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Prismatic Compass M73

Francis Barker M73 Prismatic Compass

The British Army Prismatic Marching Compass M73 has become a classic of it’s time.

I first handled one back in the Eighties and was very sad to have to hand it back at end of the exercise.

Eventually I obtained my own and I thought I would write a few words for people that have never used one to show why these are such a joy to use and still so popular today.

It is liquid filled and mainly made of brass, there is a lighter version, the M88 that is alloy but I do not begrudge the reassuring weight in my pocket.

While not completely “squaddie proof” it is well made and robust enough to not worry too much about in normal use. As with any precision instrument, a little respect in handling is wise.

You can see the mother of pearl compass dial, with it’s finely graduated scales, through the window in the lid but when opened up, a second dial is revealed with a coarser scale, marked in 5 increments. This is for setting a bearing to march and it is locked by a knurled knob at 45 from the Lubber line. 

Prismatic Compass dials
M73 Prismatic Compass

The north point on the compass dial and the marching direction on the outer dial are both marked with Tritium light sources so that a bearing can be followed in full darkness.

The observant amongst you may have noticed that the fine scale on the compass dial is reversed, this is where things get clever.

To take a sighting, we flip the small prism assembly from it’s resting place to it’s working position over the dial. It is mounted on two brass slides and you set the focus by sliding it up or down with your thumb.

Now raise the lid so that you can look through the sighting slit and align the hairline on the glass window with the object of your bearing. (You get a much clearer view of your object in reality than I can show you in these pictures.)

A rough aproximation of the view through the prism.

You should now be able to read the bearing from the compass dial by looking through the prism.

Under normal conditions it is quite possible to achieve or even accuracy with this method hand held.

The prism assembly is mounted on slides to

I usually slip my thumb through the wire ring while wrapping my fingers around the compass body for a firm but comfortable hold while taking a bearing.

There is no way to set the magnetic declination on the compass so that needs to be calculated manually. This is the same for most compasses although some do allow this setting.

Using a Prismatic Compass

The hairline in the lid can also be used for aligning the map and compass but it is not really suitable for taking a map bearing, a separate protractor / romer is better for that.

These points  may be seen as disadvantages when compared with a base-plate compass, and with good reason.

For myself, I enjoy the simple mechanics of navigation and these little foibles do not bother me in any way. In fact, I find that such calculations seem to enhance my understanding of the process. 

It’s fair to say that a prismatic compass is not for everyone and many will prefer the simple functionality of a base-plate compass.

If like me you feel that navigation is an important and also enjoyable part of your experience of the outdoors then a precision instrument like the prismatic compass, or perhaps a transit compass, is an investment that will give you accurate results and you may appreciate for many years to come.

 

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