126 CHAPTER OF VARIETIES. 



swept past the hives, seize upon the bees, and, as it seemed, with 

 difficulty devour them. I instantly recollected what Virgil has said 

 of the swallow, as well as other birds. 



" Omnia nam late vastant, ipsasque volenles 

 Ore ferunt dulcem nidis immitibus escam." 



I took my gun, and marking those swallows, which had evidently 

 seized something in passing the hives, I shot them ; I then opened 

 them carefully, and was grieved to find that, though they were 

 literally crammed with drones, there was not a vestige of a working 

 bee. Hence I concluded that swallows prey only on the drones, and 

 are consequently no enemies to bees. R. D. 



BLUE COLOUR OF THE SKY. If I might hazard my opinion of the 

 cause of this appearance, it would be the following : As the atmos- 

 phere extends upwards, its density becomes gradually less and less, 

 and of course, its power of reflecting the sun's rays in like proportion 

 diminishes, till at last, at the extreme verge, where it terminates, 

 there is no reflection at all, or total darkness. The extreme strata 

 then being most rarefied, have the least powers of reflecting the rays 

 of light, and the light thus reflected is of a bluish tint, or consists 

 principally of the blue rays. 



In this manner, a dark-brown mountain, whose surface has small 

 reflective capabilities, when seen at a distance, has a deep-blue ap- 

 pearance, exactly similar to the atmosphere. It cannot be the 

 medium of the air, through which it is seen, that renders it of this 

 colour ; for if part of the mountain be covered with snow, which 

 has strong reflective powers, this snow is still seen of a pure white 

 colour. 



It has been ascertained, too, that the atmosphere, when seen from 

 the top of a very high mountain, has a deep-blue tint, approaching 

 to black, and this tint becomes deeper the higher you ascend. 



It may be observed, also, that the centre of the atmosphere, 

 looking perpendicularly upwards, always appears of a deep-blue 

 colour, which gradually passes to a whiter appearance towards the 

 extreme verge of the horizon, or in the lower strata next the earth. 

 Here most dense air is accumulated, and here the reflection is most 

 perfect, or nearly approaching to white light j whereas perpen- 

 dicularly overhead, the rays of light pass through less of this air, the 

 reflection is fainter, and hence the deeper blue colour. WILLIAM 

 RHIND. 



