RIVERS. 77 



about half a mile. This body of water rushes at first for about 300 feet, 

 over a slope at an angle of 45 degrees in a sheet of white foam, and is then 

 precipitated to the depth of 850 feet more into a black abyss, with a 

 thundering noise. It has, therefore, a depth of 1150 feet; in the rainy 

 season the river appears to be about 30 feet in depth at the fall; in the 

 dry season it is much lower, and it is divided into three cascades of varied 

 beauty and astonishing grandeur, but the smaller streams are almost 

 dissipated in vapour before they reach the bottom. The number of 

 considerable rivers, which fall into the sea in different parts of the old 

 continent, is 430, and those of the new continent 140; but this seeming 

 disproportion is amply compensated by the vast dimensions of the latter. 

 Several attempts have been made by philosophers, to compute the quantity 

 of water which rivers discharge into the ocean, but it is a problem 

 scarcely admitting of any solution, beyond a probable approximation. 

 From the observations of Father Biccioli, on the discharge of the river 

 Po, it has been calculated, that the water poured out by all the rivers of 

 the globe is equal to 41 cubic miles daily, or 14,965 cubic miles annually; 

 this computation is probably too high. The greatest estimate of the 

 mean annual fall of rain, of dew, &c., is not above 34 inches for the whole 

 earth, and the superfices of the globe being 196,816,658 square miles, 

 this will afford a mass of water equal to 105,614 cubic miles, as the whole 

 that is precipitated on the earth. But from this must bededucted all 

 that falls on the ocean, which, if we consider the precipitation in the 

 ratio of their surfaces, will giv 38,271 cubic miles, as the water that 

 falls on the land ; of this quantity, however, at least two thrids are expended 

 in irrigating the soil, or in sustaining vegetable life, and are restored to 

 the atmosphere, by the process of evaporation; so that no more than 

 12,757 cubic miles will become the annual tribute of the rivers of the 

 globe to the ocean. This estimate differs less from that founded on the 

 tables of Cotte, than some later speculations on this subject, in which 

 we suspect that the mean annual rain and the quantity returned by the 

 rivers to the sea are considerably underrated. The length of the course 

 of 22 of the principal rivers, and the area of their respective domains or 

 basins, were calculated with much care, and given in a tabular form in 

 the Essay on Physical Geography, in the last edition of the Encyclopaedia 

 Britannica. 



