BOOK OF NATURE LAID OPEN. 161 



lengthened and prolonged. By the reflective pro- 

 perty of this iluid, which must always be in propor- 

 tion to its density, the heat of the sun, although duly 

 attempered, must be more sensibly and uniformly felt 

 nigh the surface of the earth, than in the higher re- 

 gions of the atmosphere; while, to its refractive qua- 

 lity, we arc beholden for the twilight, or that surpris- 

 ing phenomena of nature, by which we enjoy the 

 real presence of the sun when he is actually below 

 the horizon. For the better understanding of this, 

 let any person put a shilling into a basin, and then 

 retire until he can just observe its outer edge visible 

 over the inner edge of the vessel; in that position let 

 some person fill up the basin with water, the whole 

 shilling, by being seen through a denser medium, 

 will instantly become visible to the person who could 

 only before observe its outer edge. 



Were it not for the reflective quality of the air, by 

 which, indeed, the light is parted^ we would behold 

 the sun in his splendour, and observe a brightness 

 in that part of the heavens in which he happened to 

 be, but, on turning round, how cheerless would be 

 the prospect! there darkness visible would reign in 

 the heavens, although the stars and planets would 

 glimmer at noon day; and were it not for the refrac- 

 tive property of this fluid, by which the oblique rays 

 of the great luminary are broken off from a straight 

 course, and turned towards the earth, the transition 

 from the horrors of night to the light of day must 

 have been instantaneous, and, instead of those bene- 

 ficial harbingers, by which the outgoings of the mofcfc- 

 p 2 



