ly exposed to the sun's rays, are yet constantly deriving
heat from them by means of the reflection of the atmosphere. This heat,
though it produces little change on the temperature of the air which it
traverses, affords us some compensation for the heat which we radiate to
the heavens. At night, also, if the sky be overcast, some compensation
will be made to us, both in the town and in the country, though in a
less degree than during the day, as the clouds will remit towards the
earth no inconsiderable quantity of heat. But on a clear night, in an
open part of the country, nothing almost can be returned to us from
above in place of the heat which we radiate upward. In towns, however,
some compensation will be afforded even on the clearest nights for the
heat which we lose in the open air by that which is radiated to us from
the sun round buildings.

"To our loss of heat by radiation at times that we derive little
compensation from the radiation of other bodies is probably to be
attributed a great part of the hurtful effects of the night air.
Descartes says that these are not owing to dew, as was the common
opinion of his contemporaries, but to the descent of certain noxious
vapors which have been exhaled from the earth during the heat of the
day, and are afterwards condensed by the cold of a serene night. The
effects in question certainly cannot be occasioned by dew, since that
fluid does not form upon a healthy human body in temperate climates; but
they may, notwithstanding, arise from the same cause that produces dew
on those substances which do not, like the human body, possess the power
of generating heat for the supply of what they lose by radiation or any
other means."(2)

This explanation made it plain why dew forms on a clear night, when
there are no clouds to reflect the radiant heat. Combined with Dalton's
theory that vapor is an independent gas, limited in quantity in any
given space by the temperature of that space, it solved the problem of
the formation of clouds, rain, snow, and hoar-frost. Thus this paper
of Wells's closed the epoch of speculation regarding this field of
meteorology, as Hutton's paper of 1784 had opened it. The fact that the
volume containing Hutton's paper contained also his epoch-making paper
on geology finds curiously a duplication in the fact that Wells's volume
contained also his essay on Albinism, in which the doctrine of natural
selection was for the first time formulated, as C