444 



NA TURE 



[March 9, 1893 



piercing of a glass plate by a bullet from the first shock 

 step by step, until the bullet had at last emerged from 



a photographic print and even.but less clearly, of the print 

 in the text shows that these inclined air waves are made 



the confusion it had created. In Fig. 14 the glass plate 

 is seen edgeways just after the bullet has struclc it. It is 

 clear at once that the splash of glass dust backwards is 

 already four or five times as rapid as the motion of the 

 bullet forwards. A new air wave is just beginning to be 

 created in front of the glass-coated head of the bullet 

 and two highly-inclined waves, one on either side of the 

 glass, reaching about three-quarters of the way to the 

 edge, have sprung into existence. These are more 

 •clearly seen in the next figure; meanwhile it may be well 

 to point out that the fragments of paper which are follow- 

 ing the bullets have in this case — as the card was much 

 nearer to the glass plate than in those previously taken — 

 some of them lost so much of their velocity and have in 

 consequence lagged behind in a still higher proportion than 

 the others, that they are travelling at less than iioo feet 

 a second ; the more backward ones carry in consequence 

 no air waves and there is no means of teUing from 

 the photograph that they are moving at all. In Fig. 15 

 the Ijullet has struggled about half way through the 

 plate. The waves on either side of the plate have now 

 reached the edge and are on their way back to- 

 wards the centre again- They are caused in this 

 way. When the bullet strikes the plate the violent shock 

 produces a ripple or tremor in the glass which travels 

 away radially in all directions, leaving the glass quiet 

 behind. The rate at which this ripple travels may be 

 found from the angle which these new air waves make 

 with the plate, for taking any point on the plate and 

 measuring up to the point where the air wave meets the 

 plate and also the distance in air to the nearest point of 

 the inclined air wave, we get two distances, the ratio of 

 which is the ratio of the velocity of the disturbance in the 

 glass to the velocity of sound in air. But much more 

 than this is shown. An examination of the negatives or of 



NO. 1219, VOL. 47] 



up of a series of dark and light lines at a very slight 



Fu;. 15. 



inclination to the air wave itself, so that as we travel 



