March 9, 1893J 



NATURE 



445 



Fig. i6.> 



along the air wave it is alternately dark outside and light 

 outsid e. These indicate the successive positions in which 



the glass fust moved outwards to compress the air or first 

 NO. I 2 19, VOL. 47] 



moved inwards to rarefy it so that the wave length of the 

 ripple may thus be found, and finally it is seen that where 

 the waves are waves of compression on one side of the 

 plate they are waves to rarefaction on the other, 

 indicating that it was a transverse and not a mere 

 longitudinal disturbance that ran along the plate from 

 the centre outwards and back again after reflection from 

 the edge. In addition to this the fact that the reflected 

 wave is still on its inward course proves that up to this 

 time the. plate is whole, as a wave cannot be propa- 

 ;^'^ated. in a broken plate. Fig. i6 illustrates the state 

 of affairs when the bullet has travelled about five inches 

 beyond the plate. It has not yet emerged from the cloud of 

 glass dust. The new head wave is very conspicuous. In 

 the original negative, about half way between the bullet 

 and the plate, the inclined waves due to the tremor in the 

 glass plate may be detected, but they are too delicate to be 

 reproduced by the printing process. They supply the 

 information as to how long the plate remained whole or 

 rather if the bullet had been caught a little sooner before 

 these faint waves had lost so much of their distinctness 

 they would supply this information with great exactness. 

 Meanwhile the figure shows that the plate is now broken 

 up completely. It is true it is still standing, and the stem 

 air wave is seen reflected from the upper part of it, but 

 this is because the different parts have not yet had 

 time to get away ; their grinding edges, however, have cast 

 out from the surface little particles, and these are seen 

 over the whole extent of the plate. After about fifteen 

 inchesthebulletis quite clear ofthe cloud of dust (Fig. 17) ; 

 one piece only of the glass, no doubt the piece that was 

 immediately struck, has been punched out and is travel- 

 ling along above the bullet at a speed practically equal to 

 its own. I am also able to show the plate itself in this 

 and a still later stage, when at last the separate pieces 

 have begun to be visibly moved out of their position and 

 in some cases slightly turned round. 



I have merely given this evening an account of a few 

 experiments which in themselves perhaps are of little 



