218 LETTERS ON NATURAL MAGIC. 



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 explana- 

 tion 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 phenomenon 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 which are very 

 indistinctly visible, and have a tremulous motion, 

 as if they were " dancing in the air." The very 



