712 LKCTURE rVII. 



be discovered by the naked eye, being such as would fall with a velocity of about 

 a foot in asecond,if the air were perfectly at rest. But it is very probable that 

 the resistance, opposed to the motion of particles so small, may be considerably 

 greater, than would be expected from a calculation, derived from experiments 

 made on a much larger scale, and their descent consequently much slower. 



When the particles of a mist are united into drops capable of descendino- 

 ■with a considerable velocity, they constitute rain ; if they are frozen durino- 

 their deposition, they exhibit the appearance of a perfect crystallization, and 

 become snow: but if the drops already formed are frozen, either by means of 

 external cold, or on account of the great evaporation produced by a rapid de- 

 scent through very dry air, they acquire the character of hail, which is 

 often observed in weather much too hot for the formation of snow. 



It cannot be doubled but that there is a connexion between the descent of 

 the barometer and the fall of rain; but no satisfactory reason has yet been as- 

 signed for the circumstance; nor is it possible to foretel, with certainty, that 

 rain will follow any changes in the height of the barometer that have been ob- 

 served. The immediate dependence of rain, or of any other atmospherical 

 phenomena, on the influence of the moon, appears to be rendered highly im- 

 , probable, not only by mathematical calculations of the effects of the moon's 

 attraction, but also by the irregularity of the very observations, which have 

 been adduced in favour of such a connexion. But however uncertain the 

 ultimate causes of rain'may be in general, their effects in some places are suf- 

 ficiently constant, to be attributed to permanent local circumstances, and in 

 particular to the periodical recurrence of similar winds: 



In low and level countries, clouds may often begin to descend from the up- 

 per regions of the atmosphere, and may be redissolved by the warmer air be- 

 low; but when they descend in an equal degree among mountains, they fall 

 on the earth; and besides the quantity of water which they furnish for vege- 

 tation, and that which is carried off by evaporation, they afford, by means of 

 springs and rivers, a constant supply for the use of man and of other animals 

 in distant parts. The upper regions of the atmosphere are however by na 

 means the principal sources of rain in ordinary climates, since a gage placed 

 on a very high building seldom collects more than two thirds as much rain 

 as another standing on the ground below: and the effects of mountains in 



