116 Proceedings of the Royal Society 
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 con- 
ceived 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 insolation 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 varia- 
tions of temperature, and tending only slowly to bring itself into 
equilibrium with the more general changes that take place in the 
free atmosphere. 
Over and above what has been mentioned, thermal effects have 
been attributed to the vital activity of the leaves in the transuda- 
tion of water, and even to the respiration and circulation of living . 
wood. The whole actual amount of thermal influence, however, 
is so small that I may rest satisfied with mere mention. If these 
actions have any effect at all, it must be practically insensible; and 
the others that I have already stated are not only sufficient validly 
to account for all the observed differences, but would lead natu- 
rally to the expectation of differences very much larger and better 
marked. To these observations I proceed at once. Experience 
has been acquired upon the following three points : — 1. The 
relation between the temperature of the trunk of a tree and the 
temperature of the surrounding atmosphere; 2. The relation 
between the temperature of the air under a wood and the tempera- 
ture of the air outside; and, 3. The relation between the tem- 
perature of the air above a wood and the temperature of the air 
above cleared land. 
As to the first question, there are several independent series of 
