SENSITIVE FLAMES AND SOUND-SHADOWS. 17 



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 recogniz- 

 able 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 wave- 

 length 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 ^|-g- 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 ^^r 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 subse- 

 quent shock due to the succession of vibrations set up in the elas- 

 tic 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 ex- 

 ceeds that of any other known agent that has ever been similarly 

 employed. The duration of the generating impulse may be con- 

 sidered 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 

 aerial 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. 



