lowing along plains, as well as through valleys, have naturally their beds in the lowest part of the ground comprised between the opposite hills and mountains; nevertheless, the surface of the water of a river in the midst of a plain is often higher than the surface of the grounds adjacent to the banks of the river. This proceeds from the continual subsiding of the mud, &c. brought down by the stream during floods; the waters in that case usually overflowing the banks spread themselves over the plain, where they lose a great part of the swiftness of their current, which contributes greatly to the subsiding of the mud they contain; so that the farther they flow upon the plain, the clearer they grow, and the less remains to subside. From hence the greatest precipitation of mud must be in the parts of the plain nearest the sides of the river, which in length of time will raise these grounds above the rest of the plain. Again, the waters in the bed itself de-

potting incessantly a part of the mud, &c. brought down by the stream, must continually, though infrequently (for a long space of time), raise the channel and banks of the river above the rest of the plain. These causes may at last contribute to the forming of an entire new bed for the river: for as all rivers carry down in their streams more or less mud and other heterogeneous matters, which do not subside regularly in all parts alike, but must precipitate fastest where the current is slowest; there must accumulate by little and little in these parts, such banks of sand and mud as will in time hinder the current of the waters, make them reflow, and at last totally change their direction.

Canals are still more subject than rivers to have their beds raised and their currents stopped by the subsiding of mud and heterogeneous matter in different places, and especially just above their sluices; because of the sudden stagnation of the water which first begins there as often as the sluices are shut; and as there is a necessity for keeping them for the most part shut, the stagnating waters in their beds must precipitate their mud, &c. in a much greater proportion than can be done in the currents of rivers which are in a continual motion towards the sea.

Mr Mann calls centre of the current, or, more properly, line of greatest current, that line which passes through all the sections of a river, in the point where the velocity of the current is the greatest of all. If the current of a river is regular, and in a right line, its centre or line of greatest velocity will be precisely in the centre of the sections: but, on the contrary, if the bed is irregular and full of turns and windings, the centre, or line of greatest current, will likewise be irregular, and often change its distance and direction with regard to the centres of the sections through which the waters flow, approaching successively, and more or less, to all parts of the bed, but always in proportion and conformably to the irregularities in the bed itself.

This deviation of the line of greatest current from the centres of the sections through which it passes, is a cause of many and great changes in the beds of rivers, such as the following:

1stly, In a straight and regular bed, the greatest corrosion of the current will be in the middle of the bottom of the bed; because it is that part which is nearest to the line of greatest current, and at the same time which is most acted upon by the perpendicular compression of the water. In this case, whatever matters are carried off from the bottom will be thrown, by the force of the current, equally towards the two sides, where the velocity of the stream is the least in the whole section.

2ndly, If the bed is irregular and winding, the line of greatest current will be thrown towards one side of the river, where its greatest force will be exerted in proportion to the local causes which turn it aside: in short turns of a river there will be a gyration, or turning round of the stream, by reason of its heating against the outer side of the angle; this part will be corroded away, and the bottom near it excavated to a great depth. The matters so carried off will be thrown against the opposite bank of the river where the current is the least, and produce a new ground, called an alluvion.

3rdly, Inequalities at the bottom of a river retain and di- diminish the velocity of the water, and sometimes may be so great as to make them reflow: all these effects contribute to the subsiding of sand, earth, and other matters therein, which cease not to augment the volume of the obstructions themselves, and produce shallows and banks in the channel. These in time, and by a continuance of the causes, may become islands, and so produce great and permanent changes and irregularities in the beds of rivers.

4thly, The percussions of the centre of the current against the sides of the bed are so much the greater as they are made under a greater angle of incidence; from whence it follows, that the force of percussion, and the quantity of corrosion and of detriment done to the banks and weirs of rivers, and to the walls of buildings made therein, and which are exposed to that percussion, are always in a direct compound proportion of the angle of incidence, of the greatness and depth of the section together, and of the quantity of velocity of the current.

5thly, It may happen in time, that the excavation of the bottom, and the corrosion of the sides, will have so changed the form of the bed as to bring the force of percussion into equilibrium with the velocity and direction of the current; in that case, all farther corrosion and excavation of the bed ceases.

