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 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 partially 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 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 calculated 

 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 period 

 it would be receiving so much more heat as would suffice to 

 raise its mean temperature about 60° F. above what it 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 in 

 winter to produce a glacial period,^ especially as this cold would 



1 In a letter to Nature of October 30th, 1879, the Rev. 0. Fisher calls 

 attention to a result arrived at by Pouillet, that the temperature which the 

 surface of the ground would assume if the sun were extinguished would 

 be -128^F. instead of -239°F. If this corrected amount were used in 

 our calculations, the January temperature of England daring the glacial 

 epoch would come out 17° F., and this Mr. Fisher thinks not low enough to 

 cause any extreme difference from the present climate. In this opinion, 

 however, I cannot agree with him. On the contrary, it would, I think, be 

 a relief to the theory were the amounts of decrease of temperature in 

 winter and increase in summer rendered more moderate, since according 

 to the usual calculation (which I have adopted) the differences are un- 

 necessarily great. I cannot therefore think that this modification of the 

 temperatures, should it be ultimately proved to be correct (which is 

 altogether denied by Dr. Croll), would be any serious objection to the 



