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. 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.





Another Sounding Sextant

17 11 2018

A little while ago I acquired yet another sounding or survey sextant for a relatively small sum. It is based on a Cassens and Plath nautical sextant. As with most sounding sextants, it has no shades, but where the index shades would normally be mounted is a leg and where the horizon shades would be mounted is a bracket for a pentagonal prism or “penta prism”.

Over two hundred C&P surveying sextants were obtained by the US Coast Guard Service from Weems and Plath around 1978, provided with a handle that would make holding the instrument horizontally easier and stripped of the lighting system, to save unnecessary weight. In my instrument, which bore a USGCS label, the lighting system is intact and there is no provision for a modified handle.

Figure 1 shows the state of the instrument as received and it had plainly not been well loved in the autumn of its life (by the way, the hand holding it is not mine).

As found labelled

Figure 1: C & P Sounding sextant as found.

My usual method is to strip the sextant down to the last screw and washer and then to clean and repaint everything, stripping all the old paint off if necessary. As I go, I fix electrical faults, renew wiring, replace mirrors , clean optics, and re-grease moving parts. As I have elsewhere in the blog described these activities, I will not go into them here, but instead focus on the main point of difference from other sounding sextants: the pentaprism. I have given a very brief account of the use of sounding sextants in the post for 26 April, 2009, and this should be read in conjunction with the comment kindly sent by Peter Catterall.

In a pentaprism, the emerging ray is at right angles to the incident ray, and the angle between the two rays (really two parts of one ray) is independent of any rotation of the prism about an axis parallel to any of its faces.  The image is not inverted or reverted. However, if the prism is rotated about another axis, the incident and emergent rays will not be at a right angle. Although there are two internal reflections in a pentaprism, they are not total internal reflections as, say, in a 90 degree Porro prism, and so the reflecting surfaces have to be silvered. If the paint film and underlying silvering gets damaged, the damage will be apparent in the view through the prism.


Figure 2: Position of the pentaprism.

Figure 2 shows the location of the pentaprism behind the clear glass of the horizon mirror. It is located in a spring-loaded bayonet socket by means of a peg, which allows it to be placed in two positions (Figure 3). Rotating it anticlockwise locates it in the position shown in Figure 4.

Pentaprism base

Figure 3: Pentaprism base

When located in this position, if the index arm is set at 90 degrees, the two light paths should be parallel, so that a distant vertical object should form a continuous vertical, straight line when the instrument is held with the frame horizontal. For this to happen the faces of the prism must be at a right angle to the frame of the instrument, so three adjustment screws are provided to bring this about. It is a great deal easier to do this if a 2 mm diameter torus of thin, soft copper wire is placed centrally under the face opposite the adjusting screws, so as to allow a little rocking  to take place. This is a little simpler than following the official advice promulgated in the US Coast Guard Service manual, available on line here:  The prism is held in place by two rectangular “springs” which offer quite a lot of resistance to the movement of the prism when adjusting it, so it is easier simply to leave them a little proud of the prism faces and rock the prism as I have suggested. The adjusting screws then do double duty of adjusting and retaining with the springs as back-up retainers.

Any index error of the sextant must of course be allowed for, in addition to any error found with the prism in place, and normal checks for perpendicularity of the index mirror and side error made and corrected.


Figure 4: Position of prism to check index error.

Figure 5 shows the prism in its orientation in normal use and you can see that with the index arm set at 30 degrees, the rays diverge by 90 + 30 = 120 degrees, the practical limit for a normal sextant, where the reflected image is reduced to a narrow slot.

30 degrees

Figure 5: 90 + 30 degrees = 120 degrees.

Figure 6 shows how 180 degrees can be measured by setting the index arm to 90 degrees. The ability to measure large obtuse angles improves the strength of position lines when fixing the position of aids to navigation.

90 degrees

Figure 6: 90 + 90 degrees = 180 degrees.

Although the stout case could be mistaken for solid wood, it is in fact some sort of laminated wood, as witnessed by the delamination of the outer layers of the top and bottom. It seems strange that an instrument destined for use in a damp and sometimes wet atmosphere should not at least use marine grade laminates for its case. The corners are keyed mitre joints which give both a very neat appearance and very adequate strength. Note that the key should be sited as close to the inside angle as feasible, as shown in Figure 7.

Case corner

Figure 7: Keyed mitre joint in case.

Figure 8 shows the instrument less its telescope in its case. It cannot be stowed with the pentaprism in place, though with a little more thought, the pocket for the sextant handle could have been rotated anti-clockwise and moved to the left a little to give room for both the prism and the originally supplied prismatic monocular.

In case

Figure 8: Instrument in its case

The telescope with my sextant is a standard 4 x 40 C and P offering except that there is a glass in front of the objective lens that acts as an astigmatiser with stars, drawing out the point sources into lines (Figure 9). It has no effect on extended images. I cannot imagine how this works, so if any reader knows, I should be glad if they would share their knowledge with me.

I hope to be able to add two or three more posts to this blog before the end of the year, after which it will the turn of a marine chronometer to be described on my other web site, .