198 An Essay on a hither lo unolserved Properly in Light. 



day dawn ; and as the ships generally made a good offing during 

 the night, it frequently happened that, after sunrise and the air 

 became heated, we had to win seven or eight leagues directly to- 

 wards it, before we were able to see the very same land that we 

 had seen when we were more than twenty miles further off from 

 it. Here there is sufficient proof that heat rarefies the air, and cold 

 condenses it; and that the refraction of light passing through the 

 atmosphere is always in proportion to its density. 



It was remarked in Captain Ross's voyage to discover the 

 north-west passage, that the motion of a field of ice very fre- 

 quently produced the strangest optical delusions ; that the atmo- 

 sphere, in consequence, becoming suddenly condensed, the sur- 

 rounding objects would in a moment be magnified three or four 

 times beyond their ordinary size, and, on its going off, would as 

 suddenly resume their former shape. Now the very same prin- 

 ciple that will account for this phienomenon will also explain the 

 motion of the aurora horealis. In consequence of the motion of 

 an iceberg, the atmosphere may become mure condensed ; and the 

 rays of light, which before were too faint to be perceived, will be 

 condensed into a solid mass, and send down a body of light into 

 the eye : or, what is more likely, the secondary rays from the sun, 

 which before were passing outside of the eye, are now brought 

 into it in a body by the increased refraction of the atmosphere, 

 and, according to the circumstances, exhibit the various phaeno- 

 mena which have been recorded of this meteor. 



Whatever may be the true principle of the atmosphere, we can- 

 not better explain its effect, as far as regards refraction, than by 

 comparing the particles of which it is composed to balls of fungus, 

 which may be compressed into a very small space, but always en- 

 deavour to recover their original state; and this action and reac- 

 tion of the particles of air is the only way by which we can ac- 

 count for the twinkling of the stars. In the dead of the night, 

 when the aurora horealis is to be seen, the sun is in the opposite 

 hemisphere; and his heat, expanding the particles of air in the 

 torrid and temperate zones, causes them to press upon those 

 that are in the frigid zone, and compress them into a smaller 

 space : these particles always endeavour to recover their former 

 state ; and the motion of an iceberg, or the change of a hot wind 

 for a cold one, will produce a very sensible effect in those regions 

 where the refraction is so great. 



Even the variety of colour which the aurora borealis is some- 

 times said to assume, may be accounted for by the principle of 

 refraction ; for, as refraction separates the rays of colour, where 

 there is a great deal of refraction we may reasonably expect it ; 

 and if a streak of light is sometimes seen in the air unconnected 

 with any light below, we may account for it by supposing that the 



wind, 



