196 DISCUSSION OF THE DESICCATION QUESTION. 



utmost importance in its relations to the entire precipitation of the earth will 

 be readily admitted, although it would probably be difficult to give a numer- 

 ical form to the result which, as we know, must necessarily follow any varia- 

 tion of this relation. So too it is plain enough that the distribution of rain 

 and snow is largely dependent on the size and relative po.sition of the land 

 masses. An examination of any good rain-chart of the world will show this 

 at once ; and it will not require much study to enable the inquirer to per- 

 ceive that there must be other important conditions which are influential in 

 bringing about that irregularity in the precipitation to which reference has 

 already been made. In fact, it is apparent enough that the distribution 

 of the rain-fall, or the manner in which the evaporated moisture is caused 

 to return to the earth's surface, is dependent on quite a complex series of 

 conditions. 



To condense the moisture of the atmosphere, so that it may fall as rain 

 or snow, cold is required. Bodies of air must be in some way cooled down 

 below the temperature necessary to enable them to retain, in the form of 

 vapor, the moisture with which they are charged. This cooling is effected 

 by the lifting up of the moist warm air into the upper, colder regions, which 

 ordinarily happens in one or the other of two different Avays : either currents 

 moving in opposite directions come in conflict with each other and the mass 

 of air is forced upwards, aided by the ascensional power given by added 

 heat ; or, on the other hand, a stratum of moving air impinges against a 

 mountain slope, and is thus mechanically forced upward, until it becomes 

 cooled to a sufficient degree, and lets fall the moistui-e with which it is 

 charged. 



The ordinary succession of events in the tropical belt of calms, or the 

 region in which the conflicting currents of the trade-winds meet and neu- 

 tralize each other, illustrates perfectly the manner in which precipitation is 

 brought about in tlie equatorial region, in a way which is hardly at all de- 

 pendent ou the form or even the existence of land areas. It is especially 

 over the broad expanse of the Pacific that we see in perfection what may 

 be called the normal tropical rain-fall, tlie phenomena of which are so well 

 known as not to require description. This belt of constant rains is, however, 

 of no great width, and it moves with the sun across the equator; so that, on 

 its extreme northern and southern limits, there is only one rainy and one 

 dry season in each year, while, in the intermediate portions of the belt, there 

 are two alternations of dry and wet weather. 



