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Show 138 ISLAND LIFE. [PAUT I. hemisphere, and, passm· g everyw h ere over a wide ocean ' th.e y supply the moisture necessary to produce the e~ormous qt~antity of snow wh 1. c h f a lls m. th e Antarctic area. Durm. g the penod we are now dI. scussm· g, h ow ever, this state of thmgs would. hav. e b een part I. a 11 y revers ed . The south polar area, h.a vmg I. ts w•m ter I• n penr·1.w lw' n, wo uld probably have had less Ice, while t h e nor th -t empera t e and Arctic regions would have been largely ice-clad; and the north-east trades would th.erefore be stronger h th The St an ey are now. outh-westerly anti-trad. es wo. uld also b e st ranger m· the same proportion ' a.n d would . bnn.g with th.e m a great ly m· ere ased quantity of mOist.u re., which IS the pnme necessity to produce a condition of glaCiatiOn. But this is only one-half of the effect that would be produc:d, for the increased force of the trades sets up another ac~wn whwh still further helps on the accumulation of snow and .ICe. . It is now generally admitted that we owe much of o~r m1ld chmate and our comparative freedom from snow to. the mfluence ?f t~e Gulf Stream, which also ameliorates the chmate of Scandinavia and Spitzbergen, as shown by the remarkabl~ n~rthward cur: vature of the isothermal lines, so that Drontheim m N. Lat. 62 has the same mean temperature as Halifax (Nova Scotia) in N. Lat. 45°. The quantity of heat now brought into the North Atlantic by the Gulf Stream depends mainly on the superior strength of the south-east trades. When the north-east trades were the more powerful, the Gulf Stream would certainly be of much less magnitude and velocity; while it is possible, as Dr. Croll thinks, that a large portion of it might be diverted southward owing to the peculiar form of the east coast of ~outh America, and so go to swell the Brazilian current and ameliorate the climate of the southern hemisphere. That effects of this nature would follow from any increase of the Arctic and decrease of the Antarctic ice, may be considered certain; a~d Dr. Croll has clearly shown that in this case cause and effect act and react on each other in a remarkable way. The increase of snow and ice in the northern hemisphere is the cause of an increased supply of moisture being brought b.y the more powerful anti-trades, and this greater supp~y of. moisture leads to an extension of the icc, which reacts m st11l further CITAP. Vlll.) TilE CAUSES OF GLACIAL EPOCHS. 139 increasing the supply of moisture. The same increase of snow and ice, by causing the north-east to be stronger than the southeast trade-winds, diminishes the force of the Gulf Stream and this diminution lowers the temperature of the North Atl~ntic both in summer and winter, and thus helps on still further the formation i1nd perpetuation of the icy mantle. It must also be remembered that these agencies are at the same time acting in a reverse way in the southern hemisphere, diminishing the supply of the moisture carried by the anti-trades, and increasing the temperature by means of more powerful southward oceancurrents ;-and all this again reacts on the northern hemisphere, increasing yet further the supply of moisture by the more powerful south-westerly winds, while still further lowering the temperature by the southward diversion of the Gulf Stream. Surnmary of principal Causes of Glaciation.-! have now sufficiently answered the question, why the short hot summer would not melt the snow which accumulated during the long cold winter (produced by high excentricity and winter in aphelion), although the annual amount of heat received from the sun was exactly the. same as it is now, and equal in the two hemispheres. It may be well, before going further, briefly to summarise the essential causes of this apparent paradox. These are-primarily, the fact that solar heat cannot be stored up owing to its being continually carried away by air and water, while cold can be so stored up owing to the comparative immobility of snow and ice; and, in the second place, because the two great heatdistributing agencies, the winds and the ocean currents, are so affected by an increase of the snow and ice towards one pole and its diminution towards the other, as to help on the process when it has once begun, and by their action and reaction produce a maximum of effect which, without their aid, would be altogether unattainable. But even this does not exhaust the causes at work, all tending in one direction. Snow and ice reflect heat to a much greater degree than do land or water. The heat, therefore, of the short summer would have far less effect than is due to its calculated amount in melting the snow, because so much of it would be lost by reflection. A portion of the reflected heat would no |
