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Show 126 ISLAND LIFE. (PART I. however, have been very different from this, because the temperature of a place does not depend so much on .the amount of heat it receives directly from the sun, as on the am?unt brouO'ht to it or carried away from it by warm or cold wmds. We ~ften have it bitterly cold in the middle of May when .we are receiving as much sun heat as many parts of the trop1?s, but we get cold winds from the iceberg-laden North .Atlantic, and this partially neutralises the effect of the sun. s.o we of~en have it very mild in December if south-westerly wmds bnng us warm moist air from the Gulf-stream. But though the above method does not give correct results for any one time or place, it is more nearly correct for very large areas, because all the sensible surface-heat which produces climates comes from the sun, and its proportionate amount may be very nearly calculatetl in the manner above described. We may therefore say, generally, that during our northern winter, at the time of the glacial epoch, the northern hemisphere was receiving so much less heat from the sun as to lower its surface temperature on an average about 35° F., while during the height of summer of the same ~eri od it would be receiving so much more heat as would ~u~ce to raiRe its mean temperature about 60° F. above what 1t IS now. The winter, however, would be long and the summer short, the difference being twenty-six days. . We. have here certainly a superabundant amount of cold m winter to produce a glacial period, 1 especially as this cold woulu 1 In a letter to Nature of October 30th, 1879, the Rev. 0. Fisl1er calls attention to a result arrived at by Pouillet, that the temperature which the surface of the crround would assume if the sun were exti11guished would be -128° F. in0s tead of -239° F. If this corrected amount were used .m' our calculations, the January temperature of England during tho glawtl epoch would come out 17° F., and this Mr. Fisher thinks not low. enOL~g~1 to cause any extreme difference from the present clirnat~. In thts o~m10n, however, I cannot agree with him. On the contrary, 1t would, I tlunk, ?e a relief to the theory were the amounts of decrease of temperature. Jn winter and increase in summer rendered more moderate, since accordmg to the usual calculation (which I have adopted) the differences are unnecessarily great. I cannot therefore think that this modification ~f il:e temperatures, should it be ultimately proved to ?e corr~ct. ( wh1ch lS altogether denied by Dr. Croll), would be any senous obJection to the CHAP. V Jll. J TIIITI CAUSES OF GLACIAL EPOCHS. 127 be long continued; but at the same time we should havo almost tropical heat in summer, although that season would be somewh~ t shorter. How then, it may be asked, could such a climate have the effect supposed 1 Would not the snow that fell in winter be all melted by the excessively hot summer 1 In order to answer this question we must take account of certain properties of ';ater and ~ir, snow and ice, to which due weight has not been given by wnters on this subject. Prope1·ties of Air and Wetter, Snow and Ice, in relation to Olimate.-The great aerial ocean which surrounds us has the wonderful property of allowing the heat-rays from the sun to pass thro~gh it without its being warmed by them; but when the earth IS heated the air gets warmed by contact with it, and also to a considerable extent by the heat radiated from the warm earth, because, although pure dry air allows such dark heat-rays to pass freely, yet the aqueous vapour and carbonic acid in the air intercept and absorb them. But the air thus warmed by the earth is in continual motion owing to changes of density. It rises up and flows off, while cooler air supplies its place; and thus heat can never accumulate in the atmosphere beyond a very moderate degree, the excessive sunheat of the tropics being much of it carried away to the upper atmosphere and radiated into space. Water also is very mobile; and although .it receives and stores up a great deal of heat, it is for ever dispersing it over the earth. The rain which brinas down a certain portion of heat from the atmosphere, and w hi~h ?ften absorbs heat from thA earth on which it falls, flows away m streams to the ocean; while the ocean itself, constantly impelled by the winds, forms great currents, which carry off the adoption of Dr. Croll's theory o£ the Astronomical nnd Physical causes of the Glacial Epoch. 'J.1he reason of the increase of summer heat being 60° while the decrease o~ winter cold is only 35°, is because our summer is now below and our wmter above the average. A large part of the 60° increase of temperature would no doubt be used up in evaporating water, so that there would be a much. less increase of sensible heat ; while only a portion of the 35° lowe1:u~g of temperatu~e in winter would be actually produced owing to equahsmg effect of wmds and currents, and the storing up of heat by tl1e earth and ocean. |
