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 necessary 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. Ryves: "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 during 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 were 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 'Ryves 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 explosion 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