THE STARS. 145 



Now, these well-known facts do not admit of 

 a satisfactory explanation on any principle with 

 which I am acquainted, but that insisted on by 

 Mr. Hooke, namely, a perpetually varying re- 

 fractive power in the medium through which the 

 rays of light pass ; and this principle, in con- 

 sequence of the unequal distribution of heat, 

 and its constant tendency towards a more uni- 

 form diffusion, we know to be ever active in the 

 earth's atmosphere. 



What powerfully contributes to establish the 

 accuracy of this opinion, is the effect produced 

 by changes in the direction of the wind, parti- 

 cularly if attended with a considerable increase 

 or diminution of temperature. On such occa- 

 sions, and on the near approach of clouds, espe- 

 cially of that modification denominated cirro-stra- 

 tus in the nomenclature of Mr. Luke Howard, 

 I have remarked, that the stars generally twinkle 

 much ; but that this vibratory motion of their 

 light is usually greatly reduced when the wind 

 has continued to blow long and steadily from the 

 same point of the compass. 



That fluctuations, in the direction of the aerial 

 currents, facilitate the intermixture of warm and 

 cold air, and contribute materially to promote 

 the condensation of vapour into clouds, in the 



T 



