Condensation of Humidity on Solid Surfaces. S45 
than the adjacent air, when exposed after sunset ; and that, with- 
out the operation of any thing equivalent to radiation, at least 
in as far as the metal itself is concerned. If now, we suppose 
the adjacent warmer air to contain, or to acquire, such a quan- 
tity of moisture, that a deposition must necessarily take place, if 
reduced in its temperature to that of the piece of metal, and if 
we suppose this damp air to be brought, by some mechanical 
impulse, into contact with the metal, we would expect moisture 
to appear, obscuring the polished surface. This is exactly what 
occurs in nature. Even when the air is in its most tranquil 
state, it is never altogether free of motion, convolving, undula- 
tory or progressive. On the evenings more particularly refer- 
red to in these remarks, uncertain local and temporarily pro- 
gressive motions are not unusual at the lower, and also at the 
upper, boundary of the stratum of air next the earth. Dr Wells 
found occasion more than once to refer to this agitation of the 
lower air, “ even in its stillest states ;” and, he remarks, that 
<c the quantity of dew seemed to be increased by a very gentle 
motion of the air.” This he accounts for, on the principle, that 
a slight agitation of the air, when the atmosphere is pregnant 
with moisture, will bring fresh parcels of air more frequently 
into contact with the cold surface of the earth.” 
A writer in the Edinburgh Encyclopaedia * observes, that if 
the reduction of temperature was produced by evaporation, 
“ the difference between the temperature of the ground ai?d that 
of the atmosphere near it, would diminish as the air became 
moist,” &c., and that evaporation could have nothing to do with 
theTeduction of temperature “ observed on substances exposed in 
a state of dryness, and not in contact with the earth.” If, in the 
first case, the lower air is understood to remain perfectly at rest 
on such occasions, and, in the second, that its temperature is the 
same at various distances from the ground, the conclusion of this 
writer might be just. But, as neither the one nor the other is 
the case in nature, his argument seems to have no weight against 
the paramount influence of evaporation. A very small abstrac- 
tion of heat will, in certain cases, produce a copious precipita- 
tion of moisture ; and, on such occasions, if the solid body which 
* Article ‘ Meteorology.’ 
