106 



Mr. J. LeConte on 

 Fiff. 4. 



STILL WATER SURFACE 



15 lis. G.P. 



SAND-STONE REEF 



mitted by the water — and before the ascending gases of 

 explosion disturbed it — the surface of the liquid exhibited 

 numerous jets of water, rising to the height of about 3 inches 

 over the centre of the area, and diminishing in height with 

 augmenting distance from the centre. The appearance pre- 

 sented was not unlike that produced by a heavy shower of 

 rain falling on the calm waters of a lake (fig. 4). To an 

 observer in a boat floating on the adjacent water, and con- 

 sequently viewing the phenomenon from a point near the 

 water-level, there seemed to be a curious quincunx-like arrange- 

 ment of the jets. 



Explanation of the Phenomena obsekved. 



15. Greater Distinctness of Sound-Shadows in Water. — The 

 much greater distinctness of acoustical shadows in water, as com- 

 pared with those in air, appears to be pretty clearly established 

 by the experiments of Colladon; and the fact seems to be abun- 

 dantly confirmed by those which we have recorded in the prece- 

 ding pages. This is an interesting and significant phenomenon 

 in relation to the theory of sound. At first sight it might be 

 supposed that the difference is due to the greater velocity of 

 propagation of the sound-wave in water. This may have 

 been Sir John Herschel's idea when he explained Colladon's 

 results by reference to the greater elasticity of water*. 



But, as already indicated (3), according to the mathe- 

 matical theory of undulations, the intensity of the effects due 

 to the secondary waves progagated into the geometrical 

 shadow from the borders of the obstacle is not directly de- 

 pendent upon the velocity of propagation, but is properly a 

 function of the wave-length, the diffractive divergence being 

 less for short than for long waves. Hence it follows that the 

 distinctness of sound -shadows, like those of light, should 

 depend upon the shortness of the wave-length. We have 

 already seen (5) that experiment verifies this prevision of 

 theory in the case of sound-waves in air, by demonstrating 

 that acute sounds cast more distinct shadows than grave 

 * " Treatise on Sound," Encyc. Metrop., art. 102. 



