PHOTOGRAPHY OF SOUND WAVES. 355 



less than the speed of a modern rifle ball, yet ten times faster than the 

 fastest express train. The wave, even if it were stationary, could be seen 

 only by adjusting the illumination with far greater care than was neces- 

 sary in the case of the hot air, and we consequently can easily understand 

 why we never see the waves under ordinary conditions. 



While it is true that laboratory appliances are generally required to 

 render them visible, I should like at the outset to cite an example to 

 show that in the case of very loud sounds occurring in the open air 

 the wave can be perceived by the eye, without the aid of any 

 apparatus whatever. I will quote from an article by Prof. C. V. 

 Boys, which appeared in 'Nature,' June 24, 1897. Mr. Boys first 

 cites the following letter from Mr. E. J. Eyves: "On Tuesday, 

 April 6th, I had occasion, while carrying out some experiments with 

 explosives, to detonate one hundred pounds of a nitro-compound. The 

 explosive was placed on the ground in the center of a slight depression, 

 and in order to view the effect, I stationed myself, at a distance of about 

 three hundred yards, on the side of a neighboring hill. The detonation 

 was complete, and a hole was made in the ground five feet deep and 

 -even feet in diameter. A most interesting observation was made dur- 

 ing the experiment. The sun was shining brightly, and at the moment 

 of detonation the shadow of the sound wave was most distinctly seen 

 leaving the area of disturbance. I heard the explosion as the shadow 

 passed me, and I could follow it distinctly in its course down the 

 valley for at least half a mile; it was so plainly visible that I believe 

 it would photograph well with a suitable shutter." 



Professor Boys at once made preparations for photographing the 

 phenomenon at the first opportunity. On May 19th the experiment was 

 made. One hundred and twenty pounds of a nitro-compound were 

 exploded, and an attempt made to photograph the sound shadow, both 

 with the camera and the kinematograph, the latter instrument designed 

 and operated by Mr. Paul. Writing of the experiment, Professor Boys 

 says: "On the day on which I was present, about one hundred and 

 twenty pounds of a nitro-compound were detonated, and ten pounds of 

 black powder Avere added to make sufficient smoke to show on the plate. 

 As the growth of the smoke cloud is far less rapid than the expansion 

 of the sound shadow, no confusion could result from this. At the 

 time of the explosion my whole attention was concentrated upon the 

 camera, and for the moment I had forgotten to look for the 'Eyves 

 ring/ as I think it might be called; but it was so conspicuous that it 

 forced itself upon my attention. I felt, rather than heard, the explo- 

 sion at the moment that it passed. We stationed ourselves as near as 

 prudence would allow, at a distance of one hundred and twenty yards, 

 so that only about one third of a second elapsed between the detonation 

 and the passage of the shadow. The actual appearance of the ring 



