A PROPOSED SYSTEM O F RANGE FINDING FOR INFANTRY AND CAVALRY

A PROPOSED SYSTEM O F RANGE FINDING FOR

INFANTRY AND CAVALRY.

BY LIEL-~NANT

F. WHARTON GRIFFIN, ARTILLERY

CORPS.

HE range finders in use by the U. S. army to-day are T unsatisfactory with the exception of those used by the Coast artillery. In recent years numerous experiments have been made with the range finders of foreign make, such as the Bias stereoscopic binocular range finder and the Forbes telemeter, but these also have proved unsatisfactory. The essentials of a good range finder are that it should be accurate, easily carried in the field, a short time and few men required to obtain the range, and of course the less complicated the parts of the range finder and the method of obtaining the range the better. It is easier to find short ranges with accuracy than long ones with all range finders, , and for this reason this system is proposed for use by the infantry and cavalry, and if it should be tried and any one ' of the methods proves to be satisfactory a similar method will be presented for trial by the field artillery. The best range finder for troops in the field now in use is the Weldon. This range finder is used by the field artillery. and in theory it is sufficiently accurate. but unless it is in the hands of a well trained man it is very inaccurate, and even in the hands of a well trained man errors sometimes occur. The principle by means of which the range is obtained in this proposed system is similar to the method employed by the Weldon, but the instruments (there are two of them used in conjunction) are entirely different. Before describing the proposed instruments a discussion of the W u p l e will be given. An attempt has been made to eliminate all the errors that are liable to occur when using the Weldon and to make the instruments almost as accu-

RANGE FINDING.

31 rate when used by untrained men (but men who understand the principle) as it is when used by trained men.

FIRST METHOD.

This method depends upon the solution of one side of a right angle triangle; the angles of the triangle always being constant and hence the two adjacent sides of the go" angle are in proportion. Therefore in all right angle triangles containing an angle measuring 8 7 O 42t 34)' the sides adjacent to the goo angle are in proportion of one to twenty-five. The natural tangent of 87' 42' 34" is 2 j. A I It will be seen in the figure if 0 represents the point from which the rangeis'to be obtained and T represents the target,'that if a go" angle be measured at 0, and the point A be found where the angle betwe2n A T and A 0 equals 87" 42' 32). the side T 0 is 2 5 times as long as the side 0 A. The proposed instruments to be used to obtain the range consist of two instruments that will measure the goo and the 8 7 O 42' 34" angles, and a tape line from which the range can be read at once, i. e., without having to multiply the distance A 0 by 25. The instruments to be used to consist of rigid cross sights mounted on pointed staffs. One of these instruments to have the sights permanently fixed at sight angles and similar to the surveyor's cross. The surveyor's cross sights would not be the best sights for range finding, and in their place I would suggest two metal wedge-shaped sights. The front sight the same as the rear sight. The 'other instrument to have one sight at an angle of 87" 42' 34" to the other, and the same kind of sights described for the first instrument. These instruments to be

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RANGE FINDING.

detachable from their staffs and provided with the ball and socket movement similar to the Jacob's staff. The sights to be carried in leather cases swung over the shoulder. If the staiT af the go" instrument be made several feet shorter than the staE of the instrument containing the 87O H" angle and one of the sights of the goo instrument be telescopic and provided with stadia wires, ranges can be read directly by having the graduation on the staff of the 87O 42' 34'' instrument read 25 for one, If the stadia method be used, the cross arms of the ninety degree instrument should be provided with level tubes. A time saving device for the 87' 42' 34/' instrument would be a movable center similar to the transit, but having more The dimensions of the instrument and the metal or combination of metals out of which they should be made will be determined later if t h h system is approved. The instruments in their leather cases will not be much larger than the cavalry sketching case. The tape need not be over eighty yards long, for this length tape will measurea range of 2,000 yards, 80)<25=2000. This tape might be self-winding on a reel in a box and the box prodded with some arrangement to hold it in place, as a pin to be stuck in the ground. 3t will be seen from the figure that the range can be found by laying off the right angle triangle either to the right or to the left of the line T 0. Two men are required to find the range, and for convenience I will call them No. I and No. 2. No. I sets up the instrument which has the sights permanently fixed at right angles on the target. He then directs No. 2 on'the line 0 A until he has found the point A with the other instrament, and the distance 0 A, measured by the tape gives the "me. If the stadia be used the range is read as soon Bs the point A has been found, but unless 0 and A are on dry nearly the same level the tape line would have to be a. If the right angle triangle is laid off to the right of the line T 0 No. I directs No. 2 along the line OAr Play. ' . L . .

RANGE FINDING.

33 until he has found the point A', and the range is found as before explained. In order to prevent confusion on the part of No. 2 the sight which he is to point towards the target should be marked. It will be seen in the figure that this sight when used on the right of-the point 0 is revolved 87O 42' 34" from the position that it is when used on the left of the point 0. The sight to be used on the right should be marked R and the sight to be used on the left should be marked L. Then no confusion is possible. Three men could find the range quicker than two. No. 3 goes with No. 2 and uses the target sight, while No. 2 uses the other. When three men are used without the use of the stadia no staff is necessary for the 870 42' 34" instrument, for the sights can then be used as a hand instrument. I ' 5

SECOXD METHOD.

Consist in using a permanent base and measuring the angle at the point A or A'. The instrument to be used at the point 0 to be the same as the one used in the previous method without telescopic sight or level tubes. The tape to be forty yards long. At the end of this tape the second instrument is to be set up. This instrument is similar to a small azimuth instrument. but mounted on a Jacob's staff and provided with the ball and socket movement and level tubes. It should also have a movable center. The sight need not be telescopic, but like the sights described in the first method. It should also be provided with a vernier, vernier clamp and vernier tangent screws. The instrument should be set up reading oo of 0'' and pointing at 0 and the angle T A 0 should be read. This gives the range immediately if a range table has been prepared beforehand. The natural tangent of the angle multiplied by forty is the range. Thus if the angle be 8 7 O 42'34" its natural tangent is 25, and ~ ~ X ~ O = I O O O yards. The sights on this instrument should be very little above the venier plate in order to make the instrument as 0at as possible.