362 Mr.Bashforth on the Resistance of the Air to Projectiles* [Feb. 20^ 



and 1 



"=1 



All the hollow shot were fired, giving eighteen out of twenty successful 

 shots. Only a part of the solid shot prepared were fired ; and they did not 

 give nearly such good results as the hollow shot, probably in consequence 

 of the superior angular velocity of the hollow shot — because, as a 5-lb. 

 charge was used throughout, the lighter shot had a higher initial velocity, 

 and consequently a higher corresponding angular velocity. 



Tables are given showing for every round : — (1) the experimental deter- 

 mination of the time of passing each screen, supposing the first screen to be 

 passed when ^=0 ; (2) the velocities at the middle points between suc- 

 cessive screens ; (3) the weights of the shot ; and (4) the numerical values 

 of bP, where Z= 150 feet, the distance between the screens. And assuming 

 that, for a given form of head, the resistance of the air varies as the 

 square of the diameter, the mean values of 2h have been adopted for shot 

 weighing W lbs., and having a diameter of d inches, or 2E) feet. 



When a body is moving in a straight line under the action of a force 

 which varies as the cube of the velocity, it appears that the actual velocity 



at the middle of any space 2*' is such that, if another body moved over 

 the same space 2s' with a uniform velocity v\ it would describe it in the 

 same time as the first-named body. For the time t' would 



= l2.'4-6(2*'n 



uniform velocity 



25' 25' 1 



' 4-25' + 5(2.')^ 1+265' 



the actual velocity at the distance «'. 



M. Helie, in his 'Traite de Balistique' (1865), adopted, for elongated 

 projectiles, a law for the resistance of the air which varied as the velocity 

 cubed. The law was deduced from some experiments made at Gavre, when 

 a great number of velocities (v\ v") of shot fired with various charges were 

 measured at two points x metres apart. The mean values of ?;' and 



were taken and substituted in the formula ; and it was found that 



this was approximately constant, and consequently that the resistance varied 

 as the (velocity)^. The French measures and weights have been converted 

 into English measures for M. Helie's best experiment, in order to facilitate 

 comparisons with my own experiments. The contents of M. Hebe's work 

 were quite unknown to me for several months after my report on the above 

 experiments had been given in. For an ogival-headed shot struck with a 

 radius of two diam.eters M. Helie's value of 2b is 



26=-000036~ =-000000062£, 



w w 



