8* A HISTORY OF 



new Channel, but spreads over the adjacent land. " Thus," says hr v 

 *' mm are obliged to direct its course ; or, otherwise, Nature vvould 

 never have found one." He enumerates many rivers that art; cer- 

 tainly known, from history, to have been dug by men. He alleges, 

 that no salt-water rivers are found, because men did not want salt- 

 water ; and as for salt, that was procurable at a less expense than dig- 

 ging a river for it. However, it costs a speculative man but a small 

 expense of thinking to form such a hypothesis. It may, perhaps, en- 

 gross the reader's patience to detain him longer upon it. 



Nevertheless, though Philosophy be thus ignorant, as to the pro- 

 duction of rivers, yet the laws of their motion, and the nature of their 

 currents, have been very well explained. The Italians have particu- 

 larly distinguished themselves in this respect ; and it is chiefly to them 

 that we are indebted for the improvement.* 



All rivers have their source either in mountains, or elevated lakes ; 

 and it is in their descent from these that they acquire that velocity 

 which maintains their future current. At first their course is gene- 

 rally rapid and headlong ; but it is retarded in its journey, by the 

 continual friction against its banks, by the many obstacles it meets to 

 divert its stream, and by the plains generally becoming more level as 

 it approaches towards the sea. 



If this acquired velocity be quite spent, and the plain through whicb 

 the river passes is entirely level, it will, notwithstanding, still continue 

 to run from the perpendicular pressure of the water, which is always 

 in exact proportion to the depth. This perpendicular pressure is 

 nothing more than the weight of the upper waters pressing the lower 

 out of their places, and, consequently, driving them forward, as they 

 cannot recede against the stream. As this pressure is greatest in the 

 deepest parts of the river, so we generally find the middle of the 

 stream most rapid ; both because it has the greatest motion thus com- 

 municated by the pressure, and the fewest obstructions from the banks 

 an either side. 



Rivers thus set into motion are almost always found to make their 

 own beds. Where they find the bed elevated, they wear its substance 

 away, and deposit the sediment in the next hollow, so as in time to 

 make the bottom of their channels even. On the other hand, the wa- 

 ter is continually gnawing and eating away the banks on each side ; 

 and this with more force as the current happens to strike more directly 

 against them. By these means it always has a tendency to render 

 them more straight and parallel to its own course. Thus it continues 

 to rectify its banks, and enlarge its bed ; and, consequently to diminish 

 the force of its stream, till there becomes an equilibrium between the 

 force of the water, and the resistance of its banks, upon which both 

 will remain without any further mutation. And it is happy for man 

 that bounds are thus put to the erosion of the earth by water ; and 

 that we find all rivers only dig and widen themselves but to a certain 

 degree.t 



In those plains| and large valleys where great rivers flow, the bed 

 nf the river is usually lower than any part of the valley. li"t t often 



S. Guglielinini della Natura de Fiumi, passim. f Ibid 



I Buffon, de Fleuves, passim, vol. ii. 



