STUDY IX. 239 



The air when loaded with vapours, we farther 

 fay, is refrangible. The farther we advance to the 

 North, the more elevated does the Sun appear 

 over the Horizon, above the place which he ac- 

 tually occupies in the Heavens. The Dutch ma- 

 riners, who pafled the Winter of 1597, in Nova- 

 Zembla, after a night of feveral months, faw the 

 Sun re-appear fifteen days fooner than they ex- 

 pecfted his return. All this is very well. But if 

 vapours render the air refrangible, why is there no 

 Aurora, nor twilight, nor any durable refradion of 

 light whatever, between the Tropics, not even on 

 the Sea, where fo many vapours are exhaled, by 

 the confiant adion of the Sun, that the Horizon 

 is fometimes quite involved in mift by them. 



The light is not refraâ;ed,-fays another Philo- 

 fopher, by the vapours, but by the cold ; for the 

 refradion of the Atmofptiere is not fo great at the 

 end of Summer, as at the end of Winter, at the 

 autumnal Equinox, ns at the vernal. 



I admit the truth of this obfervation ; however, 

 after very hot days in Summer, there is refradion to 

 the Norths as well as in our temperate Climates, 

 and there is none between the Tropics : the cold, 

 therefore, does not appear to me to be the mecha- 

 nical caufe of refradion, but it is the final caufe of 



it. 



