OBSTACLES TO THE SOUND-WAYE8. 2G5 



of different parts of the soil. In calm air, whether dry or 

 mingled with vesicular vapours equally distributed, sound- 

 waves are propagated without difficulty. But when the air 

 is crossed in every direction by small currents of hotter 

 air, the sonorous undulation is divided into two undulations 

 where the density of the medium changes abruptly ; partial 

 echoes are formed that weaken the sound, because one of 

 the streams comes back upon itself; and those divisions of 

 undulations take place of which M. Poisson has developed 

 the theory with great sagacity.* It is not therefore the 

 movement of the particles of air from below to above in the 

 ascending current, or the small oblique currents that we 

 consider as opposing by a shock the 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 undu- 

 lations may cross each other in water, as in air, without 

 being disturbed in their propagation : little movements may, 

 as it were, 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. Durijig 

 the day there is a sudden interruption of density wherever 

 small streamlets of air of a high temperature rise over parts 

 of the soil unequally heated. The sonorous undulations are 

 divided, as the rays of light are refracted and form the 

 mirage wherever strata of air of unequal density are con- 

 tiguous. 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 M. 

 Biot has well explained, by the interposition of bubbles of 

 carbonic acid gas, why a glass filled with champagne is not 

 sonorous so long as that gas is evolved, and passing through 

 the strata of the liquid. 



In support of these ideas, I might almost rest on the 

 authority of an ancient philosopher, whom the moderns do 

 not esteem in proportion to his merits, though the most dis- 

 tinguished zoologists have long rendered ample justice to the 

 Bugacity of his observations. " Why," says Aristotle in his 

 curious book of Problems, "why is sound better heard 



* AnuaJas de Chimie, torn, vii, p. 293. 



