556 LECTURE LVII. 



tables. Such mists are sometimes attended by a smell, resembling that 

 which is occasioned by an electric spark. Rain falling after a dry season 

 deposits, when it has been suffered to stand, some particles of foreign 

 matter which it has brought down from the atmosphere. There must in- 

 deed frequently be a multiplicity of substances of various kinds floating in 

 the air ; the wind has been found to carry the farina of plants as far as 30 

 or 40 miles, and the ashes of a volcano more than 200. It only requires 

 that the magnitude of the particles of any substance be sufficiently reduced 

 in size, in order to render them incapable of falling with any given velo- 

 city ; and when this velocity is very small, it may easily be overpowered 

 by any accidental motions of the air. The diameter of a sphere of water, 

 falling at the rate of one inch only in a second, ought to be one six hundred 

 thousandth of an inch, which is about the thickness of the upper part of 

 a soap bubble at the instant when it bursts ; but the particles of mists are 

 incomparably larger than this, since they would otherwise be perfectly 

 invisible as separate drops : the least particle that could be discovered by 

 the naked eye, being such as would fall with a velocity of about a foot in 

 a second, 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 experi- 

 ments made on a much larger scale, and their descent consequently much 

 slower. 



When the particles of a mist are united into drops capable of descending 

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

 during their deposition, they exhibit the appearance of a perfect crystal- 

 lization, 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 descent through very dry air, they acquire the cha- 

 racter of hail, which is often observed in weather much too hot for the 

 formation of snow. 



It cannot be doubted but that there is a connexion betwen the descent of 

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

 assigned 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 observed. The immediate dependence of rain, or of any other atmo- 

 spherical phenomena, on the influence of the moon, appears to be rendered 

 highly improbable, 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 sufficiently 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 

 upper regions of the atmosphere, and may be redissolved by the warmer 

 air below ; but when they descend in an equal degree among mountains, 

 they fall on the earth ; and besides the quantity of water which they fur- 

 nish for vegetation, and that which is carried off by evaporation, they 



