ELECTRIC-SPARK PHOTOGRAPHS. 173 



seen by the eye. The box seen just to the right of the ritle with a coil 

 of wire upon it is the one in wlueh the revolving mirror was fixed, and 

 in which the trails of sparks made near the door at the end of the 

 passage were photographed. The apparatus for photographing the 

 bullets was put together and set up by Mr. Barton, a student, whose 

 very skillful help in the matter and afterwards during the experiments 

 I found of very great value. 



The first photograph which I am able to shov\^ was taken at Christmas, 

 before the apparatus just described was put together. It was taken to 

 see if the idea would practically succeed, merely using for the purpose 

 bits of wire and other things to be found in any laboratory, which were 

 set up in a dark room in less than au hour. The first shot was suc- 

 cessful, but the sharpness of the photograph is not what it might be, 

 owing to the fact that I used, for the sake of the brilliant light, a spark 

 taken between magnesium terminals. However, the bullet is clearly 

 enough defined, as are the wires which it has just struck. This is a 

 photograjih of a pistol ballet traveling only 750 feet a second. You 

 will notice that unlike that taken by Prof. Mach, which represented a 

 shot going at a much higher speed, this photograph shows no atmos- 

 pheric phenomena surrounding" the bullet. I would only add, in con- 

 nection with this photograph, that by some accident the wad remained 

 attached to the bullet in this case forming the enlarged tail. I do not 

 know if this often happens; it must, if it does, seriously disturb the 

 flight of the projectile, and introduce an anomaly that might not easily 

 be accounted for. 



The next photograph, PL iv, fig. 1, shows a bullet which has just 

 left a Martini-Henry rifie. This is taken with the apparatus in its latest 

 form, and the bullet appears perfectly sharp. There is no sign of any 

 movement whatever in so far as the bullet itself is concerned. But 

 now that we are dealing with a higher speed, namely, 1,295 feet a sec- 

 ond, there is evidence of the movement of the bullet in the form of a 

 wave of compressed air in front and of other waves at the side of and 

 behind the bullet. I shall explain this in a moment, but I would rather 

 first show another photograph, PI. iv, tig. 2, of a bullet travelling at a 

 still higher speed, a inagazine-rifle bullet travelling about 2,000 feet a 

 second, in which these air waves are still more conspicuous, and in 

 which a glance is sufficient to make it evident that the waves are much 

 more inclined to the vertical than in the previous case. 



Now, as it may not be evident why these waves of air are formed, 

 why their inclination varies with the speed, or why existing- they are vis- 

 ible at all, a short explanation may not be out of place, more especially 

 as they form a most interesting feature in the remaining photographs 

 that I have to exhibit, which can not, as a matter of fact, be properly 

 interpreted without frequent reference to them. 



I would first ask you to examine some still water into which a needle 

 lield vertically is allowed to dip. If you move the ueeuie very slowly 



