Compass

[Woftam]

This is the other new toy I got recently. My wife pointed one out to me in an antique shop on our recent trip to Tasmania. They only wanted $800 for it
After telling her I thought this was well overpriced I decided to do a little browsing when I got home. Picked this up for less than a quarter of the price of the one in Tasmania.

[Niner]

That's interesting. Verners Pattern, isn't it? WWI issue compass?

One thing to note with your compass is that the two rows of numbers are true bearing in the large figures and the reciprocal bearing in the small figures in line with them. Later military compasses in the 20th Century have mils on the outside of the dial and degrees on the inside. Artillery always used the mils. 6400 mils equal 360 degrees. The mils were finer mesurements of the degrees and not reciprocal.

[Tom-May]

Woftam,

Am I right in thinking that the compass is of the Air filled pattern (with a brake for the card) as opposed to the later oil filled models?

Tom

[Woftam]

Yes Tom,
it is air "damped" as opposed to oil damped. There is a latch that engages with the closing of the lid to lock the card and a plunger style button almost opposite to brake the rotation when trying to get a bearing.
In the fourth last photo (just before the photos that include the leather case) you can see the latch at 1 o'clock, the plunger at 8 o'clock and another feature at 11 o'clock - you wouldn't know what that is by any chance ?

[Tom-May]

Hi Woftam,

Sorry for the delay in replying - scanner trouble.

Is this of any use to you? It's lifted straight out of the 1906 manual.

Regards

Tom





Manual of Map Reading and Field Sketching – General Staff – 1906)

The Prismatic Compass

In a prismatic compass the bearings of the card are read through a prism which enables the observer to see the distant object and the reading at the same time. There are many different patterns of prismatic compass. One form is the Service Prismatic Compass (Mark V). (See Plate 11.)
The instrument consists of a magnetic needle balanced on a pivot, and carrying a dial divided into degrees, contained in a metal box, round which is a brass ring graduated to show every fifth degree and the points of the compass. The metal cover, C, opens on a hinge, and is fitted with a glazed window, W, on which is traced a fine black line, V, for use as a sighting-vane. Opposite the hinge of the cover is fitted a prism, R, through which can be read the graduated edge of the dial, while at the same time an alignment of the object and of the sight-vane on the cover is observed through the slit above it. The prism should be moved up or down in its slot till the figures on the dial are properly focussed. A clamping-screw, S, is provided for clamping the needle when not in use and a “check-spring”, A, for checking its oscillations when observing. A brass ring, B, is attached for convenience in holding it.
The dial is “luminous” for night work, the North point is marked with a large diamond shaped figure. A revolving glass is fitted over the compass dial, and on the glass is a black direction mark, radiating from the centre, at the end of which is a small brass “setting-vane”, the latter working over an external arc graduated to 360o. A brass screw, J, is used to clamp the glass. On the inside of the cover are two luminous patches, PP, which gives a good alignment of this instrument at night when it is held in the hand with the cover wide open. There are two small holes, HH, in the brass window-edge of the cover, so that, if the glass breaks, a horse-hair can be run between them, and an extemporised sight-vane be utilized.
The compass dial is graduated with two sets of figures which read eastward of the meridian, or from left to right, like the hands of a watch. The outer set is for use with the prism, and commences at South in order that the bearing of the object may appear under the eye. Thus the vane being directed on an object which is 50o from North, it is that number measured from South which appears under the prism; or in other words, 180o is over the North end and 360o over the South end of the needle. The inner set of figures is of use for direct readings (i.e. without the prism) as for compass marching, or when the instrument is used with the plane-table.
The prismatic compass gives “bearings” and not “angles”. The horizontal angles between any distant objects are obtained by taking the difference of their observed bearings.
Example.- The angle between two points A and B , is required. Their bearings are observed to be respectively 50o and 110o. The angles required :- 110o – 50o = 60o.

[joseyclosey]

I had forgotten that i had recently bought a copy of the 1929 edition of that book Tom, mine shows the MkVIII compass.

Joe

[Woftam]

Yes Tom that answers a few questions. Unfortunately it doesn't identify the feature marked J, which is the one I was interested in. Fiddling with it, it seems to be a clamp that when loosened allows rotation of the glass. Not sure why but that is what it seems to do.

[Tom-May]

Hi,

Sorry, yes, 'J' is the clamp for the top (movable) glass cover. This allows the luminous stripe (on the top cover) to be used to allow the compass to be set for marching on bearings at night.

Tom