Page:Popular Science Monthly Volume 36.djvu/51

Rh capable of producing such sharp shadows, Dr. Le Conte advances what seems to be the only tenable theory, and one which equally explains the observations of Colladon on the clicking sound of a distant bell as heard in water. In the absence of any recognizable pitch for pitch implies a series of impulses recurring in regular order there is no means of determining wave-length in these cases. But whatever this may be, the wave-length is equal to the product of the time consumed in generating the wave and the velocity of propagation. Thus, assume the initial pitch of a bell to be 220 vibrations per second. We may compute the wavelength either by considering that 220 waves are strung out over a distance of 1,120 feet, making each a trifle more than five feet long, or we may say that the time consumed in generating each wave is of a second, and that this impulse is propagated at the rate of 1,120 feet per second, which would be a little over five feet in of a second. The blow of the hammer on Colladon's bell was almost instantaneous, and the intensity of the first shock thus given to the water was far greater than that of any subsequent shock due to the succession of vibrations set up in the elastic bell-metal. The distance through which this intense sound would be propagated might be expected greatly to exceed that traversed by the subsequent weaker vibrations. The generating blow was so brief that the wave-length could only be short; and hence comparatively well-defined sound-shadows were produced at a distance. In the case of the dynamite explosions under water this reasoning holds with yet greater force. If the duration of the generating impulse be only a millionth of a second, and the velocity of propagation in water be 4,700 feet per second, the resulting wave-length would be only about of an inch. The quickness of action manifested in the explosion of dynamite exceeds that of any other known agent that has ever been similarly employed. The duration of the generating impulse may be considered indefinitely small, certainly immeasurably small. The sharpness of the sound-shadows it produces in water indicates a wave-length that can not exceed a small fraction of an inch.

The production of sharp sound-shadows in air is of more recent date than the experiments in water. In 1880 a dynamite-factory near San Francisco was destroyed by the explosion of its contents. On a large building three miles away many panes of window glass on the side toward the explosion were broken, and two shocks were felt, one conducted by the air and the other by the ground. In the acoustic shadow cast by this building, nearly nine hundred feet away on the side remote from the explosion, no aërial shock was experienced, though that from the ground was distinctly felt. The shortness of the air-wave due to exploding dynamite sufficiently accounts for the sharpness of the shadow.