OBSTACLES TO THE SOUND-WAVES. 
265 
of different parts of the soil. In calm air, whether dry or 
singled with vesicular vapours equally distributed, sound- 
waves are propagated without difficulty. But when the air 
18 crossed in every direction by small currents of hotter 
a ir, 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 
hquid will form circles around the centre of percussion, 
e ven when the liquid is agitated. Several kinds of undu- 
* a tions may cross each other in water, as in air, without 
being disturbed in their propagation : little movements may, 
ss 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. During 
"be day there is a sudden interruption of density wherever 
Sttiall streamlets of air of a high temperature rise over parts 
the soil unequally heated. The sonorous undulations are 
c byided, as the rays of light are refracted and form the 
Jbirage wherever strata of air of unequal density are eon- 
^guous. The propagation of sound is altered when a 
a tratu In of hydrogen gas is made to rise in a tube closed 
?F. °ne end above a stratum of atmospheric air ; and M. 
° l °t has well explained, by the interposition of bubbles of 
° ar bonic acid gas, why a glass filled with champagne is not 
Sonorous so long as that gas is evolved, and passing through 
^ 1 strata of th! liquid. 
kn support of these ideas, I might almost rest on the 
Othority of an ancient philosopher, whom the moderns do 
e steem m proportion to his merits, though the most dis- 
biguished zoologists have long rendered ample justice to the 
opacity of his observations. “Why,” says Aristotle in his 
“brious book of Problems, “why is sound better heard 
* Anuaiss de Chimie, tom. vii, p. 2#*- 
