398 NOTES OF A- NATURALIST. 



receives one-half of the amount of solar radiation which reaches 

 the earth, and in addition, on Mr. Croll's hypothesis, one twenty- 

 fifth part of that which reaches the southern hemisphere. It 

 follows that the heat available to raise the southern hemisphere 

 above the temperature of space stands to that which is received 

 by the northern hemisphere in the ratio of 24 : 26, and that the 

 mean temperature of the southern hemisphere should be 

 298*5 X i§, or 275*5° above the temperature of space ; so that, 

 in ordinary language, the mean temperature of the southern 

 hemisphere should be 36'5° Fahr. If the fact corresponded with 

 this result of theory, it would not be necessary to invoke in- 

 creased eccentricity of the earth's orbit to account for the 

 extreme cold of one hemisphere, seeing that the actual con- 

 ditions would suffice to completely alter their relative tempera- 

 tures. 



It occurs to me, however, that, on further consideration, Mr. 

 Croll would reduce his estimate of the volume of heated water 

 transferred from the southern to the northern hemisphere ; but 

 even if that estimate were reduced by one-half, we ought to 

 find in the southern hemisphere a mean temperature of 47'8° 

 Fahr., or nearly 12 degrees lower than that of our hemisphere. 



We have already seen that, so far as climate depends on the 

 relative position of the earth and the sun, we ought to find in 

 the southern hemisphere climates of a more extreme character, 

 with hotter summers and colder winters, than those to which 

 we are accustomed. If it be true that through the agency of 

 ocean-currents a considerable amount of heat is transferred to 

 the northern hemisphere, that circumstance might serve to 

 account for the fact that the summers of the southern are not 

 generally hotter than those of the northern hemisphere ; but it 

 would, at the same time, tend to aggravate the severity of the 

 southern winters. 



At the time of the publication of Mr. Croll's earlier memoirs, 

 there existed a general belief that the southern hemisphere was 

 in fact notably cooler than our portion of the globe, and he 

 naturally referred to the supposed fact as harmonizing with the 

 general conclusions drawn by him from theory. But, imperfect 

 as our knowledge of the southern hemisphere still is, a good 

 deal of information has been obtained of late years. The only 

 stations south of the fiftieth degree of latitude from which we 



