10 OF CLOUDS. CHAP. 1. 3. 



atmosphere, and become fleecy and irregular 

 in their appearance, with their surfaces full of 

 protuberances. In changeable weather, they 

 partake of the vicissitudes of the atmosphere, 

 and evaporate almost as soon as formed, or 

 quickly change into other modifications. The 

 quick formation and subsiding of this cloud 

 may be observed too in fair weather, with 

 easterly and varying breezes, particularly in 

 spring; but, in fair and settled weather, they 

 keep pace in some measure with the diurnal 

 temperature, they form soon after sunrise, ar- 

 rive at their maximum in the middle of the 

 day, and become very convenient skreens to 

 intercept the rays of the sun; and they subside 

 in the evening. It was this circumstance which 

 probably led to a conjecture of the particular 

 cause of their production, which appears as fol- 

 lows: The sun's rays warming first the surface 

 of the earth, and their radiation causing warmth 

 to be propagated upward, this warmth converts 

 water on the earth's surface into vapour, which 

 rises and exerts its elastic force on that which 

 the nocturnal decrease of temperature had not 

 decomposed, and which therefore remained 

 diffused. The latter, in passing through the 

 atmosphere, to give place to that from below, 



