Dr. Croll's Hypotheses on Geological Climates. 227 



tion of the temperatures which obtained during high excen- 

 tricity and winter in aphelion ; and repeatedly he admits that 

 the temperature of a place, other things being equal, is pro- 

 portional to the heat received from the sun. To make us 

 quite sure of his meaning, he has a table in ' Climate and Time/ 

 p. 320, where he gives the value of excentricity for different 

 periods, and the midwinter temperature of Great Britain for 

 periods of great excentricity and winter in aphelion. So, for 

 example, it was, according to him, — 6°*3 F. 850,000 years 

 ago, when the excentricity was 0*0747; l°-3 F. 210,000 years 

 ago, excentricity 0*0575, &c. The temperatures of Great 

 Britain are evidently given only as an illustration ; and there 

 is no doubt that a similar decrease of temperature was expe- 

 rienced, according to him, even in the midst of the Atlantic 

 Ocean. Admitting even the present mean winter tempera- 

 tures there to be 6° F. higher than in Great Britain at the 

 same latitude, we should then have, at the highest excentricity, 

 with winter in aphelion — 0°'3 F. and 4°'7 F. 210,000 years 

 ago ; i. e. temperatures which were possible only if the ocean 

 was covered with solid ice — which is an impossibility with any- 

 thing like the present geographical conditions ; and Dr. Croll 

 repeatedly admits that they have not changed since the glacial 

 epochs. 



It is exceedingly strange that Dr. Croll has not tried how 

 his method works when applied to the existing mean tempe- 

 ratures of different latitudes. The mean temperatures at in- 

 tervals of 10° have been calculated by different scientists. I 

 use the calculation of Ferrel as the most recent. The mean 

 temperature of January may be regarded as the result of the 

 position of the earth towards the sun at the winter solstice. 

 Thus, if Ave compare the mean January temperature of 50° 

 and 60° N., we have 



50° K, 21-3, 60° N., 1*7. 



The quantity of solar heat received at the winter solstice at 

 the 60th parallel N. is but 0"35 of that at the 50th parallel K 

 Thus, if the temperature of the former was less in proportion 

 to the quantity of solar heat received, it should be = — 147°* 9 F. 

 It is easy to see how large the discrepancy is. 



Dr. Croll ascribes the relatively small decrease of tempera- 

 ture with latitude to the influence of ocean-currents, which 

 abstract warm water from the tropics and bring it to high 

 latitudes. But on the 60° N. there is considerably more land 

 than on the 50° N., and air over the land under 60° N. is 

 colder than over the sea. 



But to be quite sure of getting beyond the influence of 



