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Show 124 ISLAND LIFE. (PAin' 1, eight and a half millions of miles between ou~ distance from the sun in aphelion and perehelion (as the most distant and nearest points of the earth's orbit are termed). At a hundre~ an~ ~fty thousand years back it had decreased somewhat-to six m1lhons of miles; but then it increaseu again, till at two hundred thousand years ago it was ten and a quarter, and at two hundred and ten thousand years ten and a half millions of miles. By reference to the accompanying diagram, which includes the last great period of excentricity, we find, that for the immense period of a hundred and sixty thousand years (commencing !(PROBABLE DURATION OF' THE CLACIAt: EPOCH'>, I I I I I I I I I I THOUSAND YEARS ACO F"ROt.f A.D.I800, DIAGRAM OF EXOENTRICITY AND PRECESSION. The dark and light bands mark the phases of precession, the dark showing short mild winters. and the light lon¥ cold winters, the contrast being greater as the cxcentricity is higher. The horizontal dotted line marks the present excentricity. The figures show tho maxima and minima of excentricity during the last 300,000 years from Dr. Croll's Tables. about eighty thousand . years ago) the excentricity was very great, reaching a maximum of three and a half times its present amount at almost the remotest part of this period, at which time the length of summer in one hemisphere and of winter in the other would be nearly twenty-eight days in excess. Now, during all this time, our position would change, as above described (and as indicated on the diagram), every ten thousand five. hundred years; so that we should have alternate periods of very long and cold winters with short hot summers, and short mild winters with long cool summers. In order to understand the important effects which t.hi~ would produce we must ascertain OlJAI'. VIII.] THE CAUSES OF GLACIAL EPOCHS. 125 two things-first, wha.t actual difference of temperature would be caused by varying distances of the sun. and secondlv what are the properties of snow and ice in regard to ~limate. " ' Difference of Terftperature ca?tsed by varying distances of the S1m. -On this subject comparatively few persons have correct ideas owing to the unscientific manner in which we reckon heat by our thermometers. Our zero is thirty-two degrees below the freezing point of water, or, in the centigrade thermometer, the freezmg point itself, both of which are equally misleading when apphed to cosmical problems. If we say that the mean temperature of a place is 50o F. or 10° 0., these figures tell us nothing of how much the sun warms that place, because if the sun were withdrawn the temperature would fall far below either of the zero points. In the last Arctic Expedition a temperature of -74° F. was registered, or 106o below the freezing point of water; and as at the same time the earth, at a depth of two feet, was only-13o F. and the sea water + 28o F., we may be sure that even this intense cold was not near the possible minimum temperature. By various calculations and experiments which cannot be entered upon here, it has been determined that the temperature of space, independent of ~olar (but not of stellar) influence, is about -239o F., and physicists almost universally adopt . this quantity in all estimates of cosmical temperature. It follows, that if the mean temperature of the earth's surface at any time is 50o F. it is really warmed by the sun to an amount measured by 50 + 239 = 289o F., which is hence termed its absolute temperature. Now during the time of the glacial epoch the greatest distance of the sun in winter was 97 i millions of miles, whereas it is now, in winter, only 91 millions of miles. But the quantity of heat received from the sun is inversely as the square of the distance, so that it would then be in tee proportion of 8,281 to 9,506 now, or nearly one eighth less than its present amount. The mean temperature of England in January is about 39° F., which equals 278° F. of absolute temperature. But the above named fraction of 278° is 36°, representing the amount which must be deducted to obtain the January temperature during the glacial epoch, which will therefore be 3° F. Our actual temperature at that time might, |