142

On Evaporation , Rain, &c.

mounts to 40.4, and in rhe latter to 28.1.—The reason of this seems to be, that, in the for-mer period, the temperature of the air is de-creasing, and consequently its capacity for vapouralso; which circumstance is an additional causeof the precipitation of vapour. In the latterperiod, the capacity of the air for vapour is in-creasing, which occasions a less precipitation.

When a precipitation of vapour takes place,a multitude of exceedingly small drops form acloud, mist, or fog; these drops, though 800times denser than the air, at first descend veryslowly, owing to the resistance of the air, whichproduces a greater effect as the drops are smaller^as may be proved thusLet d — the diameterof a small drop, and nd — that of a larger ; thenthe resistances, being as the squares of the dia-meters when the velocity is given, will be as d %and n x d z , respectively; but the magnitudes areas d l to n 3 d 3 , or as x to n 3 , whence, if the largedrop be divided into others of the same magni-tude as the small one, the number will be — n 3 ,and the resistance to them falling, as n 3 d z , whilstthe resistance to an equal mass in one drop is asn*d r ; consequently, the resistance to the largedrop is to the resistance of all the small ones,moving with the same velocity, as the % diameterof one small drop is to the diameter of the largeone, and the force being constant, the time offalling through a given space will be greater

when