ON THE THERMAL
removed from all such radiative and absorptive influences, it
is plain that all three should have been equally exposed to the
sun or kept equally in shadow. As the observations were made,
they give us no notion of the relative action of earth-surface
and forest-surface upon the temperature of the contiguous
atmosphere; and this, as it seems to me,, was just the crux
of the problem. So far, however, as they go, they seem to
justify the view that all these actions are the same in kind,
however they may differ in degree. We find the forest heating
the air during the day, and heating it more or less according as
there has been more or less sunshine for it to absorb, and we find
it also chilling it during the night ; both of which are actions
common to any radiating surface, and would be produced, if
with differences of amount and time, by any other such surface
raised to the mean level of the exposed foliage.
To recapitulate :
1st. We find that single trees appear to act simply as bad
conductors.
2nd. We find that woods, regarded as solids, are, on the
whole, slightly lower in temperature than the free air which
they have displaced, and that they tend slowly to adapt them-
selves to the various thermal changes that take place without
them.
3rd. We find forests regarded as surfaces acting like any
other part of the earth's surface, probably with more or less
difference in amount and progression, which we still lack the
information necessary to estimate.
All this done, I am afraid that there can be little doubt
that the more general climatic investigations will be long and
vexatious. Even in South America, with extremely favourable
conditions, the result is far from being definite. Glancing
over the table published by M. Becquerel in his book on
climates, from the observations of Humboldt, Hall, Boussin-
gault, and others, it becomes evident, I think, that nothing
can be founded upon the comparisons therein instituted ; that
22