Chap. I-

GEOLOGY.

639

and Apennines , and all mountainous regions, are usually covered with snow, so that thereis a perpetual humidity in the loftier regions of the earth, until we arrive at that stationwhere, for the greatest part of the year, the whole is congealed by extreme cold.

The Seine has in the summer not more than 3 feet depth of water, thougli in time ofhigh floods it has been known to rise nearly 23 feet. The Po, in Italy , during floods doesnot increase in breadth, but quadruples its height, so that the quantity of water poureddown in one day is equal to eight times that which flows on ordinary occasions. Theriver Thames drains an area of a little more than 5000 square miles, and taking the averagedepth of rain that falls in a year at 24 inches, it has been computed that 239,765,120,000cubic feet of water annually pass down this great natural drainage into the ocean, to beagain returned by evaporation.

Were it not for floods, the beds of rivers and their banks would undergo little alteration :when, however, they occur, as they do in many districts periodically, the large stones aremoved forward, and rounded by attrition, whilst their debris, and the fine gravels and sandswhose specific gravity varies little from that of the water, are carried along by the forceof the current, and are not deposited till they meet with still water, or are taken out to sea,where the motion of the fresh water is opposed or staid altogether.

Beds of rivers are constantly raised by the stones, gravel and sand brought down at thetime of flood : this is evident, when we take into consideration the enormous quantities ofballast dredged from our rivers, in order to keep their channels open for the purposes ofnavigation. It thus becomes necessary to elevate the banks, which is often continuedbeyond the limits that are beneficial to the drainage of the neighbouring land. When abar or shoal is thrown up, if not cleared away, it acts like a dam or artificial wall, pre-venting first the free passage of the heavy stones, which are consequently deposited untilthe bed is raised to the level of the impediment. In many instances where the foundationsof buildings have been laid considerably above the level of the water, w*e find them nowsunk one or more stories below it, and the whole drainage of a city destroyed by cithernatural impediments in the slope of rivers, or some artificial contrivance to benefitmachinery constructed on its banks. Throughout England the proprietors of mills havebeen suffered to increase their fall, not by dredging the bed of the tail-water, but by raisingthe banks through which the water was conducted to the sill; thus destroying the useof a river as the natural drain of a country, by elevating its bed above the ordinary levelof that part of the valley where the mill is situated. Wherever a head of water is createdin a valley, it does an infinite mischief in penning back, by its weight, the springs whichendeavour to find a vent at the foot of the hills, and which the river, when left to itself,would carry off.

Mill-dams, when thrown across a stream, occasion a deposition of all that is brought down,and thus elevate the upper parts of the beds, as well as affect all the tributary streams that arewithin its influence. When a succession of drains occur, they materially change the naturalslope of the bed ; for whatever the water tumbles over occasions it to acquire an increasedvelocity, and deepens the channel for some distance, pushing as it were the bed forwards,so that if the section of such a stream were taken, we should find ascending concavitiesrising to the level of each successive dam instead of a regular slope. When the velocity ofa stream depends upon its fall, it is materially altered by the introduction of a dam ; andwhen the velocity is diminished the natural slopes in the bed are all changed, and newdepositions are the consequence. In muddy streams, where no impediment occurs in theway, or any dam offers itself to oppose the force of the floods, the whole trunk becomesscoured out, and the natural slope is maintained. Where the slope of the bed of a rivervaries, there we always have a difference in the velocity of the running waters. By aug-menting the force of a stream, any deposits may be pushed onwards; and this may be doneby uniting several others with it, thus increasing its height, or by making the courseshorter through which it flows, which has the effect of distributing its fall over a lessdistance. Gravels will not always move forwards with the ordinary power that is exertedupon them ; they will generally accumulate to such a degree as to drive the river from itschannel, and find some new course. This is not the case with rivers flowing over a bedof sand, where the current is seldom changed from its original direction. When a riverthrough a gravel is shortened, by making it flow in a straighter direction, the bed in theupper part is lowered, and the portions pushed forward elevate the bed below the pointwhere the cut terminated, which will be the case with every successive portion, until inthe course of time it ceases to admit the vessels or boats, which formerly navigated it, tothe serious injury of the surrounding neighbourhood; such are the too frequent results, whenimprovements, so called, are suggested or undertaken by persons ignorant of the elementswhich they have to manage.

In straight rivers we find the gravel more easily pushed forward than in those whichhave a meandering course, and it is first deposited at the bottom, at the greatest distance,where it gradually raises the lower parts, and then those higher up the stream : this in timerequires the embankments to be elevated, to prevent an overflow when floods occur.