62 DISCUSSIONS IN CLIMATOLOGY. 



The fact that the equator at present is not hotter 

 when the earth is in perihelion, instead o£ being an 

 objection to the theory that the glacial period was due 

 to an increase of eccentricity, as is supposed by some, 

 is in reality another strong argument in its favour, 

 for it shows that a much less amount of eccentricity 

 would suffice to induce a commencement of glacial 

 conditions in the northern hemisphere than would 

 otherwise be required, were it not for the circum- 

 stances to which reference has been made. This 

 objection, like many others which have been urged 

 against the theory, arises from looking too exclusively 

 at the direct effects of eccentricity. 



There is another cause which must also tend to 

 lower the January and raise the July temperature of 

 the equator, viz., the northern trades pass farther south 

 in January than in July, and consequently cool the 

 equatorial regions more during the former than the 

 latter season. This general tendency of the trades to 

 lower the temperature of the equatorial regions more 

 in January than in July is, of course, subject to modi- 

 fications from the monsoons, the rainy seasons, and 

 other local causes ; nevertheless, so long as the present 

 distribution of land and water endures, so long will 

 eccentricity have a counteracting effect upon the tem- 

 perature of the air at the equator, which but for that 

 would be hotter in July than in January. 



No knowledge whatever as to the intensity of the 

 sun's heat can be obtained from observations on the 

 temperature of the air at the equator. The compara- 

 tively cold air flowing in from the temperate regions 

 has not time to be fully heated by the sun's rays 

 before it rises as an ascending current and returns to 

 the temperate regions from whence it came. More 

 than this, these trades prevent us from being able to 



