THE EARTH. 



f>9 



His reasons arc, that when a new spring 

 breaks forth, the water does not make itself 

 a new channel, but spreads over the adjacent 

 land. " Thus," says he, " men are obliged 

 to direct its course ; or, otherwise, Nature 

 would never have found one." He enume- 

 rates many rivers that are certainly 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 less 

 expense than digging a river for it. How- 

 ever, it costs a speculative man but a small 

 expense of thinking to form such an hypothe- 

 sis. It may, perhaps, engross the reader's 

 patience to detain him longer upon it. 



Nevertheless, though Philosophy be thus 

 ignorant, as to the production of rivers, yet 

 the laws of their motion, and the nature of 

 their currents, have been very well explain- 

 ed. The Italians have particularly distin- 

 guished themselves in this respect; and it is 

 chiefly to them that we are indebted for the 

 improvement." 



All rivers have their source either in moun- 

 tains, or elevated lakes; and it is in their 

 descent from these that they acquire that ve- 

 locity which maintains their future current. 

 At first their course is generally 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 which the river passes 

 is entirely level; it will, notwithstanding, 

 still continue to run, from the perpendicular 

 pressure of the water, which is always in ex- 

 act proportion to the depth. This perpen- 

 dicular pressure is nothing more than the 

 weight of the upper waters pressing the low- 

 er out of their places ; and, consequently, 

 driving them forward, as they cannot recede 

 against the stream. As this pressure is great- 

 est in the deepest parts of the river, so we 

 generally find the middle of the stream most 

 rapid ; both because it has the greatest mo- 

 tion thus communicated by the pressure, and 



S. Guglielmiui della Natui-a de Fiumi, passim. 

 " Ibid. 



the fewest obstructions from the banks on 

 either side. 



Rivers thus set into motion are almost al- 

 ways found to make their own beds. Where 

 they find the bed elevated, they wear its sub- 

 stance away, and deposit the sediment in the 

 next hollow, so as in time to make the bot- 

 tom of their channels even. On the other 

 hand, the water 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 ren- 

 der them more straight and parallel to its 

 own course. Thus it continues to rectify its 

 banks, and enlarge its bed ; and, consequent- 

 ly, 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. 1 " 



In those plains" and large valleys where 

 great rivers flow, the bed of the river is usu- 

 ally lower than any part of the valley. But it 

 often happens, that the surface of the water 

 is higher than many of the grounds that are 

 adjacent to the banks of the stream. If, after 

 inundations, we take a view of some rivers, 

 we shall find their banks appear above wa- 

 ter, at a time that all the adjacent valley is 

 overflowed. This proceeds from the frequent 

 deposition of mud, and such like substances, 

 upon the banks, by the rivers frequently 

 overflowing; and thus, by degrees, they be- 

 come elevated above the plain ; and the wa- 

 ter is often seen higher also. 



Rivers, as every one has seen, are always 

 broadest at the mouth, and grow narrower 

 towards their source. But what is less known, 

 and probably more deserving curiosity, is, 

 that they run in a more direct channel as they 

 immediately leave their sources; and that 

 their sinuosities and turnings become more 

 numerous as they proceed. It is a certain 

 sign among the savages of North America, 



c Buflbn, de Fleuves, passim, vol. ii. 



R 



