, Dr. CroWs Theory of Glacial and Warm Periods. 89 



-emphatically, is due to an entirely different cause, namely, 

 the fact that the equator of heat of the earth is situated to 

 the north of the real equator. I will on this point quote the 

 lucid notice of Reclus : " The complete system of the 

 winds," he says, " inclines towards the northern hemisphere, 

 and it is to the north of the line that the girdle of the equa- 

 torial calms is in all seasons developed. This phenomenon, 

 which might seem at first sight strange, results from the 

 grouping of the greater part of the continental lands in the 

 northern hemisphere, and from the difference of temperature, 

 which must be, for our part of the world, the result of this 

 unequal distribution of solid and liquid. It is also in the 

 northern hemisphere that we find the Desert of Sahara, 

 that immense extent where wooded tracts are relatively few, 

 and where the reflection from the burning sands and rocks 

 vaporizes the clouds which the atmospheric currents bring. 

 The Sahara, and, in a less degree, all the tropical countries 

 of the northern hemisphere, act as a great centre, towards 

 which the aerial masses flow. It results from the tables 

 drawn up by Dove that the mean temperature of the year 

 is more elevated (92 Fahr.) towards the 10th degree N.L., 

 than it is at the equator itself (91*5 F.) while the mean 

 balance is stronger towards the 20th degree of Lat. (94 

 Fahr.) than in any other region of the world." Reclus adds : 

 " The high temperature of the continents thus forces the 

 southern winds to encroach upon the northern systems." 

 (Reclus, The Ocean : translated by Woodward, I., 243-4.) 

 This northern position of the equator of heat is not only the 

 mean of its several movements, but we shall find, as a matter of 

 fact, that it never goes south of the equator at all. "When 

 the sun," says Reclus, "after the 21st of September, crosses 

 the equatorial line to tend towards the tropic of Cancer, the 

 centre of the trade winds, and consequently of the band of 

 calms, moves at the same time towards the north ; on the 

 contrary, when the sun returns to the tropic of Capricorn, 



