachting Sextant.

17 02 2016

This post also appears under the category “Box sextants”. All figures may enlarged by clicking on them. Use the back arrow to return to the text. 

Francis Barker, born in 1819, flourished in Clerkenwell Road, London from about 1840 until his death from tuberculosis in 1875. He was succeeded by his sons and the firm continued under his name until about 1932, when it was taken over, eventually by Pyser CGI of Edenbridge, Kent. Barker’s main products were magnetic compasses and sundials, though they did branch out into jewellery making in the late nineteenth century. Magnetic marching compasses continue to be made under the Francis Barker name. Probably in the second half of the 1970s they began to produce a yachting sextant and production probably continued for about ten years. Celestaire bought the entire remaining stock of 36 instruments in the late 1980s and disposed of all of them. They are relatively rare.

The sextant was a box or drum sextant and was contained in a heavy saddle leather case, retained by two press studs (Figure 1). It is quite difficult to extract it from its case without pulling on the strap and putting its stitching at risk. The small handbook reommends putting the strap around the user’s neck to guard against accidentally dropping it overboard.

In case

Figure 1. Sextant in its case.

Figure 2 shows the instrument out of its case, and Figure 3 shows the general arrangement with the principal parts labelled.

OLYMPUS DIGITAL CAMERA

Figure 2: Sextant out of its case.

GA

Figure 3: General arrangement.

The operating parts are contained within a light alloy drum 76 mm in diameter and 50 mm high. The main scale, 45 mm in radius, is divided to half degrees, and there is a vernier divided to single minutes. The setting knob contains a planetry drive, which gives a slow motion when rotated slowly and a fast motion when rotated more quickly.

There are two interchangeable peep sights (Figure 4), each with a 1.5 mm in diameter hole. One is provided with a shade for use when the horizon is bright. Many older users may have small central cataracts in their eyes and they may find that the hole in the sight is small enough to cause a shadow of their cataracts to partially obscure their view, in which case there is nothing to be lost by opening out the hole to, say, 2 mm, to allow more peripheral rays to by-pass the cataract. The two levers shown bring index shades into the light path.

GA with sights

Figure 4: Peep sights and shades.

The internal arrangement for these shades is shown in Figure 5. According to Ken Gebhart of Celestaire, there are at least 18 units in ciculation in which full glasses were installed by mistake, instead of the half-glasses shown.

Shades

Figure 5: Horizon shades.

Figure 6 shows the general internal arrangement.

GA internal

Figure 6; General internal arrangement.

The index mirror is adjusted for perpendicularity by the usual method (Figure 7) and Figure 8 shows how the mirror bracket is tilted by means of opposed screws which rock it about a horizontal axis formed by the heads of two grub screws.

Perp adjust

Figure 7: Aligning index mirror for perpendicularity.

Index mirror mount

Figure 8: Index mirror mounting and adjustments.

The method of adjusting the horizon mirror for index error can be seen reflected in the index mirror in Figure 7 and a close up is shown in Figure 9. Side error is taken care of by the same method of rocking about an axis by two opposed screws , while index error is removed by rotating the whole mirror bracket via a metal gear meshing with a nylon gear.

Horizon mirror adj

Figure 9: Horizon mirror adjustments.

The squared heads of the adjusting screws are on the face of the sextant and are adjusted by means of a key that unscrews from its nearby housing (Figure 10). The slotted screw must be tightened before removing side error and then slackened off a little to allow adjustment for index error. This may introduce some more side error, so the cycle may need to be repeated a few times, after which the slotted screw is carefully re-tightened.

Adjusting screws

Figure 10: Heads of adjusting screws

The scales are read with the help of a plano-convex lens of about 25 mm focal length (Figure 11). The divisions lack a little of the crispness seen in earlier box sextants. The rather small radius of the arc, of 45 mm as opposed to about 160 mm for a full size vernier sextant, makes deciding which line on the vernier coincides with a line on the main scale somewhat problematical. In the photograph, which gives a fair representation of what is actually seen, at least three lines coincide, so perhaps the best that can be done is to decide which two pairs just do not coincide and to chose the lines midway between the two pairs.

Scales

Figure 1. Sextant in its case.

If you enjoyed reading this post, you may also enjoy my books “The Nautical Sextant” and “The Mariner’s Chronometer” (www.chronometerbook.com).

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A Box Sextant

7 01 2010

All figures may enlarged by clicking on them. Use the back arrow to return to the text.

For a sextant enthusiast, to own a box sextant after handling  full sized instruments gives a lot of pleasure. These dainty and ingenious little instruments, measuring only 75 mm in diameter, can be slipped into a pocket, but be quickly brought out to make a reading with a precision of around an arc-minute. They are equipped with shades that allow sun and moon observations and some models come with a small telescope. They are especially handy for taking horizontal angles on land.

Unfortunately, there are many so-called reproductions on the market, and it may not be easy for the inexperienced to distinguish them from the genuine article. One should look for fine, usually straight  knurling on the adjusting knobs, crisp edges to the index arm and magnifier arm, sharp corners on the bracket that holds the latter, smooth, ground edges to the mirrors and sweet operation of the various adjustments. A bright sky should just be visible through the darkest shade (usually red) and should not be visible at all when used in combination with the second shade (usually blue). The two together should allow comfortable viewing of the full sun.

