+-------------------------------------------------------------+ | Wind. | N. | N.E.| E. | S.E.| S. | S.W.| W. | N.W.| | |-----+-----+-----+-----+-----+-----+-----+-----| |Var. in Wood |+0.60|+0.26|+0.26|+0.04|-0.04|-0.20|+0.16|+0.07| |Var. in Wind |-0.30|-2.60|-3.30|-1.20|+1.00|+1.30|+1.00|+1.00| +-------------------------------------------------------------+
From this curious comparison, it becomes apparent that the variations of the difference in question depend upon the amount of variations of temperature which take place in the free air, and on the slowness with which such changes are communicated to the stagnant atmosphere of woods; in other words, as Herr Rivoli boldly formulates it, a forest is simply a bad conductor. But this is precisely the same conclusion as we have already arrived at with regard to individual trees; and in Herr Rivoli's table, what we see is just another case of what we saw in M. Becquerel's--the different progression of temperatures. It must be obvious, however, that the thermal condition of a single tree must be different in many ways from that of a combination of trees and more or less stagnant air, such as we call a forest. And accordingly we find, in the case of the latter, the following new feature: The mean yearly temperature of woods is lower than the mean yearly temperature of free air, while they are decidedly colder in summer, and very little, if at all, warmer in winter. Hence, on the whole, forests are colder than cleared lands. But this is just what might have been expected from the amount of evaporation, the continued descent of cold air, and its stagnation in the close and sunless crypt of a forest; and one can only wonder here, as elsewhere, that the resultant difference is so insignificant and doubtful.