ATMOSPHERE. [Lesson xvt. 



to protract his stay with us after he is set in the 

 evening. This is an admirable contrivance to 

 shorten the long and dismal nights in the frigid 

 zones, and thus to add to the comfort of the in- 

 habitants of those forlorn regions. Varenius re- 

 lates in his geography, that " to those Hollanders 

 " who wintered in Nova Zemlla, the Sun was 

 " visible, in a clear sky, sixteen days before it ac- 

 (t tually rose above the horizon, being yet four de- 

 ** grees below it." And Hook, speaking of the 

 same circumstance, says, " The night in that 

 *' place shortened no less than a whole month j 

 ^which must needs be a very great comfort, to 

 *' all such people as live very far towards the 

 * North and South poles, where length of night, 

 t( and want of seeing the Sun, cannot but be very 

 <l tedious and irksome." 



Though it is rather foreign from the immediate 

 subject of this Lesson, yet I shall here relate a few 

 properties of refraction ; as it will somewhat tend 

 to elucidate what is above-mentioned with regard 

 to the twilight. By numerous experiments we 

 find that the rays of a luminous body or even of 

 any visible object, when they fall upon a medium 

 or diaphonous body of air or water of a different 

 .density from- that from whence they first proceeded, 

 do not afterwards go directly in the same straight 

 lines, but are- broken or bent, and proceed as 

 though they had been propagated from another 

 point. And if the medium on which they fall be 

 denser than the first, they are bent towards a line 

 perpendicular o the, surface -whereon they fall 



at 



