80 THE PHYSICAL GEOGRAPHY OF THE SEA. 



over a large space of the ocean. It has no room for escape but 

 in the upward direction {^ 105). It expands as it ascends, and 

 becomes cooler ; a portion of its vapor is thus condensed, and 

 comes down in the shape of rain. Therefore it is that, under 

 these calms, we have a region of constant precipitation. Old 

 sailors tell us of such dead calms of long continuance here, of 

 such heavy and constant rains, that they have scooped up fresh 

 water from the surface of the sea. 



116. 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. 



Consider the great rivers — the Amazon and the Mississippi, for 

 example. We see them day after day, and year after year, dis- 

 charnrinsr an immense volume of water into the ocean. 



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

 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. 



117. We see simply, in the waters that are discharged by these 

 rivers, the amount by w^hich the precipitation exceeds the evapor- 

 ation throughout the whole extent of valley drained by them ; and 

 by precipitation I mean the total amount of w^ater that falls from, 

 or is deposited by the atmosphere, wiiether as dew^, rain, hail, or 

 snow. 



The springs of these rivers (^ 87) are supplied from the rains 

 of heaven, and these rains are formed of vapors which are taken 

 up from the sea, that " it be not full," and carried up to the mount- 

 ains through the air. 



" Note the place whence the rivers come, thither they return 

 again." 



118. Behold now the waters of the Amazon, of the Mississippi, 

 the St. Lawrence, and all the great rivers of America, Europe, 

 and Asia, lifted up by the atmosphere, and flowing in invisible 

 streams back through the air to their sources among the hills 

 (§ 87), and that through channels so regular, certain, and well de- 



