226 



SCIENCE 



[N. S. Vol. LI. No. 1314 



^ Screen B Screen C ScrcetN: .D s>-o^e,, r 



r««,, \ 7? I Type i-i 



'Screen F 

 Type /-/ 



■Screen G 

 Type /-/ 



5CREEN H Screen I 

 T^PE l-l Type HI 



Fig. 2. 



is based on the assiimption that the torque 

 due to the air is proportional to sin 0. Our 

 air stream experiments throw doubt upon this 

 assumption but the English experimenters, 

 who have made the most complete studies of 

 the rotational motions of projectiles that we 

 know of, seem to confirm it. 



These motions of precession and nutation 

 of a projectile can be studied by firing 

 through a number of cardboard screens spaced 

 at equal distances along the line of fire. As 

 has been said, the English have been the fore- 

 most investigators in the work. At Aberdeen, 

 under the immediate supervision of Mr. R. H. 

 Kent, a very extended study, following in 

 general the English method, is being made 

 of the stability of projectiles. Cardboard 

 screens are placed at distances of 20 feet from 

 one another for some distance from the gun, 

 then at 100 feet, then at 20 feet again towards 

 the end of the path. A careful study was 

 made of cardboard so as to obtain a kind 

 which would give a clean cut hole. The 



lantern slide (Fig. 2) shows the variation of 

 the major axis of the hole for eight con- 

 secutive 20-foot screens. 



It wiU be seen from Fig. 2 that the major 

 axis of the hole in screens B and made by 

 the 3.3 inch projectile is about 3.6 inches, and 

 between those screens the angle of the major 

 axis has turned through about 60°. At screens 

 D, E, F, the major axis is about 3.5 inches 

 and it turns rapidly. Here the yaw is a 

 minimum and the rapid motion of the axis is 

 in accord with the theory governing nutation. 



If the projectile were moving in a vacuum 

 or if the air forces did not influence the 

 motion, the precessional velocity ip' (considered 

 imiform) woidd be given by 



AN 



B(l +.C0S ( 



AN 

 2B ■ 



For the projectile in question iV = 

 per second. 



220 turns 



1 = 1/6. 



Hence </>'= 220/12 = 18.3 turns per second. 



