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Show A NOTATIONS AND ADDITIONS. 113 from it. The result of thi unequal exchange will thus be a loss of temperature for the second stratum of leaves also. A similar operation will continue from stratum to stratum until all the leaves of the tree, by greater or less radiation, as modified by their diversity of position, have passed into a state of stable equilibrium, of which the law can be deduced by mathematical analysis. In this manner, in the long and clear nights of the equinoctial zone, the forest air contained in the intervals between the strata of leaves becomes cooled by the process of radiation; and by reason of the great quantity of its thin appendicular organs or leaves, a tree, the horizontal section of whose summit would measure for example 2000 square feet, would act in diminishing the temperature of the air equivalently to a space of bare or turf-covered ground several thousand times greater than 2000 square feet (Asie Centrale, t. iii. pp. 195- 205 ). I have sought thus to develope in detail the complicated effects which make up the total action of extensive forests upon the atmosphere, because they have been so often touched upon in reference to the important question concerning the climates of ancient Germany and Gaul. As in the Old Continent European civilization has had its principal seats on a western coast, it could not but be early remarked that, under equal degrees of latitude, the opposite eastern coast of the United States was several degrees colder in mean annual temperature than Europe, which is, as it were, a projecting western peninsula to Asia, as Brittany is to the rest of. France. But in this remark it was forgotten that these differences decrease from the higher to the lower latitudes, in such manner that they almost entirely disappear from 30° downwards. For the west coast of the New Continent, exact thermometric observations are still almost entirely wanting; but the mildness of the winters in New California shows that the west coasts of America and Europe, under the same parallels of latitude, probably differ little from each other in mean annual temperature. The subjoined table shows what arc the corresponding mean annual temperatures, in the same geographical latitudes, of the west coast of Europe and the cast coast of the New Continent. 10* |
