1560

THEORY AND PRACTICE OF ENGINEERING.

Book II.

first portions trod gently down, the remainder spread and levelled over it. Where thereis an impervious bottom, the drains are often cut 4 feet or more in depth; they are thenmade proportionately wider, and filled up within 12 or 14 inches of the surface; wherethe depth does not exceed 5 feet, a width of 2 feet at top is amply sufficient; the widthshould, however, be increased 4 inches for every foot in depth, and that at the bottom be20 inches. A great variety of tiles are now used for the purpose of draining; some arecylindrical, the upper half being perforated with small holes to allow the water to passinto the channel formed by laying one-half of the pipe over the other; they are verydurable and effective when properly laid down.

When the land abounds with springs, or is subject to the oozing out of subterraneouswaters, the draining must be effected by a different method. Springs have their origin fromthe accumulation of the rain water, which, falling upon the earth, after passing throughsoils of a porous nature, descends to a bed of clay, close gravel, or some other water-bearingstratum, upon which it glides or runs down the sloping bed, until it crops out at the surfaceof the ground, generally in some valley, where it forms a run of water. In the chalk dis-tricts, the water rises to the slant line of its drainage, or rather stands at the level at whichit can drain off; and where a considerable detritus has been deposited upon its bed, thenatural oozing out of the water from these springs is prevented, and being penned back, theyare forced to find an opening elsewhere.

Wherever a ditch is cut on the slope of a hill at a level, to tap the strata in which thewater stands, there it will flow out and effectually drain the soil above it, as has been provedby constant experience; in sinking a shaft or tunnel in the chalk districts, below theordinary standing of water in the neighbouring wells, the whole has been drained dry formany miles : therefore, to drain a valley which suffers from floods or inundations producedfrom such causes, it is only necessary to have a catch-water drain made at the foot of thehills which skirts it, and cut sufficiently deep to receive the water which will pour into it.

Descending waters are readily got rid of, by collecting them into a stream beforethey injuriously overspread the plains below; there are many instances of a valuable levelbeing ruined by the filling of the ditches with water, producing weeds and vegetable matter,which in decaying give out gases of the worst kind, in consequence of the natural drain orriver being raised or penned up, which elevates the level of the springs by forcing thembackwards; whenever this occurs a new catch-water drain should be sunk, and a free dis-charge for all the waters which descend from the high ground kept open. If this drainpasses through sand or a porous soil, it must be puddled at the sides and bottom.

It is not uncommon in many parts of Kent, where the chalk is capped with a stiff clayand flints, to drain the surface by sinking wells through the clay, and filling them up withstones : the water which falls upon the surface is directedinto these artificial cavities, and led off to the chalk, where itsinks to the level of the water-bearing stratum, or to theheight of its slant line, which finds an outlet or natural dis-charge at the toe of the hills, or in some fissures of the chalk.

This clay is supposed to be the alumina held by the chalkbefore its decomposition, or rather before it was dissolved,and carried away by the waters which acted upon it • that agreat height has been reduced from both the North and SouthDowns is evident from the quantity of flints which remain onthe surface, and the loftiest parts of these ranges, which aremost exposed to the action of the weather, abound withthem. Wherever pits can be sunk to arrive at the chalk orthe limestone, there is no difficulty in obtaining a free andperfect drainage.

When a morass or a peaty ground is to be drained, theseveral strata which it reposes upon should be examined byboring, and if, as is often the case, a layer of clay intervenesbetween the limestone and the mossy surface which holds thewater, by tapping the clay in various places well selected, thewhole of the water will be discharged and sink away.

Tunnelling under rivers presents very considerable difficulty,as has been experienced in that under the Thames, and itcould not have been effected, but for the shield contrived byMr. Brunei , which consisted of twelve frames, each havingthree cells that could be opened and shut at pleasure.

. Each of the twelve frames, one of which is shown in thecut, had boards 3 feet in length and 4 inches in thickness,called polling-boards; these were supported and maintainedin their position by screws, to the number of fifty-four ineach frame. When it was required to take out the earth,one of these boards only was removed at a time by the