442 



NA TURE 



[March 9, 1893 



now attracting so much interest in connection with the 

 .propagation of electrical waves. 



On returning to the non-reflection of the air wave in 

 the upper part of the figure we have here, I imagine, 

 optical evidence of what goes on in a whispering gallery. 

 The sound is probably not reflected at all, but runs 

 round almost on the surface of the wall from one part to 

 another. 



We are now in a position to see how the reflection or 

 non-reflection of air waves produced by a passing bullet, 

 when they meet with some solid body, may produce a 

 practical result which might be of importance in some 

 cases. Suppose a bullet to be passing near and parallel 

 to a wall. Then if the velocity of the bullet and its 

 distance from the wall are such that the head wave meets 

 the wall at an angle at which it can be reflected, especially, 

 as in the case of Fig. ii, if the reflected ray can only 

 return into the path of the bullet after it has gone, then 

 no influence whatever can be exerted upon the bullet 

 by its proximity to the wall. If, however, the head wave 

 would, if undisturbed, meet the wall at such an angle 



bullet has left the muzzle the imprisoned powder gases, 

 under enormous pressure, rush out, making a draught 

 past the bullet of the most tremendous intensity tending 

 obviously to drive it forward. While this draught does 

 most assuredly hurry the bullet on its forward course, it 

 does not tend to make it spin round any faster. Now if 

 the bullet were not hurried on at all after it left the 

 muzzle it would, travelling as in a screw of the same pitch 

 all the way from the breach of the rifle up to the point 

 at which it is photographed, have turned round a certain 

 number of times which depend upon the distance travelled 

 and the pitch of the screw. If, however, the longitudinal 

 motion is hurried and the rotational is left unaltered the 

 pitch will be lengthened outside the barrel and the 

 rotation will have been less for any position than it 

 would have been if the bullet had not been accelerated 

 in this way. If, therefore, we can find to what extent 

 the bullet has turned actually at the place at which 

 it has been photographed, we can find the apparent 

 rotational lag and so working backwards get a measure 

 of the velocity acquired after leaving the muzzle. In 



that it could not be reflected, as for instance, in Fig. 12, 

 when the head wave can be reflected by neither of the 

 walls between which the bullet is passing, obviously the 

 wave will become stronger and the resistance which it 

 ofl"ers will, I imagine,becomegreater,andif in this case the 

 upper plate be removed this extra resistance will be one- 

 sided and must tend to deflect the bullet. This is quite 

 distinct from the well-known effect of a bayonet upon 

 the path of a bullet ; when a bayonet is fixed the rush 

 of powder gases between the bullet and the bayonet is 

 quite sufficient to account for the deflection which every 

 practised marksman allows for. 



I have devised a method by which a problem of some 

 difficulty, about which authorities are, I believe, by no 

 means in accord, may be solved with a fair degree of 

 certainty. The problem is this, to find what proportion 

 of the velocity of a bullet is given to it after it has left 

 the barrel, or, what comes to the same thing, to find the 

 position in front of the barrel at which the speed is a 

 ^maximum. The cause of this is evident. When the 



NO. 



I 2 19, VOL, 47] 



I order to accomplish this I drilled a series of holes 



! transversely through the bullet, each one at an angle to 



j the previous one, the whole series being such that to 



[ whatever extent the bullet had twisted, one at least, and 



perhaps two, would allow the light of the spark to shine 



I through it upon the photographic plate. Then from the 



I photograph it is easy to see through which hole the light 



shone, and knowing in what position this was in the 



breach, it is easy to find what fraction of half a turn 



over or above any whole number of half turns the bullet 



has twisted. Strictly the measure should be made at 



different distances to eliminate all uncertainty, but the 



only shot I have taken was sufficient to show that there 



was a rotational lag equivalent, according to the measure 



made by Mr. Barton, to something under a two per 



cent, acceleration outside the barrel. I do not attach 



any importance to this figure ; the experiment was made 



with a view to see if the method was practicable and this 



it certainly is. I would recommend, where accuracy is 



i required, that having found as above about how much the 



