130 ISLAND LIFE 



by the sun to an amount measured by 50 + 239 = 289° 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 98 J millions of miles, whereas it 

 is now, in winter, only 91^ millions of miles, the mean 

 distance being taken as 93 million 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 the proportion of 

 8,372 to 9,613 now, or nearly one seventh less than its 

 present amount. The mean temperature of England in 

 January is about 37° F., which equals 276° F. of absolute 

 temperature. But the above-named fraction of 276° is 237, 

 the difference, 39, representing the amount which must be 

 deducted to obtain the January temperature during the 

 glacial epoch, which will therefore be — 2° F. But this is 

 a purely theoretic result. The actual temperature at that 

 time might 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 

 amount brought to it or carried away from it by warm or 

 cold winds. We often have it bitterly cold in the middle 

 of May when we are receiving as much sun heat as many 

 parts of the tropics, but we get cold winds from the 

 iceberg-laden North Atlantic, and this largely neutralises 

 the effect of the sun. So we often have it very mild in 

 December if south-westerly winds bring 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 will be 

 more nearly correct for very large areas, because all the 

 sensible surface-heat which produces climates necessarily 

 comes from the sun, and its proportionate amount may be 

 very nearly calculated in the manner above described. We 

 may therefore say, generally, that during our winter, 

 at the time of the glacial epoch, the northern hemi- 

 sphere was receiving so much less heat from the sun 

 as was calculated to lower its surface temperature on an 

 average about 39° F., while during the height of summer 

 of the same period it would be receiving so much more 

 heat as would suffice, other conditions being equal, to raise 

 its mean temperature about 48° above what it is now. 



