THE ATMOSPHERE. 85 



160. To distribute moisture over the surface of the earth, and 

 to temper the climate of different latitudes, it would seem, are 

 two great offices assigned by their Creator to the ocean and the 

 air. 



161. When the northeast and southeast trades meet and pro- 

 duce the equatorial calms (§ 135), the air, by the time it reaches 

 this calm belt, is heavily laden with moisture, for in each hemi- 

 sphere it has traveled obliquely over a large space of the ocean. 

 It has no room for escape but in the upward direction (§ 136). It 

 expands as it ascends, and becomes cooler ; a portion of its vapor 

 is thus condensed, and comes down in the shape of rain. There- 

 fore it is that, under these calms, we have a region of constant 

 precipitation. Old sailors tell us of such dead calms of long con- 

 tinuance here, of such heavy and constant rains, that they have 

 scooped up fresh water from the surface of the sea. 



162. The conditions to which this air is exposed here under 

 the equator are probably not such as to cause it to precipitate all 

 the moisture that it has taken up in its long sweep across the 

 waters. Let us see what becomes of the rest ; for Nature, in her 

 economy, permits nothing to be taken away from the earth which 

 is not to be restored to it again in some form, and at some time 

 or other. 



163. Consider the great rivers — the Amazon and the Missis- 

 sippi, for example. We see them day after day, and year after 

 year, discharging immense volumes of water into the ocean. 



"All the rivers run into the sea, yet the sea is not full." — EccL, 

 i., 7. Where do the waters so discharged go, and where do they 

 come from ? They come from their sources, you will say. But 

 whence are their sources supplied ? for, unless what the fountain 

 sends forth be returned to it again, it will fail and be dry. 



164. We see simply, in the ^waters that are discharged by 

 these rivers, the amount by which the precipitation exceeds the 

 evaporation throughout the whole extent of valley drained by 

 them ; and by precipitation I mean the total amount of water that 

 falls from, or is deposited by the atmosphere, whether as dew, 

 rain, hail, or snow. 



165. The springs of these rivers (§ 112) are supplied ft-om the 



