258 Mr Harvey’s Observations on the formation of Mists, 
land; and the volumes of air reposing over each. In conse- 
quence of the unequal cooling powers of the land and water, the 
former will first have its temperature reduced below that of the 
air ; and although by this diminution the equality of tempera- 
toe between the two volumes of air will be destroyed, and a 
condition created favourable to the formation of mist ; still, as 
the cooling of the first volume, and the mingling of the two are 
not cotemporaneous acts,, dev/ will be the first deposited. 
In proportion, however, as the land radiates freely, with the 
same rapidity must the temperature of the superincumbent air 
be diminished, and the equilibrium between it and the atmo- 
sphere hovering over the water be disturbed. The rapid forma- 
tion of dew, is therefore accompanied by circumstances favour- 
able to the quick formation of mist ; and it hence becomes pro- 
bable, that, under such conditions, mist will be formed at an 
earlier period of the night, than when the land radiates less co- 
piously, and dew is deposited in less abundance. 
If, in consequence of the interposition of a canopy of clouds, 
the cooling of the land and water should be checked, and an equa- 
lity of temperature be restored between the two masses of air, 
and the bodies on which they respectively repose, the deposi- 
tion of dew will be suspended, and likewise all tendency to the 
farther formation of mist. The entire dissipation of the latter 
may likewise result from the change. 
Or, if we suppose a temporary interposition of clouds, and 
conceive the land to radiate its heat only at intervals, corre- 
sponding changes of temperature will immediately follow ; an 
increase of heat taking place when the sky is obscured, and a 
diminution when its clearness is restored. Dr W ells has furnished 
a case, wherein the temperature of grass increased 15° in the 
short space of forty-five minutes, and which was at the same time 
accompanied by an elevation in the temperature of the air of three 
degrees and a half. Nor is it improbable but that examples 
sometimes occur, of dew and mist being deposited in the for- 
mer part of the night, and both disappearing before the mor- 
ning ; or, as it may sometimes happen, the former be preserved, 
and the latter dissipated. The first of these phenomena may 
take place, when a portion of the night, favourable to the for- 
mation both of mist and dew, is succeeded by a brisk wind ; 
