Chap. I.

GEOLOGY.

641

Rivers near their discharge often obtain a much greater velocity than they had at their •commencement, from the increase of their body of water.

The slopes or beds of rivers differ materially in their inclination, and we often findthem cutting deep chasms through lofty mountains, or taking their course through ravinesor fissures caused by some convulsive movement of the earth itself. Where the riverpasses over or through a district of alkaline or calcareous rocks, which are soluble in water,the carbonic acid of the water dissolves them, and wears away by degrees these apparentlyhard and indestructible substances, often acting at the base of a hill or mountain, under-mining them, and masses fall into the torrent, to be carried lower down the next flood.

When two rivers fall into one channel, the height of the water does not increase in pro-portion to the body by which it is augmented; so that when a considerable quantity ofwater is added to a stream, there is only an increase of velocity. If the height of thesections of twenty or more tributary streams were added to that of the trunk they fallinto, this fact would be made evident. When the Mayne, which is more than half thesize of the Rhine , unites with that stream, there is no apparent increase, and where dividedinto two or more channels, its height is not lowered. The Inn falls into the Danube , andboth rivers, nearly equal in size, then pursue one course, without becoming either broaderor deeper.

The Po has no apparent increase after it has received the Secchio and Panaro; and theTiber receiving the Teverone is neither deepened nor widened, and so it is with other rivers.Pliny , in one of his letters to the emperor Trajan , observes that the canal cut by Nerva todraw off'the superfluous waters of the Tiber , did not in any degree prevent the inundationsof which it was the cause. The two sections above and below this canal, called the1'iumicino, are nearly of the same breadth; the depth in the upper is 7 feet 4 inches, andthe whole section is a rectangle; the depth of the lower is 6 feet 8 inches on one side, andIff on the other; but when the areas of the two sections arc accurately computed, theyare found to be similar.

Rivers which carry sands in the lower parts of their beds have less slope than at theupper; their declivity diminishes in proportion to the distance they have run from theirsources; this is caused by the diminution of the size of the particles as they progress, whichconsequently require less force to push them forward, and are borne to the very ex-tremity before they are deposited; where these light substances are floated, less declivityis needed to keep the bed of the stream clear of obstruction. The body of water beingthe same, the slope of the bottom may be said to diminish in proportion as the matterbrought down becomes smaller, and is more easily moved onwards on this account.

Rivers which are increased by their union with others that are less require less fall thanbefore; for if the slopes of all the small streams which unite to form one be measured, itwill be found that their declivity is much greater before than after their junction, so thatthe greater the ordinary body of water in a river, the less will be the slope of its bed.

Whenever the freshes of a tributary stream fall into another, the recipient flows back,depositing its sediment above the mouth, as well as below it, if the assistance which theformer receives from the low waters is insufficient to compensate for the difference of fallwhich the tributary encounters in passing from its own into the common bed.

It appears to be a common law that the greater the quantity of water a river carries, theless will be its fall, and the greater the force of the stream, the less will be the slope of itsbed, and the slope of the bottom of rivers diminishes in proportion, as the body of water isincreased.

Tacitus relates in his Annals , that when a proposition was made to the Roman senate todivert into other channels all the rivers which flow-ed into the Tiber , the opinion of Pisowas followed ; he advised that no alteration should be made, since every one might see thatnature -knew how to provide for her wants much better than could be done by art; sheassigned to rivers their sources, boundaries, and limits the most suitable. Nature, however,exhibits at times singular phenomena, where rivers discharge themselves into the sea, byspreading their waters over its surface. At a considerable distance from the land, theyoften run over a bottom having a very small declivity, but which at the mouth of the riveris bent downwards, forming a deep concavity; this is the case with some of the largestrivers, where the tides are apparent at a considerable distance up them.

The sea-water during the time of flood-tide, entering the river, and at ebb returning,helps to produce this effect, and to render the section of the bed a concave line, by sweepingaway all the deposits lodged where this action takes place; so that shoals are not formed solong as a river can keep its mouth open on a flat shore, the particles brought down beingdeposited either above or below the spot where they discharge themselves.

Deltas at the mouth of rivers arise from the deposit of the detritus they carry intheir course ; this in time rises to the surface, forms a bar, turning the river into anotherchannel. On a flat coast there is usually the most deposit, and sometimes what is broughtdown is carried into a current in the ocean, and afterwards deposited on some other coast,

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