Over and above what has been mentioned, thermal effects have been attributed to the vital activity of the leaves in the transudation 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 naturally 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 temperature of the air outside; and, 3, The relation between the temperature 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 observations; and I may remark in passing, what applies to all, that allowance must be made throughout for some factor of specific heat. The results were as follows:--The seasonal and monthly means in the tree and in the air were not sensibly different. The variations in the tree, in M. Becquerel's own observations, appear as considerably less than a fourth of those in the atmosphere, and he has calculated, from observations made at Geneva between 1796 and 1798, that the variations in the tree were less than a fifth of those in the air; but the tree in this case, besides being of a different species, was seven or eight inches thicker than the one experimented on by himself.[48] The variations in the tree, therefore, are always less than those in the air, the ratio between the two depending apparently on the thickness of the tree in question and the rapidity with which the variations followed upon one another. The times of the maxima, moreover, were widely different: in the air, the maximum occurs at 2 P.M. in winter, and at 3 P.M. in summer; in the tree, it occurs in winter at 6 P.M., and in summer between 10 and 11 P.M. At nine in the morning in the month of June, the temperatures of the tree and of the air had come to an equilibrium. A similar difference of progression is visible in the means, which differ most in spring and autumn, and tend to equalise themselves in winter and in summer. But it appears most strikingly in the case of variations somewhat longer in period than the daily ranges. The following temperatures occurred during M. Becquerel's observations in the Jardin des Plantes:-- Date. Temperature of Temperature in the Air. the Tree.