60 DISCUSSIONS IN CLIMATOLOGY. 



The reason of this is obvious. Nearly all the land is 

 in the northern hemisphere, while the southern 

 hemisphere is for the most part water. The surface 

 of the northern or land-hemisphere, for reasons which 

 have been discussed in the last chapter, becomes 

 heated in summer and cooled in winter to a far greater 

 extent than the surface of the southern or water 

 hemisphere. Consequently, when we add the July or 

 midsummer temperature of the northern to the July 

 temperature of the southern hemisphere, we must get 

 a higher number than when we add the January or 

 midwinter temperature of the former to the January 

 temperature of the latter. For example, the mean 

 July temperature of the northern hemisphere, 

 according to Dove ("Distribution of Heat on the 

 Surface of the Globe") is 70 o, 9, and that of the 

 southern hemisphere 53°*6 ; add the two together and 

 we have 124°'5, which gives a mean for both hemi- 

 spheres of 62 0, 3. The mean January temperature of 

 the northern hemisphere is 48 0, 9, which, added to 

 59°'5, the mean January temperature of the southern 

 hemisphere, gives only 108 o, 4, or a mean of 54 0, 2. 

 Consequently the air over the surface of the globe is 

 hotter in July by 8° than in January, notwithstanding 

 the effects of eccentricity. It is obvious that, were it 

 not for the counteracting effects of eccentricity, the 

 difference would be much greater. Ten thousand years 

 ago, when eccentricity and the distribution of land 

 and water combined to produce the same effect, the 

 difference must have been far greater than 8°. 



But it will be asked, How can this affect the air 

 over the equator, which is not situated more on the 

 one hemisphere than on the other ? It is true that 

 those causes have but little direct effect on the air at 

 the equator, but indirectly they have a very powerful 



