This paper was originally published in the Slide Rule Gazette, Issue 17, Autumn 2016 by the UK Slide Rule Circle.
Figure 1 - Marsch Zirkel, upper side
This instrument was a chance find on eBay a few years ago and in fact performs both functions. The upper side is shown in Figure 1 and it can be clearly seen that it consists of a series of scales combined with a divider. It was intended to be used in conjunction with a map to show (calculate?) the length of time it would take infantry or cavalry to travel from one point to another. This side of the instrument is graduated for a map scale of 1:100,000. The outer scale is in km and reads in increments of 0.1 km to 10 km. The remaining scales are times in hours and minutes. The outermost of these is for infantry and it can be deduced that the marching pace is 5 km/hour. The other three are for cavalry. Progressing inwards these are respectively at the walk, trot and gallop. The other side (Figure 2) is similar but is graduated for a map scale of 1:80,000. The scale here is limited to 9 km by the geometry and size of the instrument. From the face of the instrument it can be seen that it was designed by Prim. Lieut. Brenske and patented (in 1891), DRP61509. Figure 3 shows the first side with the divider open.