THE CLIMATIC ZONES. 37 



placing a general survey of climate in the forefront 

 of our work and in opening this branch of our sub- 

 ject, it may be desirable to commence with a short ex- 

 planation of what are called " The Climatic Zones. " 



The two great factors in the regulation of climates, 

 briefly stated, may be regarded as heat, and moisture. 

 These attain their maximum at the equator. The 

 equatorial regions, therefore, have the highest mean 

 temperature of any part of the earth, and also, as we 

 shall presently show, the heaviest rainfall. Sometimes 

 this zone is spoken of by geographers as the Region 

 of Perpetual Rains, because for some distance on each 

 side of the equator heavy rains occur at short intervals 

 throughout the whole year. Beyond that again, both to 

 the northwards and southwards, we come to regions 

 where the rains become intermittent ; part of the year 

 being dry and hot, and the remainder subject to 

 periodic wet seasons. 



Here we locate other zones, and so on, to denote 

 every well-marked change of climate that occurs, until 

 we reach the Polar regions, of almost perpetual cold. 



It goes almost without saying, however, that there 

 are no fixed limits at which hard and fast lines can 

 be drawn round the earth, marking the exact points 

 where each substantial change of climate occurs. 

 Climates as we know, are variable, so that even in 

 the same district, they differ materially one year with 

 another. So also the boundaries of Climatic Zones, 

 wheresoever they may be placed, are apt to shift from 

 time to time, rendering it impossible to say with 

 certainty exactly where such zones begin or end. A 

 good example of this is to be found in the limits of 

 "The Desert Zone." 



