70 



propagation of the sonorous undulations. A 

 shock, given to the surface of a liquid, will form 

 circles around the centre of percussion, even 

 when the liquid is agitated. Several kinds of 

 undulations may cross each other in water, as in 

 air, without being disturbed in their propaga- 

 tion ; little movements may ride over each other, 

 and the real cause of the less intensity of sound 

 during the day appears to be the interruption of 

 homogeneity in the elastic medium. During the 

 day, there is a sudden interruption of density, 

 wherever small streamlets of air of a high tem- 

 perature rise over parts of the soil unequally 

 heated. The sonorous undulations are divided, 

 as the rays of light are refracted, and form the 

 mirage (looming), wherever strata of air of un- 

 equal density are contiguous. The propagation 

 of sound is altered, when a stratum of hydrogen 

 gas is made to rise in a tube closed at one end 

 above a stratum of atmospheric air ; and Mr. 

 Biot has well explained by the interposition of 

 bubbles of carbonic acid gas, why a glass filled 

 with Champagne wine is little sonorous so long as 

 the gas is evolved, and continues to pass through 

 the strata of the liquid. 



In announcing these ideas, I might almost 

 rest on the authority of an ancient philosopher, 

 whom the moderns continue to treat with dis- 

 dain, though the most distinguished zoologists 

 have long rendered ample justice to the saga- 



