176 ELECTRIC-SPARK PHOTOGRAPHS. 



was not sufficient to be noticeable, astbediflterence in the acuteness of 

 the cones certainly is. 



Going back now to the photographs, the next one was taken with 

 the view of illustrating the eftect on the inclination of the waves of 

 the velocity of the bullet. In this case the bullet was aluminum; it 

 was only one-seventh the weight of the regulation bullet. In conse- 

 quence of its lightness it travelled about half as fast again as the ordi- 

 nary bullet (not V- 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 con- 

 sequence 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 lit- 

 tle pieces that were cut out were driven forward at a high speed, but, 

 being lighter than the bullet, they soon lost a large part of their veloc- 

 ity; they had in consequence lagged behind when they were photo- 

 graphed, but though travelling more slowly (they were still going at 

 more than 1,100 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 illustrated in the i^liotograph. 

 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 vapor of ether, a mixture which 

 is very dense, and through which sound in consequence travels 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 inju- 

 riously upon the photographic plate. The increased inclination of the 

 waves is very evident in Plate vi. 



These waves, revealed by photography, have a very important eftect 

 on the flight of projectiles. Just as in the case of waves produced by 

 the motion ot a ship, which, as is well known, become enormously more 

 energetic as the velocity increases, and which at high velocities pro- 

 duce 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 rajiidly 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 interesting to see 

 whether the analogy between the behavior of the two classes of waves 

 might be even nearer than has already appeared, and on turning to the 

 beautiful researches of Mr. Scott Russell, published in the report of the 

 British Associatiou for the year 1844, in which he gives a very full 



