Sound-Shadoics in Water. 107 



sounds. Does this principle apply to sound-shadows in 

 water ? 



Some physicists have attempted to explain the phenomenon 

 of the great distinctness of sound-shadows in water, as in- 

 dicated by the observations of Colladon (8), by assuming that 

 the lengths of the sonorous waves propagated through water 

 are much shorter than those transmitted through air*. But no 

 reason is given for this fundamental assumption, other than 

 that it is required by the demands of the theory of undulations, 

 in order to account for the more perfect shadows in water. 

 It evidently would be vastly more philosophical to establish as 

 a matter of fact the greater shortness of the sound-waves in 

 water, and thus to verify the deductions of theory. This we 

 shall endeavour to accomplish. 



16. Measurement of TTave-Ienaths. — With regard to con- 

 tinuous or musical sounds, we have the means of very 

 readily determining the wave-length ; for it is equal to 



-^ -, — • „^ : . It is evident, therefore, that, the 



A umber ot \ lbrations 



number of vibrations or musical pitch of the sonorous body 

 remaining the same, the wave-length in water, so far from 

 being shorter, must be more than four times as long as they 

 are in air. Hence, according to theory, if Colladon's obser- 

 vations had been within the radius (200 metres) at which the 

 musical tone of the sonorous bell was heard, the sound-shadows 

 would have been less distinct than in air. Unfortunately, 

 Colladon does not inform us at what distance from the 

 vibrating bell his observations in relation to the acoustical 

 shadows were made ; so that it is impossible to apply this 

 critical test of the theory of shadows. But the presumption 

 is, that the observations were made in the neighbourhood of 

 Thonon (while the sonorous bell was placed at Eolle), at a 

 distance of 13,487 metres from the source of sound, these 

 being the arrangements during the execution of the experi- 

 ments for determining the velocity of sound in the waters of 

 the Lake of Greneva. At all distances from the bell greater 

 than 200 metres, as we have seen (9), the sound lost its 

 musical character and became short and sharp, like two knife- 

 blades struck together. Hence, under the assumption that the 

 observations on sound-shadows were made very far beyond 

 the limits at which musical tones were transmitted, we are 

 precluded from determining the wave-length by the number 

 of vibrations. It appears that in water grave sounds are more 

 rapidly suppressed or damped than acute sounds ; so that at 



* Vide W. H. C. Bartlett's •' Elements of Xatural Philosophy,' "Acoustics 

 and Optics, 1 ' 4th ed., N. Y. 1866, p. 7-5, ait. 66. 



