SOUNDS DURING THE NIGHT. 277 



clear sky, but indistinct, and attended by a long-continued 

 roll like thunder, when a cloud covered a considerable 

 part of the horizon. It is no doubt owing to the same 

 cause, namely, the reflection from the clouds, that the 

 thunder rolls through the heavens, as if it were produced 

 by a succession of electric explosions. 



The great audibility of sounds during the night is a 

 phenomenon of considerable interest, and one which had 

 been observed even by the ancients. In crowded cities or 

 in their vicinity, the effect was generally ascribed to the 

 rest of animated beings, while in localities where such 

 an explanation was inapplicable, it was supposed to arise 

 from a favourable direction of the prevailing wind. Baron 

 Humboldt was particularly struck with this phenomenon 

 when he first heard the rushing of the great cataracts of 

 the Orinoco in the plain which surrounds the mission of 

 the Apures. These sounds he regarded as three times 

 louder during the night than during the day. Some 

 authors ascribed this fact to the cessation of the humming 

 of insects, the singing of birds, and the action of the 

 wind on the leaves of the trees, but M. Humboldt justly 

 maintains that this cannot be the cause of it on the 

 Orinoco, where the buzz of insects is much louder in the 

 night than in the day, and where the breeze never rises 

 till after sunset. Hence he was led to ascribe the pheno- 

 menon to the perfect transparency and uniform density 

 of the air, which can exist only at night after the heat of 

 the ground has been uniformly diffused through the 

 atmosphere. When the rays of the sun have been beating 

 on the ground during the day, currents of hot air of 

 different temperatures, and consequently of different 

 densities, are constantly ascending from the ground and 

 mixing with the cold air above. The air thus ceases to 

 be a homogeneous medium, and every person must have 

 observed the effects of it upon objects seen through it 



