grove,

ferved, that fprings are rarely, if ever, on the fummits of mountains, but on their fides, a little way down, thereby giving the waters room to collect; that the greatest rivers originate from the higheft, mountains, they attracting, stopping, and condenfing the clouds, which, yielding to them their contents, furnish fources for the moft copious ftreams; that where there are moft clouds, are the highest mountains, the most rapid torrents, the greatest, and the longest rivers. It seems further fupported, by the peculiarity attending that immenfe ridge of mountains the Andes: on one fide, they fearce know rain; a few clouds, now and then, are their diftant vifitants--here, rife gentle brooks, rilis, and rivulets, which meander through the and ornament the plain. But on the other fide the Andes, prodigious maffes of clouds are perpetually forming, beating against the rocks, befieging, as it were, this barrier, which forbids their paffage, and tormenting, with ftorms, this boundary placed by Nature. What is the confequence? At many hundred miles from the fea, rife thofe floods which are rivers at their origin, and which receiving many like themselves, roll on, in one united deluge, to the ocean. While, therefore, this cause feems adequate to the purpofe, we fhall not incline to that theory, which fuppofes fubterraneous waters furnished by the fea to be raifed into fteam, and fo converted into fprings : fince it most fails where moft wanted, i. e. in accounting for the origin of the greatest rivers, which are always farthest from the fea, and moft highly elevated.

It is not uncommon, elfewhere, for rivers which rife near

each

cach other, to iffue widely different; and in our own country, the fprings which furnish the Thames and the Severn, rise at no very great distance from the fame place.

With regard to the nature of rivers, we obferve, that in whatever direction the ridge of the mountain runs which furnifhes the ftream, the ftream itself takes an oppofite courfe ; never following the bearing of the land from whence it iffues. As to the ufual laws of motion in rivers, they are chiefly as follow:-[Vide Guglielmini del fiumi.]

All rivers have their fources either in mountains, or elevated lakes; and in their defcent from thefe, they acquire that velocity which maintains their future currents. At first their courfe is generally rapid and headlong; but is retarded in its journey by the continual friction against its banks, by the many obftacles it meets to divert its ftream, and by the plains generally becoming more level as it approaches towards the fea.

If this acquired velocity be quite spent, and the plain through which the river paffes be entirely level, it will, notwithstanding, ftill continue to run, from the perpendicular preffure of the water, which is always in exact proportion to the depth. This perpendicular preffure is nothing more than the weight of the upper waters preffing the lower out of their places, and, confequently, driving them forward, as they cannot recede against the ftream. As this preffure is greateft in the deepest parts of the river, fo we generally find the middle of the ftream moft rapid; both because it has the

greatest

greatest motion thus communicated by the preffure, and feweft obftructions from the banks on either fide.

Rivers thus fet into motion almost always make their own beds. Where they find the bed elevated, they wear its fubftance away, and deposit the fediment in the next hollow, fo as in time to make the bottom of their channels even. On the other hand, the water is continually abrading and eating away the banks on each fide; and this with more force, as the current happens to strike more directly against them. By thefe means, it always has a tendency to render them more strait and parallel to its own courfe. Thus it continues to rectify its banks, and enlarge its bed; and, confe quently, 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 us that bounds are thus put to the erosion of the earth by water; and that we find all rivers only dig and widen themfelves but to a certain degree.

In those plains and large vallies where great rivers flow, the bed of the river is ufually lower than any part of the valley. But it often happens, that the furface of the water is higher than many of the grounds that are adjacent to the banks of the ftream. If, after inundations, we take a view of fome rivers, we fhall find their banks appear above water, at a time that all the adjacent valley is overflown. This proceeds from the frequent depofition of mud, and fuch like fubftances, upon the banks, by the rivers frequently overflowing;

and

and thus, by degrees, they become elevated above the plain ; and the water is often feen higher alfo.

Rivers, as every body has feen, are always broadeft at the mouth; and grow narrower towards their fource. But what is lefs known, and probably more deferving curiosity, is, that they run in a more direct channel as they immediately leave their fources; and that their finuofities and turnings become more numerous as they proceed. It is a certain fign among the favages of North America, that they are near the fea, when they find the rivers winding, and every now and then changing their direction. And this is even now become an indication to the Europeans themfelves, in their journies. through thofe tracklefs forefts. As thofe finuofities, therefore, increafe as the river approaches the fea, it is not to be wondered at, that they fometimes divide, and thus difembogue by different channels. The Danube difembogues into the Euxine by seven mouths; the Nile, into the Mediterranean, by the fame number; and the Wolga, into the Cafpian, by feventy. It often happens that the ftream of a river is oppofed by one of its jutting banks, by an island in the midft, the arches of a bridge, or fome fuch obftacle. This produces, not unfrequently, a back current; and the stream having paffed the arch with great velocity, pufhes the water on each fide of its direct current. This produces a fide current, tending to the bank; and not unfrequently a whirlpool; in which a large body of waters is circulated in a kind of cavity, finking down in the middle. The central point of the whirlpool is always loweft, because it has leaft motion: the other parts

are fupported, in fome measure, by the violence of theirs and, confequently, rife higher as their motion is greater; fo that towards the extremity of the whirlpool must be higher than towards the centre.

It might be supposed that bridges, dams, and other obftacles in the current of a river, would retard its velocity. But the difference they make is very inconfiderable. The water, by thefe ftoppages, gets an elevation above the object; which, when it has furmounted, gives a velocity that recompenfes the former delay: Iflands and turnings alfo retard the course of the stream but very inconsiderably. Any cause which diminishes the quantity of water, moft fenfibly diminishes the force and velocity of the ftream.

The rivers run to the ocean; to the ocean we now accompany them: it is among the fublimeft objects of Nature. So lemn and grand, if calm, smooth, and ferene, it plays its variously refracted tints of changing greens, and blues, and greys, as the light falls upon it, or fhadows of clouds, &c. or if it be agitated by furious winds, and roll its buoyant eddies forward to the fhore, roaring as they advance, hiffing as they retire, feeming ready to overwhelm the very earth, yet yielding and changing their direction at every obstacle.-It fills the mind with fenfation! Human skill has availed itself of this element alfo, proud and licentious though it be, fickle and unstable, wild and unreftrainable, yet we plow the waters, and we navigate the ocean. It is, perhaps, the greatest of human triumphs! a ftriking demonstration of the superi ority of reafon, which chooíes the beft adapted means to ac

complish