GEOGRAPHICAL DISTRIBUTION — CLIMATOLOGY. 99 



in Africa, the Sahara and Kalahari deserts north and south of the 

 equatorial zone. In the western section there are — the arid region 

 of northern Mexico in North America ; the almost treeless regions 

 known as the savannahs of Venezuela and Guiana, the Campos of 

 Brazil and the Pampas of Bolivia and Argentina. Besides these larger 

 tracts, there are many other places where, owing to local causes, the 

 tropical rains are either intercepted or so greatly reduced in quantity 

 that orchid life cannot exist. In all these tracts the atmosphere is 

 not only almost always dry, but the daily thermometric range is also 

 too great to admit of any but the scantiest vegetation to exist and 

 that of certain types only. 



The aggregate area of these dry and arid tracts is probably not 

 less than one-half of the whole region, which reduces the actual 

 area over which the tropical orchids are spread to about one-fourth 

 of the land surface of the globe. 



The general climatic phenomena of the region are dependent on 

 the vertical position of the sun in respect to the earth ; but the 

 sun does not remain vertical over the same parallel of latitude owing 

 to the obliquity of the earth's axis to the plane of the ecliptic, the 

 gi-eat circle that traces the annual course of the sun in the heavens ; 

 the limits of the sun's annual excursion on each side of the equator 

 are indicated by the tropics, which are nearly 23^ degrees north 

 and south of it. The sun is thence north of the equator one half 

 of the year and south of it the other half. Now where the sun 

 is vertical its heating power is greatest, and there accordingly the 

 aerial currents known as the trade winds originate, evaporation is most 

 rapid and the precipitation of rain the greatest. The heated air as 

 it ascends is accompanied by the vapour raised by evaporation and 

 which is lighter than the ascending air ; both expand as they 

 ascend and both part with a portion of the heat with which they 

 were first charged until the vapour is sufficiently chilled to be 

 precipitated first as cloud, then as rain. The parallel over which 

 the sun is vertical with a narrow space on each side of it is known 

 as "the i-egion of calms." Of course this belt shifts with the 

 annual course of the sun, and is thence at and near the equator 

 twice in the year ; and hence it is that the equatorial climate is 

 more equable than in othei- parts of the region ; the variations in 

 temperature, both annual and diurnal, are least and the rainfall is 

 most regular and continuous. 



