100 PHYSICAL GEOGKAPHY OF THE SEA, AND ITS METEOROLOGY. 



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

 invisible streams back through the air to their sources among the 

 hills (§ 191), and that through channels so regular, certain, and 

 well-defined, that the quantity thus conveyed one year with the 

 other is nearly the same : for that is the quantity which we see run- 

 ning down to the ocean through these rivers ; and the quantity 

 discharged annually by each river is, as far as we can judge, nearly a 

 constant. 



268. We now begin to conceive what a powerful machine the 

 Powerful machinery, atmospherc must bc ; and, though it is apparently 

 so capricious and wayward in its movements, here is evidence of 

 order and arrangement which we must admit, and proof which we 

 cannot deny, that it performs this mighty office with regualrity 

 and certainty, and is therefore as obedient a law as is the steam- 

 engine to the will of its builder. It, too, is an engine. The South 

 Seas themselves, in all their vast intertropical extent, are the boiler 

 for it, and the northern hemisphere is its condenser (§ 24). The 

 mechanical power exerted by the air and the sun in lifting water 

 from the earth, in transporting it from one place to another, and in 

 letting it down again, is inconceivably great. The utilitarian who 

 compares the water-power that the Falls of Niagara would afibrd if 

 applied to machinery, is astonished at the number of figures which 

 are required to express its equivalent in horse-power. Yet what is 

 the horse-power of the Niagara, falling a few steps, in comparison 

 with the horse-power that is required to lift up as high as the clouds 

 and let down again all the water that is discharged into the sea, not 

 only by this river, but by all the other rivers and all the rain in 

 the world? The calculation has been made by engineers, and, 

 according to it, the force for maMng and lifting vapom' from each 

 area of one acre that is included on the surface of the earth is equal 

 to the power of 30 horses. 



CHAPTEE Y. 



270-303. RAINS AND RIVERS. 



270. KrvERS are the rain-gauges of nature. The volume oi 

 Sn^£uges-thft?n ^^tcr annually discharged by any river into the 

 largest. ° sca cxpresscs the total amount by which the precipi- 



tation upon the valley drained by such river exceeds the evapora- 

 tion fi'om the_ sam3 valley during the year. There are button 