6thly, This gives the reason why when one river falls into another almost in a perpendicular direction, and makes with it too great an angle of incidence, this direction is changed in time, by corrosions and alluvions, into an angle much more acute, till the whole comes into equilibrium.

7thly, So great and such continued irregularities, from local causes, may happen in the motion of a river as will entirely change its ancient bed, corrode thro' the banks where they are exposed to the greatest violence of percussion of the stream, and open new beds in grounds lower than what the old one is become.

8thly, Hereupon the state of the old bed will entirely depend on the quantity of water, and on the velocity and direction of the current in the new one; for immediately after this division of the waters into two beds is made, the velocity of the current in the old one will be diminished in proportion to its less depth. In consequence thereof, the waters therein will precipitate more of their mud, &c. in equal spaces than they did before; which will more and more raise up the bottom, sometimes even till it becomes equal with the surface of the stream. In this case, all the water of the river will pass into the new bed, and the old one will remain entirely dry. It is well known, that this has happened to the Rhine near Leyden, and to many other rivers.

9thly, Hence the cause of the formation of the new branches and mouth, whereby many great rivers discharge their waters into the sea.

But in proportion as a river that has none of these obstructions in its bed, approaches towards its mouth, we see the velocity of its current augment, at the same time that the declivity of the bed diminishes, the causes of which have been explained above. It is for this reason that inundations are more frequent and considerable, and do more damage in the interior parts of a country, than towards the mouths of most rivers.

In the Po, for example, the height of the banks made to keep in the waters, diminishes as the river approaches to the sea. At Ferrara, they are 20 feet high; whereas, nearer the sea, they do not exceed 10 or 12 feet, although the channel of the river is not larger in one place than in the other.

The mouths of rivers, by which they discharge their waters into the sea, are liable to great variations, which produce many changes in them.

1st, The velocity and direction of the current at these mouths are in a continual variation, caused by the tides, which alternately retard and accelerate the stream.

2ndly, During the flowing of the tide, the current of the river is first stopped, then turned into a direction entirely contrary throughout a considerable extent: if we may believe M. de Buffon, there are rivers in which the effect of the tides is sensible at 150 or 200 leagues from the sea.

3rdly, This state of things is a cause of a great quantity of sand, mud, &c. being precipitated and accumulated in the channel near the mouth. This continually raises and widens the bed, and at last changes it entirely into a new place, or at least opens new mouths to discharge the waters at. The Rhine, the Danube, the Volga, the Indus, the Ganges, the Nile, the Mississippi, and many other rivers, are instances of this.

4thly, All these effects are less sensible at the mouths of little rivers, as their currents oppose no sensible obstacle to the flowing of the tides; so that the ebb carries off again what the flow had brought in.

Whenever the course of a river throughout a considerable extent of country, approaches towards a right line, its current will have a very great rapidity; and the velocity wherewith it runs diminishing the effect of its natural gravitation, the middle of the current will rise up, and the surface of the river will form a convex curve of sufficient elevation to be perceived by the eye; the highest point of this curve is always directly above the line of greatest current in the stream.

On the contrary, when rivers approach near enough to their mouths for a sensible effect to be produced in them by the flowing of the tides; and also, when in other parts of their course they meet with obstacles at the sides of their channel; in both these cases the surface of the water at the sides of the current is higher than in the middle, even though the stream be rapid. In this situation of things, the surface of the river forms a concave curve, the lowest point of which, or that of inflection, is directly over the line of greatest current. The reason thereof is, that there are in this case two different and opposite currents in the river; that whereby the waters flow towards the sea, and preserve their motion therein even to a considerable distance; and that of the waters which re-mount, either by the flowing of the tide, or by their meeting with local obstructions, which form a counter current, so much the more sensible as the flowing of the tide is stronger, or as the percussion of the water is made against greater obstructions, and in a direction nearer to a perpendicular to them. From both these causes, the greater of which by far is that of the tides, the water near the sides of the channel, where the velocity of the descending stream is naturally the least, takes a contrary direction, and runs back in the river, while that that in the middle continues to flow on towards the sea. This counter current is what the French call a remous.

