Range Finders Useful in 1915

Tuesday, April 20, 1915

In trenches, St. Julien

The Battalion War Diarist wrote for this day: “Improving trenches, also working on reserve trenches northeast of and just outside St. Julien”. [1]

THIS DAY IN RMR HISTORY: “Referring to shelling of Ypres on April 20th, a Signaller of the 14th writes in his diary as follows:- ‘ I went into the town during the bombardment to see what it looked like. Nearly all the buildings in the market place had already been destroyed and the café where I had breakfast this morning was spread all over the square. In a corner of the square a group of civilians and soldiers were loading wounded into ambulances. Close by another group were working feverishly with pick and shovel recovering bodies buried in the debris of ruined buildings Here and there dead horses lay across the sidewalks and in the roadway. The few women I saw were all hysterical and running about like mad things. Later, in the evening, I went up again. An unnatural calm hung about the town. The civilians seemed awed and terrified, walking close to the walls, and crouching down every time a shell screeched overhead. It is difficult to describe that awful calm. The people seemed afraid to speak and every step they took they would put their feet down as if afraid to make the slightest noise. I hope I shall never see such a sight again.’” [2]


20 April 15

“The range-finder, which is of the utmost importance in war, is an instrument for ascertaining the distance to any visible point on the landscape from the position occupied by the observer or operator.

Range-finders are of two types: the double-observer type, such as the Mekometer, or Telemeter, used in the British service; and the ‘one-man’ type. In both cases the distance to the object (or range) is found by triangulation, the angles being taken from the ends of a known base – a very short base of about three feet in the case of the one-man range-finder, and a normal base of fifty yards for the Mekometer artillery instrument and twenty-five yards with the infantry instrument.

In the Mekometer * the base is the known length of chord stretched between two instruments, held by the two observers. The man with the reading instrument sights an object of which the range is required. The second man advances until he can, through his right-angling instrument, see both the same object and the sighting-vance on his partner’s instrument. When these two coincide, he shouts ‘On,’ and the first man, by turning the range-drum on his instrument until he also makes the reflection of the observer’s sighting-vance coincide with the object seen in the instrument, is then able to read the range off the range-drum in yards. This instrument was employed at the time of the South African War, but owing to its having a very long base (25-50 yards), and requiring two men to operate it, was found extremely difficult to use because of the lack of cover.

In one-man range-finders, like the one illustrated above, the base is a bar, or frame, of short length, with a telescope mounted at each end, and having an eye piece in the middle into which the rays are reflected. With this instrument, measuring only 37 inches long by 3 inches in diameter, and weighing 5 ½ lbs, one man is able rapidly and accurately to take ranges of objects up to 20,000 yards distant. In taking the range the operator directs the telescopes of the instrument on to a clearly defined object, and by turning the range-drum the right-hand telescope is inclined inwards until the two images seen in the central eye-piece coincide. The range given on the drum can then be read.

A typical one-man range-finder is illustrated, diagrammatically, above. It shows the two telescopes already mentioned running at right-angles to the single eye-piece fixed in the center of the range-finder tube. The rays from the distant object entering the end apertures of the range-finder base, are received by the left and right prisms and transmitted through the left and right objectives towards the central reflectors, which reflects them outwards through the eye-piece. The observer, looking into the eye-piece, will see the field of view divided by a thin ‘dividing line.’ Anything seen above this horizontal line is formed by the left-hand telescope, and that seen below the dividing line, by the right-hand telescope. By turning a drum, these images can be brought into coincidence, and the correct range can be read from the range-drum.

An artillery range-finder must be a good horseman, having a good eye for country, keen eyesight, and above all, steady nerves. Any nervousness while taking a range may easily give an error of a hundred yards or more, which might make the difference between victory and defeat. The advantage is with the battery which drops the first shell on the right spot at the correct range.

Figure 1 shows the external appearance of the one-man range-finder (Marindin). Figure 2 is a cross-section view. Figure 3 shows the instrument in use, while figure 4 shows the images in coincidence.”[4]

* Note: Mekometer: a device that accurately measures distance by measuring the polarization of a reflected beam of light. [5]

[1]   War Diary, 14th Canadian Battalion, The Royal Montreal Regiment, April 20, 1915.  Library and Archives Canada, Ottawa, http://data2.collectionscanada.ca/e/e044/e001089715.jpg
[2]   R.C. Featherstonhaugh, The Royal Montreal Regiment 14th Battalion C.E.F. 1914-1925, Montreal, The Gazette, Printing Co., Ltd., 1927, pg.37.
[3]   “The Range-Finder, A Simple But Wonderful Instrument,” The Citizen, Ottawa, Ontario, Saturday, April 24, 1915, pg. 16, col.
[4]   Ibid
[5]   Collins Dictionary,  http://www.collinsdictionary.com/dictionary/english/mekometer