SENSITIVE FLAMES AND SOUND-SHADOWS. 39 



The secondary waves, which are started at the upper edges of the 

 cutting, reach the ear and give still a good idea of the character 

 of the noise and position of the train. The range of the ear greatly 

 exceeds that of the eye, not only in relation to the variety of wave- 

 lengths by which it may be impressed, but yet more as to varia- 

 tions of intensity. Just as sunshine and shadow during the day 

 indicate merely variations in illumination without the complete 

 extinction of light, so noise and sound-shadow are merely relative 

 terms, the latter not necessarily implying the complete extinction 

 of sound ; for in air diffraction usually plays so important a part 

 as to forbid complete extinction, and to prevent all sharpness of 

 definition at the edges of the shadow. 



When the medium is water instead of air, some new phenomena 

 are noticeable. In 1826 Daniel Colladon's classic experiments on 

 the velocity of sound in water were performed on the Lake of Ge- 

 neva. The source of sound was a large bell, from which vibra- 

 tions were conducted through the water several miles away to an 

 elastic membrane stretched across the expanded opening of a par- 

 tially submerged hearing-trumpet. They were thus given to the 

 air within the trumpet and conveyed to an ear applied at its smaller 

 end above the water. A bell when struck sends forth a vari- 

 ety of tones, and it is often hard to determine which of these is 

 most prominent. Usually that of deepest pitch is the slowest to 

 die away in air, and often it penetrates to the greatest distance. 

 Colladon made the remarkable observation that in water the low- 

 er tones are conducted off to but a short distance before their 

 energy ceases to produce the sensation of sound ; while the initial 

 stroke is propagated much further, and is then perceived as a 

 short, sharp, almost clicking sound, without definite musical char- 

 acter. Placing the hearing-trumpet behind a wall which project- 

 ed out into the water, the decrease of intensity was much greater 

 than under similar conditions in air, and the demarkation of the 

 region of shadow was decidedly more noticeable. 



Still more interesting than the experiments of Colladon were 

 those made in the Bay of San Francisco in 1874 by Prof. John 

 Le Conte and his son, Mr. Julian Le Conte. The source of sound 

 was not such as would give a definite pitch, like a bell, but the 

 quick, violent, single impulse due to the explosion of dynamite 

 employed in the blasting of rocks which obstructed the channels. 

 The intensity of the shock thus propagated was such as to be felt 

 as a blow on the feet of a person seated in a boat three hundred 

 feet or more from the detonating cartridge, and to kill hundreds 

 of fish. Several vertical posts or piles, each about a foot in diame- 

 ter, projected from the ground out of the water in the neighbor- 

 hood. A stout glass bottle was suspended in the water about a 

 foot in the rear of one of these piles (Fig. 1), within the geomet- 



