242 HABITS OF BIRDS. 



greater. There is another circumstance which evinces 

 that birds have a prodigious power of voice ; the 

 cries of many species are uttered in the higher regions 

 of the atmosphere, where the rarity of the medium 

 must consequently weaken the effect. That the rare- 

 faction of the air diminishes sounds is well ascertained 

 from pneumatical experiments ; and I can add from 

 my own observation, that, even in the open air, a 

 sensible difference in this respect may be perceived. 

 I have often spent whole davs in the forests, where I 

 was obliged to listen closely to the cries of the ddgs 

 df shouts of the hunters ; I uniformly found that 

 the same noises Were much less audible during the 

 heat of the day; between ten and four o'clock, than 

 in the evening, and particularly in the night, whose 

 stillness would make hardly any alteration, since in 

 these sequestered scenes there is nothing to disturb 

 the harmony but the slight buzz of insects, and the 

 chirping of some birds. I have observed a similar 

 difference between the frosty days in winter and the 

 heats of slimmer. This can be imputed only to the 

 variation in the density of the air. Indeed, the dif- 

 ference seems to be so great, that I have often been 

 unable to distinguish, in mid-day, at the distance of 

 six hundred paces, the same voice which I could, at six 

 o'clock in the morning or evening, hear at that of twelve 

 or fifteen hundred paces. A bird may rise at least to 

 the height of seventeen thousand feet, for it is there 

 just visible. A flock of several hundred storks, 

 geese, or ducks, must mount still higher, since, not- 

 withstanding the space which they occupy, they soar 

 almost out of sight. If the cry of birds, therefore, 

 may be heard from an altitude of above a league, we 

 may reckon it at least four times as powerful as that 

 of men or quadrupeds, which is not audible at more 

 than half a league's distance on the surface. But 

 this estimation is even too low ; for, besides the dis- 



