69 



plied to the forests of the Oroonoko, where the 

 air is constantly filled by an innumerable quan- 

 tity of moschettoes, where the hum of insects is 

 much louder by night than by day, and where 

 the breeze, if ever it be felt, blows only after 

 sunset. 



1 rather think, that the presence of the sun 

 acts upon the propagation and intensity of the 

 sound by the obstacles, which they find in the 

 currents of air of different density, and the par- 

 tial undulations of the atmosphere caused by 

 the unequal heating of different parts of the 

 soil. In calm air, whether it be dry, or mingled 

 with vesicular vapours equally distributed, the 

 .sonorous undulation is propagated without diffi- 

 culty. But when the air is crossed in every 

 direction by small currents of hotter air, the so- 

 norous undulation is divided into two undula- 

 tions, 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 undu- 

 lations take place, of which Mr. Poisson has 

 recently developed the theory with great saga- 

 city *. It is not therefore the movement of the 

 particles of air from below to above in the as- 

 cending current, or the small oblique currents, 

 that we consider as opposing by a shock Ihe 



* Annates deChimic, torn, vii, p. 293. 



