440 



NATURE 



[March 9, 1893 



ON ELECTRIC SPARK PHOTOGRAPHS; OR, 

 PHO TOGRA PH Y OF FL YING B ULLE TS, &-c. , 

 BY THE LIGHT OF THE ELECTRIC SPARKS 



II. 



GOING back now to the photographs, the next one 

 was taken with the view of illustrating the effect 

 on the incHnation of the waves of the velocity of the 

 bullet. In this case the bullet was aluminium ; it was only 

 one-seventh the weight of the regulation bullet. In con- 

 sequence of its lightness it travelled about half as fast 

 again as the ordinary bullet (not Vr times as fast as it 

 would have done if the pressure of the powder-gases had 

 been the same in the two cases), and in consequence of 

 the higher speed the inclination of the waves is still 

 greater than in the previous case. Further, in this 

 case the bullet was made to pierce a piece of card 

 shortly before it was photographed. The little pieces 

 that were cut out were driven forward at a high speed, 

 but, being lighter than the bullet, they soon lost a large 



only about half as fast as it does in air, and which will 

 not explode or even catch fire when an electric spark is 

 made within it, or directly act injuriously upon the 

 photographic plate. The increased inclination of the 

 waves is very evident in Fig. lo. 



These waves, revealed by photography, have a very 

 important effect on the flight of projectiles. Just as in 

 the case of waves produced by the motion of a ship, 

 which, as is well known, become enormously more ener- 

 getic as the velocity increases, and which at high veloci- 

 ties produce as a matter of fact an effect of resistance to 

 the motion of the ship of far greater importance than the 

 skin friction, so in the case of the air waves produced by 

 bullets ; in its flight the resistance which the bullet meets 

 with increases very rapidly when the velocity is raised 

 beyond the point at which these waves begin to be 

 formed. This being the case, I have thought it might be in- 

 teresting to see whether the analogy between the behav- 

 iour of the two classes of waves might be even nearer 

 than has already appeared, and on turning to the beautiful 



part of their velocity ; they had in consequence lagged 

 behind when they were photographed, but though travelling 

 more slowly (they were still going at more than i loo feet a 

 second) they yet made each its own air wave, which 

 became less and less inclined as the bits lagged more 

 and more behind ; each, moreover, produced its own 

 trail of vortices like that following the bullet. The 

 well-known fact that moving things tend to take the 

 position of greatest resistance, to avoid the effect of 

 which the bullet has to be made to spin, is also illus- 

 trated in the photograph. The little pieces that are large 

 enough to be clearly seen are moving broadside on, and 

 not edgeways, as might be expected. 



In order to illustrate the other fact that the angle of 

 the waves also depends on the velocity of sound in the 

 gas, I filled the box with a mixture of carbonic acid 

 gas, and the vapour of ether, a mixture which is very 

 dense, and through which sound in consequence travels 



1 Lecture delivered at the Edinburgh meeting of the British Association by 

 C. V. Boys, F.R.S. Continued from page 421. 



researches of Mr. Scott Russell, published in the Report 

 of the British Association for the year 1844, in which 

 he gives a very full report on water waves and their 

 properties, I found that he had made experiments and 

 had given a diagram showing what happens when a soli- 

 tary wave meets a vertical wall. The wave, as would be 

 expected, is, under ordinary conditions, reflected perfectly, 

 making an angle of reflection equal to the angle of inci- 

 dence, and the reflected and incident waves are alike in 

 all respects. This continues to be the case as the angle 

 gets more and more nearly equal to a right angle, i.e. 

 until the wave front, nearly perpendicular to the wall, runs 

 along nearly parallel to it. It then at last ceases to be 

 reflected at all. The part of the wave near the wall 

 instead gathers strength, it gets higher, it therefore 

 travels faster, and so causes the wave near the wall to run 

 ahead of its proper position, producing a bend in the wave 

 front, and this goes on until at last the wave near the 

 wall becomes a breaker. 



In order to see if anything similar happens in the case 



NO. 12 19, VOL. 47] 



