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Experimental Apparatus

Reference was made in the earlier pages to experimental methods and measurements, in connection with hysteresis data and velocity determinations. A brief description was also given of the method of timing the illumination for photographing arrows in flight.

Among the items of measuring equipment indispensable to the scientific archer are the balance, for weighing arrows, and a "spine tester," which is actually a stiffness tester. The balance should be capable of precision to 0.1 g (about 2 grains). Almost any laboratory balance of the "triple beam" or "trip scale" type is suitable. Stiffness tests are readily made on a set of shafts by the usual laboratory devices for measuring flexure of beams under load. For measuring the small deflections in these tests I have used screw micrometers, an optical lever and an improvised multiplying indicator. The shaft is placed on supports 24 to 26 in. apart, and its deflection measured when a fixed load of a pound of two is applied at the midpoint.

The ballistic pendulum for measuring arrow velocity is readily constructed and used, and it is ideally suited for the purpose. The bob is made of a wooden box with an opening about 10x10 in. and a depth of 12 in. This is filled with sheets of corrugated board, packed tightly side by side edge on, with the corrugations running in the direction of impact of the arrow. The box is supported in the usual way, with two similar bifilar suspensions, spread at the upper ends to promote stability. Somewhat greater accuracy is obtainable with a high speed spark chronograph.

In velocity testing it is desirable to shoot under reproducible conditions. An aid to this end is a shooting machine, of which various experimenters have designed various kinds. All have the common features of providing reproducibility of length of draw and a mechanical release. The clamp for the bow should be adjustable sidewise, to secure flight of the arrow in the direction of aim. As in hand shooting, the same interval should obtain, in successive shots, between completion of the draw and the instant of release.

High speed photography can still contribute materially to an understanding of some of the problems of "internal ballistics" pertaining to both bow and arrow. The modern speed-flash outfit employing Edgerton lamps immediately comes to mind as a superior aid in such studies. High speed motion pictures at 1000 frames per second should be used to supplement the "stop-motion" speed-flash pictures. This rate is easily attained in several designs of motion picture equipment.