We are all familiar with compasses that are marked in cardinal points: North, East, South, West, and the inter-cardinal points: North East, North North East, East North East etc., and the compasses marked in 360 degrees where North is zero or 360 degrees, South 180 degrees etc., but there is another unit of measurement that has been in common use in Francis Barker military compasses since the mid-1960s.

I will give a brief description of the mils system and how it works, purely out of interest for compass collectors who may not be aware how mils work and why they are used.

The mils system has been around since the beginning of the twentieth century, but the American military formally adopted it in the 1960s and it is now widely used, in one form or another, by most armies worldwide.

Measuring angles is important for navigation, map making and artillery, all of which are very important in the military world. The measurement of angles is the area of mathematics known as trigonometry, or geometry.

There are two pi radians in a circle, where pi is the ratio of the diameter of a circle to its circumference. Pi is roughly equal to 3.1416, but the decimal places go on to infinity, it is an irrational number. Simplistically explained, one pi radian is a segment of a circle that has a rounded edge that is the same length as the straight sides.

From the radian we get the mil, or mil-radian. This is another unit used to define angles. These are used by the military because an angle of n mils is n units wide at a distance of 1000 of the same units. So when viewed at 1000 metres a 3 metre long vehicle will appear to be 3 mils in length.

There are 6283.1853 mils in a circle, but the U.S. military standardised this to 6400 mils to simplify things, so that North is seen as 6400, south is 3200 and so on.

The Russian and Arab countries decided to make things even easier, and they developed compasses with 6000 mils, which look just like a watch face. North is 6000 or zero, East is 1500, South is 3000, West is 4500 etc., just like minutes on a watch face. These compasses have been produced by Francis Barker since the early 1970s and are called the DICI model.

The mils system fits in well with artillery, rifle and binocular optics that have reticles or graticules, scales visible in the sights that work as distance markers. A man, who we can assume to be approximately 2 metres tall, seen through an optical sighting system as 4 mils tall will be at 500 metres, but if he was seen as 1 mil tall he would be at 2000 metres distance.

The French came up with an idea to make a compass with 400 units or "grads" but it never really caught on, and Francis Barker never produced any.

The three Francis Barker M-73 compasses above appear very similar, but the one of the left is calibrated in 360 degrees and simultaneously 6400 mils (dual-use), you can see the number 36 at the top of the index ring; the centre compass is calibrated in the standard 6400 mils, with 64 at the top of the index ring, and the compass on the right has the Russian / Arabic calibration of 6000 mils - see the 60 at the top of the index ring.

Many compasses are also calibrated in both mils AND degrees and are known as dual-use compasses. The perfect example is the currently produced Francis Barker M-88 Prismatic Compass which, like nearly all dual-use compasses, is calibrated primarily in mils, with degrees as the secondary calibration.

The M-73 above left was made as a one-off special order item for a private client who then never took possession of it, and it is unusual in that it is calibrated primarily in degrees with mils as the secondary unit of measurement.

Mils can appear confusing, but using them is exactly the same as using a compass calibrated in 360 degrees, you just think in different numbers. Mils calibration has the advantage of working well with artillery calculations, as the same unit is used to calculate distance, vertical angles and horizontal angles.