316 HISTORY OF 



surface to the evaporating rays. A dry frost, in 

 some measure, assists the quantity of evapora^ 

 tion ; as the quantity of rays are found to be no 

 way diminished thereby. Moist weather alone 

 prevents evaporation ; for the rays being absorb- 

 ed, refracted, and broken, by the intervening 

 moisture before they arrive at the surface, cannot 

 produce the effect, and the vapour will rise in a 

 small proportion. 



Thus far we have accounted for the ascent of 

 vapours ; but to account for their falling again, is 

 attended with rather more difficulty. We have 

 already observed, that the particles of vapours 

 disengaged from the surface of the water, will be 

 broken and attenuated in their ascent by the re- 

 flected, and eyen the direct rays, that happen to 

 strike upon their minute surfaces. They will, 

 therefore, continue to ascend, till they rise above 

 the operation of the reflected rays, which reaches 

 but to a certain height above the surface of the 

 earth. Being arrived at this region, which is cold 

 for want of reflected heat, they will be condensed, 

 and suspended in the form of clouds. Some va- 

 pours, that ascend to great heights, will be frozen 

 into snow ; others, that are condensed lower 

 down, will put on the appearance of a mist, which 

 we find the clouds to be when we ascend among 

 them, as they hang along the sides of a mountain. 

 These clouds of snow and rain, being blown about 

 by winds, are either entirely scattered and dis- 

 persed above, or they are still more condensed by 

 motion, like a snow-ball that grows more large 

 and solid as it continues to roll. At last, there- 



