third influence had better not be insisted upon, however, as it is counteracted by—1. The immense amount of latent heat liberated in the condensation of the vapor; and, 2. The interception by these fogs of the heat radiated from the earth. A substitute may be offered for it, viz., by rendering the atmosphere diathermous, and therefore incapable of absorbing the solar rays or of intercepting the radiation and reflection from the earth. In consequence of the operation of these agencies, the air in snow-covered regions seldom rises above the freezing-point, and the solar heat is conveyed away into space and utterly lost to the earth; for, if a portion of it is absorbed by the snow and ice during the hours of sunshine, it is not rendered sensible, owing to the high latent and specific heat of these forms of water, and is radiated away, unchecked by any "protecting blanket of vapor" (which Professor Tyndall shows to be so efficacious in protecting the earth from radiation, but which can not exist above snow-covered regions), during the succeeding hours of darkness.
This waste of solar energy, in turn, still further curtails the already short summer, and permits the same causes to operate with increased efficiency during the succeeding season. Moreover, the reaction of each effect upon its cause is such as to strengthen the cause, and the interaction of all the agencies is such as to increase the efficiency of each. Each winter would thus add to the snow which had remained unmelted during the intervening summer, until the accumulation of snow was checked by the absence of vapor for condensation and precipitation; for, as pointed out by Tyndall, the presence of large quantities of vapor is the first essential for the formation of extensive glaciers.
It has been objected to the theory, that this picture has been overdrawn—that no such slight cause could so seriously disturb the equilibrium of the seasons; it has even been shown mathematically that the heat of a few days in summer would melt the total accumulation of the previous winter. The answer to these objections is, that in such calculation the operation of the agencies just described was disregarded, and hence that its results are unreliable; that, though the solar intensity is greater over polar regions in summer than in the tropics, as shown by Meech, it is not sufficient to melt the annual accumulation of ice, else this ice never could have accumulated to so vast an extent as to annually send forth thousands of colossal bergs to be melted in temperate seas; that not only in the Alps, but even in the almost tropical Himalayas, where the sun shines with undiminished intensity throughout the year, the direct effect of the solar energy is so far below the accumulation of congealed vapor that the ice is only prevented from piling up indefinitely by that property which enables it to flow down to lower levels where the conditions described do not exist; that even in the northern portions of our own country the slight annual film of snow retards the coming of summer by weeks if not months;
