The Machine-Gun Clinometer, M1917 (fig. 20)

a. Description- This instrument is used to lay the machine gun in elevation, or when the gun is laid, for measuring its angle of quadrant elevation. The machine-gun clinometer, complete, consists of the clinometer and its carrying case.

The clinometer itself consists of a sector-shaped frame to which is pivoted a radial arm which carries the level holder. The lower and rear edges of the frame are accurately machined so as to form a right angle. The sector-shaped or arc portion is notched and graduated every 20 mils, 840 mils above and 160 mils below the zero or normal graduations. Similar graduations, are on each side of the frame. One end of the radial arm is pivoted to the frame, opposite the arc, while the other is fitted with a plunger carrying an index, which engages the notches on the arc and which is held against the are by a spring contained within the radial arm. Both sides of the radial arm are graduated into 20 equal spaces and numbered every 5 spaces from zero to 20. Each space represents an angle of 1 mil when used in conjunction with the scales on the frame. The level holder slides along the curved top surface of the radial arm, carries the index marks used in conjunction with the scales on the sides of the radial arm, and can be clamped in position by the thumbscrew.

b. To lay the gun at a given elevation- Push the plunger toward the pivot; set the index line to the next lowest 20-mil reading on the main scale and set the index on the level holder opposite the graduation on the radial arm so that the sum of the two settings equals the elevation desired. Place the base of the clinometer on the top of the receiver, to the right of and alongside the latch, with the are to the rear if the desired elevation is plus, and with the are to the front if the desired elevation is minus. Operate the elevation mechanism of the gun until the level bubble is midway between the lines on the level vial.

c. To measure the angle of quadrant elevation- The gun having been laid, place the clinometer on the receiver as described above and move: the radial arm and level holder until the bubble is midway between the lines on the vial. The sum of the readings indicated by the indexes on the level holder and plunger is the angle of quadrant elevation at which the gun is laid.

d. To test the clinometer- To find the error of the clinometer, place the clinometer on any straight surface which is not inclined more than 40 mils from the horizontal. Set the radial arm and the level holder so that the bubble is centered and note the reading. Assume a, reading of 6 mils below zero (plunger, index 20 below zero and level holder, index at 14). Having marked carefully the position of the clinometer, reverse it end for end, placing it on the same part of the surface. Again set the radial arm and level holder so that the bubble is centered. Assume that the clinometer now reads 2 mils above zero. Then the mean (half the algebraic difference) is plus or minus 4 mils; that is, the surface on which the clinometer was placed is inclined at an angle of 4 mils elevation in one direction and 4 mils depression in the other. The clinometer reads 6 mils below zero when it should read 4 below, and 2 above when it should read 4 above, so that the clinometer has an error of minus 2 mils. To put any given quadrant angle of elevation on the gun, the clinometer should be set so that the graduations read 2 mils less, numerically, than the given quadrant elevation whether that quadrant angle of elevation is actually elevation or depression; that is, to put 22 mils elevation (plus 22 mils) on the gun the clinometer should be set at plus 20 and placed with the are to the rear, and to put 22 mils depression (minus 22 mils) on the gun it should be set at 20 and placed with the arc to the front. On the other hand, if the clinometer has an error of plus 2 mils, the clinometer should be set so that the graduations read 2 mils more, numerically, than the given quadrant angle of elevation whether that quadrant angle of elevation is actually elevation or depression; that is, to put 22 mils elevation on the gun, set the clinometer at 24 and place it on the gun with the arc to the rear, and to put 22 mils depression on the gun, set the clinometer at 24 and place it on the gun with the arc to the front. To level the gun, the clinometer should be set at 2 mils below zero.

e. Care and preservation- (1) Special care should be exercised to prevent burring, nicking, or denting the toothed portion of the are, the curved top surface of the radial arm, or the smooth surfaces of the bases on the frame. (2) The clinometer should always be removed from the gun before firing.

Above: A photo from an Army training manual showing a clinometer in use.
Right: A training manual illustration.
Source: Craig Johnson

A machine gun crew in training. A clinometer can be seen on the belt of the soldier at the far right.
Source: National Archives via Craig Johnson

Sources: The text was taken verbatim from the War Department Field Manual FM 23-55, Basic Field Manual, Browning Machine Gun, United States Government Printing Office, Washington, 1940.

The photo from the field manual from which the above article was taken was was poor, so I replaced it with my own clinometer. The original photo is at right.