The scale is usually divided on silver, each degree being divided into halves, with a vernier reading to one minute. The arc radius is about 46 mm. Look for crisp regular engraving of the numerals (Figure 1) rather than the uneven, stamped numbering with rounded edges often seen in  reproductions. The numbering on the vernier usually omits the fives, and a scale with crowded numerals reading 5, 10, 15 etc is quite likely to be a fake. The scale is read from the centre of the index arm, the opposite of a nautical sextant, which is read towards the centre.

Scales 001

Figure 1: Scales of genuine antique.

A maker’s name  may be of little help. There must be thousands of so-called “Stanley” and “Kelvin-Hughes” instruments around, the latter usually dated 1918, nearly thirty years before  the real company came into being. Stanley was a highly respected firm of instrument makers and suppliers, founded in 1853 and trading well into the second half of the twentieth century. Older instruments are engraved “Stanley, London” in beautiful copperplate, though they were probably made (and later labelled) by Heath and Co, with whom Stanley eventually merged in 1926. There are box sextants bearing the name of Elliott Brothers and many other nineteenth century instrument makers and most of these are genuine, but it is important to look at the general workmanship as well as the name. Stanley is the name that has been most abused.

The second photo shows a genuine box sextant by Stanley, alonside its big brother from the same period, a vernier sextant of eight inches radius by Crichton of London, dated to about 1850. The lid unscrews to expose the controls, and is screwed on to the base to act as a handle in use.

 

The next photo shows the underside of the browned bronze base of the instrument. The nib is used to slide the cover aside. This opens the slot through which shades emerge when not in use, as will be seen in a later picture.

Figure 3: Base.

The general view below shows some of the main features. Rotating the control knob moves the index arm and the vernier over the scale while rotating the index mirror. Note the fine knurling and the crisp edges of the pin holes in the central screw. The magnifierarm, its bracket and the index arm also have crisp edges and the screw slots are narrow, the screw heads polished. Sliding the nib brings the peephole into position and a knurled screw is provided above the peephole to attach the telescope when it formed part of the kit (in some makes, the telescope screwed into the hole). Next to this screw is the mirror-adjusting tool, which screws into place. The levers for bringing the shades into and out of position are to the bottom left of the photo. When they are not in position, they project through the slot described in the preceding paragraph. At the top end of the scale can be seen the two square-headed screws which are used to adjust out side error.

The next photograph shows many of these features from a different viewpoint, that also shows the window opposite the peephole and the head of the screw used to adjust for index error.

For those of you who dare not take their instruments apart, in the next picture I have done it for you, by removing three screws from the periphery of the base plate. The shades are raised out of the way. In use, the head of a limit screw ensures their correct position. The index mirror, its bracket and keeper are mounted on a bearing and are rotated by the toothed sector or rack by means of the control pinion, the business end of the control knob. The horizon mirror sits on a base that can be rocked by means of two spring loaded screws to remove side error and the sub-base below it can be rotated by a further spring loaded screw to remove index error.

The next picture shows another view of the interior to show more details of the horizon mirror arrangement.

Thanks to the kindness of Bill Whiteley, I am now (October 2013) able to add a few sentences about the origins of the box sextant.

A memoir of a meeting appeared in The New Monthly Magazine, Vol 24  1828, of “…the late James Allan mathematical instrument maker in London, who died in the year 1821, compiled by the late Rev. Thomas Macfarlane, minister of Edinkillie, with an introductory letter by Sir Thomas Dick Lauder, Bart. Mr Allan was a native of the parish of Edinkillie, who procured to himself a considerable portion of fame by the discovery of several simple, but most accurate methods of graduating mathematical instruments. The pocket sextant, which gained him the prize and encouragement of the Society of Arts of London, was exhibited on the table of the institution at this meeting. It now belongs to Sir Thomas Dick Lauder.”

 A box sextant presented by the 4th Duke of Gordon to his son in 1813, signed by Allan, is now in the Royal Engineers Museum. This gives us a date before which the instruments were being made.
 
In Nov 1800 James Allan is a shopman (?foreman) lately in the employ of the famous Jessie Ramsden receiving a legacy from Mr Ramsden of twenty pounds. It seems that Allan remained at the Piccadilly workshop, (which had been inherited by Matthew Berge from  Jessie Ramsden,) and was in a position to operate independently. In Nov 1809 he presented to the Royal Society of Arts an improvement on the dividing machine created in 1775 by his former employer Jessie Ramsden. Meantime Berge had amongst other matters, been actively miniaturising the moderately short-lived bridge sextant an instrument which in all probability involved the attention of James Allan.
 There currently exists no firm date for Allan’s creation of his box sextant, However we might assume it to be around the time of his improvements to the dividing machine.
I hope to be able to add a little more about the origins of the box sextant when Bill has had a chance to trawl through the Proceedings of the Royal Society of Arts in the British Library.
12 November 2016: Richard Paselk , Professor emeritus at Humbolt University has kindly just send me this link describing some more of the history of the box sextant: http://www2.humboldt.edu/scimus/AvH_HSU_Centenial%20Exhibit/Box_Sextant/BoxSextant.htm 

Finally, this picture, showing the instrument in use, gives another impression of its size.

If you enjoyed reading this post or found it helpful, do let me know and if you have a “doubtful instrument” I will be happy to view a photo and advise.