A wood, then, may be regarded either as a _superficies_ or as a _solid_; that is, either as a part of the earth's surface slightly elevated above the rest, or as a diffused and heterogeneous body displacing a certain portion of free and mobile atmosphere. It is primarily in the first character that it attracts our attention, as a radiating and absorbing surface, exposed to the sun and the currents of the air; such that, if we imagine a plateau of meadow-land or bare earth raised to the mean level of the forest's exposed leaf-surface, we shall have an agent entirely similar in kind, although perhaps widely differing in the amount of action. Now, by comparing a tract of wood with such a plateau as we have just supposed, we shall arrive at a clear idea of the specialities of the former. In the first place, then, the mass of foliage may be expected to increase the radiating power of each tree. The upper leaves radiate freely towards the stars and the cold inter-stellar spaces, while the lower ones radiate to those above and receive less heat in return; consequently, during the absence of the sun, each tree cools gradually downward from top to bottom. Hence we must take into account not merely the area of leaf-surface actually exposed to the sky, but, to a greater or less extent, the surface of every leaf in the whole tree or the whole wood. This is evidently a point in which the action of the forest may be expected to differ from that of the meadow or naked earth; for though, of course, inferior strata tend to a certain extent to follow somewhat the same course as the mass of inferior leaves, they do so to a less degree--conduction, and the conduction of a very slow conductor, being substituted for radiation. We come next, however, to a second point of difference. In the case of the meadow, the chilled air continues to lie upon the surface, the grass, as Humboldt says, remaining all night submerged in the stratum of lowest temperature; while in the case of trees, the coldest air is continually passing down to the space underneath the boughs, or what we may perhaps term the crypt of the forest. Here it is that the consideration of any piece of woodland conceived as a solid comes naturally in; for this solid contains a portion of the atmosphere, partially cut off from the rest, more or less excluded from the influence of wind, and lying upon a soil that is screened all day from isolation by the impending mass of foliage. In this way (and chiefly, I think, from the exclusion of winds), we have underneath the radiating leaf-surface a stratum of comparatively stagnant air, protected from many sudden variations of temperature, and tending only slowly to bring itself into equilibrium with the more general changes that take place in the free atmosphere.