REPLY TO CRITICS. 61 



influence. The air is continually flowing in to the 

 equatorial regions from both hemispheres. In fact, 

 the air which we find there is derived entirely from 

 the temperate regions. In July we have the northern 

 trades coming from a hemisphere with a mean tem- 

 perature as high as 70 o- 9, and the southern trades 

 coming from a hemisphere with a mean temperature 

 not under 53°, while in January the former trades 

 flow from a hemisphere as low as 50°, and the latter 

 from a hemisphere no higher than 60°. Consequently, 

 the air which the equatorial regions received from the 

 trades must have a higher temperature in July than 

 in January. The northern is the dominant hemisphere; 

 it pours in hot air in July and cold air in January, 

 and this effect is not counterbalanced by the air from 

 the opposite hemisphere. The mean temperature of 

 the air passing into the equatorial regions ought, 

 therefore, to be much higher in July than in January, 

 and this it no doubt would be were it not, let it be 

 observed, for the counteracting effects of eccentricity. 

 The tendency of the present distribution of land and 

 water, when our northern winter occurs in perihelion, 

 is to counteract the effects of eccentricity. But ten 

 thousand years ago, when our winters were in 

 aphelion, that cause would co-operate to intensify 

 the effects of eccentricity. In fact, it would actually 

 more than double the effects then produced by 

 eccentricity. Now if the influence of the present 

 distribution of land and water is so oreat as not 

 merely to counteract but to reverse the effects of 

 eccentricitv to the extent of making the mean tern- 

 perature of the earth 8° warmer in July than in 

 January, it is not surprising that it should be sufficient 

 to make the equatorial regions at least as warm in 

 the former as in the latter period. 



