THE RIVER. 145 



the rotten fens," is one of the most disagreeable things 

 that can well be imagined. But the river carries off all 

 these, and runs pure and limpid; and thus its motion 

 is an instrument as well as an emblem of life. Nor are 

 the advantages confined to the river, while it is a 

 rapid stream winding its way among hills and uplands ; 

 they continue through all its course ; and, in the puri- 

 fication of the air especially, have their full effect when 

 it has sunk down nearly to a level estuary, enjoys the 

 benefit of a tide from the sea, and is useful for the 

 purposes of navigation. 



Take, as an instance, the British metropolis waiving 

 the benefit that its commerce derives from the river, 

 and the utter impossibility of carrying on that com- 

 merce without it. Suppose, for a moment, that nearly 

 a million and a half of human beings, with all their 

 domestic animals, and their fires and furnaces, and other 

 means of contaminating the air, were huddled together 

 on a plain, elevated only a few feet above the level of 

 the sea, and surrounded by marshes, which London 

 once partially was, and always would have been, had it 

 not been for the drainage of the Thames, and the gra^ 

 dual elevation of the banks of that river, by the debris 

 that it is constantly bringing down. It would have 

 been a region of death, instead of the healthy place 

 which, in spite of all its magnitude, it is. The tide in 

 the Thames not only produces a constant current, and 

 therefore change of air, in the direction of the river ; 

 but the sea and the land air that it ultimately brings, 

 occasion, by their difference of temperature, a play 

 of cross wind to and from the hills on the north 

 and south ; and thus the river puts in motion currents, 

 o 