An island in the middle of a river produces the same effect as obstructions at the sides, regard being had to the difference of situation of each.

Eddies and whirlpools in rivers, in the centre of which there appears a conical or spiral cavity, and about which the water turns with great rapidity and sucks in whatever approaches it, proceed in general from the mutual percussion of these two counter currents; and the vacancy in the middle is produced by the action of the centrifugal force, whereby the water endeavours to recede, in a direct ratio of its velocity, from the centre about which it moves.

If rivers persevered always nearly in the same state, the best means of diminishing the velocity of the current, when it is found too great for the purposes of navigation, would be by widening the canal; but as all rivers are subject to frequent increase and diminution, and consequently to very different degrees of velocity and force in the current, this method is liable to produce very detrimental effects; for, when the waters are low, if the channel is very large in proportion, the stream will excavate a particular bed, which, according to the irregularities of the bottom, will form various turnings and windings with regard to the principal bed; and, when the waters come to increase, they will follow, to a certain degree, the directions which the bottom waters take in this particular bed, and thereby will strike against the sides of the channel, so as to destroy the banks and cause great damages.

It would be possible to prevent in part the bad effects proceeding from the current striking against the banks, by opening, at those places where it strikes, little gulps into the land, dug in such a form and direction as that the striking current should enter and circulate therein, so as to destroy, or at least greatly diminish, its velocity. This effect would be felt for a considerable way down the river.

This same method might probably be used with success against the destruction of bridges, weirs, &c., by the violence of the stream during floods. Such gulps being dug into the outer-side of those turnings in the river which are immediately above the place to be secured from the violence of the stream, would successively diminish its velocity, its force and dangerous effects, a considerable way down the river. It is true, this method might contribute to produce an overflowing of the river upon the grounds adjacent to those artificial gulps, this being a natural consequence of the decrease of the velocity of the current in those places; and it would remain to be considered whether those local inundations, or the danger of destruction of the bridges or edifices in the river, were the lesser evil.

The nature of inundations, and the manner of their formation, merit a particular attention in this place.

While the volume of water in the bed of a river increases, the velocity of the current increases in proportion; but from the moment that part of this water overflows the bed, the velocity thereof begins to diminish, and does so more and more, the farther it flows and spreads on the plain. So that the overflowing being once begun, it is a natural consequence, that the inundation should continue for several days; for though the volume of water brought down by the flood during that time should decrease, yet, as the quantity of what runs off decreases likewise, from the great decrease of velocity in what overflows the plains, it will continue to produce the same effect as if the volume of water coming down had not diminished, until the whole of the stream be everywhere contained again within the bed of the river. When that is become the case, the waters that have overflowed the plain will decrease theron, by gradually and slowly running off, and also by evaporation, till they wholly disappear. If this was not so, we should see rivers overflow for an hour or two, and then return again within their beds, a thing contrary to general observation; for we constantly see inundations, once begun in flat countries, last for several days together, although in the mean while the rain ceases, and the quantity of water coming down diminishes. This must be so, because as the overflowing diminishes the velocity, and consequently the quantity of water carried off, it has the same effect as if a greater quantity still continued to come down.

It may not be useless to remark here, that if the wind blows directly contrary to the current of the river, the overflowing will be greater than it would have been otherwise, because this accident diminishes the velocity of the stream; but, on the contrary, if the winds blow in the same direction with the current of the river, the inundation will be less than otherwise, and sooner at an end; because this accidental cause augments the velocity of the stream.

River Water. This is generally much softer and better accommodated to economical purposes than spring-water. For though rivers proceed originally from springs, yet, by their rapid motion, and by being exposed during a long course to the influence of the sun and air, the earthy and metallic salts which they contain are decomposed, the acid flies off, and the terrestrial parts precipitate to the bottom. Rivers are also rendered softer by the vast quantity of rain-water, which, passing along the surface of the earth, is conveyed into their channels. But all rivers carry with them a great deal of mud and other impurities; and, when they flow near large and populous towns, they become impregnated with a number of heterogeneous substances, in which state the water is certainly unfit for the purposes of life; yet, by remaining for some time at rest, all the feculencies subside, and the water becomes sufficiently pure and potable.