Condensation of Atmospheric Humidity on Solid Surfaces. 89 
they receive from it by radiation. But similarly dense clouds, 
if very high, though they equally intercept the communication 
of the earth with the sky ; yet being, from their elevated situa- 
tion, colder than the earth, will radiate to it less heat than they 
receive from it, and may, consequently, admit of bodies on its 
surface becoming several degrees colder than the air.” But dense 
clouds, though at times they may be comparatively near to the 
earth, never (excepting, perhaps, on some very extraordinary oc- 
casions) approach within a great distance of the low plains, — 
their elevation being commonly such, that, at that height, the tem- 
perature of the air must be very considerably below that of the 
lower atmosphere ; — otherwise, the fact, now generally under- 
stood to be well ascertained, that the temperature of the air dimi- 
nishes about 1° for every 800 feet of elevation, would be incorrect. 
Where the basis is so purely hypothetical, it is more surprising 
that the theory which is built upon it should be made to account, 
plausibly at least, for so much, than that it should seem to fail 
in some points, confessedly not free of obscurity. 
When we observe a cloud passing at some considerable dis- 
tance above the earth, and surrounded on all sides by transpa- 
rent air, we are apt to imagine that the increase of moisture is 
confined to the space within the circumference of the cloud. 
This, however, is not necessarily the case. The body of air occu- 
pying the interval between the cloud and the surface of the 
earth, during the passage of the former, may be more humid 
than that body of air which preceded it, and than that which 
follows next in succession. And when we consider what the 
source is, from which the moisture of the atmosphere is original- 
ly derived, we can readily comprehend how this state of increas- 
ed humidity may extend from the surface upwards. When the 
temperature of solid bodies at the surface of the earth increases 
during the passage of a cloud, the cold produced by evapora- 
tion is diminished, and this may proceed from the passage of a 
more humid body of air ; the upper boundary of which is so 
moist, as necessarily to produce a cloud at that elevation. Again, 
as every portion of the stratum of air next the earth is not ne- 
cessarily, and at all times, of equal temperature, and equally 
damp, especially when there is not a steady current of some 
force and duration, the increase of temperature indicated by 
