When a small portion of the tunnel is excavated, the casing of brickwork is begun from the mouth of it, and continued a small piece at a time, following closely the excavation. The brickwork is generally begun at each side, and carried up to the height at which the arch springs; an inverted arch is then constructed from one side to the other to form the floor, and “centreings” resting upon uprights are placed, to “turn” the arch upon. These centreings are only about two feet deep, and span the arch from side to side; the men work with their backs to the part being excavated, and are able to reach across the narrow centreing to lay the bricks. When completed all round, and the spaces between the brickwork of the arch and the surface of the excavation filled up with earth, the centreing is shifted further in and a fresh ring of brickwork commenced, the excavation going on at the same time. In many tunnels, large perpendicular shafts are sunk down for the purposes of ventilation and lighting. DRAINS. Illustration: DRAIN (‡ DIAGRAM). Illustration: DRAIN-PIPES AND TILES. Drains are constructed for the purpose of carrying off from the surface of the ground all superfluous fluid, whether it may arise from want of a proper exit for the rain, from springs rising in the ground to be drained, or from water coming from some higher level. Drains are made in towns to carry off the rain from the surface, and all liquid refuse from the houses. When the surface of any portion of land has to be drained of its accumulated water, where the soil is of an impervious nature, like clay, it is usual to cut deep furrows from the higher to the lower parts and fill these up with gravel stones or brushwood; the moisture which collects in these is made to flow into the lowest part, and there form a pond, from which it evaporates as fast as the drains supply it; or if the quantity be too great, a main drain or ditch carries it onwards to some stream or river. The furrows before being filled up with any porous substance are often laid down with “drain-tiles;” tiles bent into the form of half a cylinder, and placed with two edges on a flat tile at the bottom of the drain, or pieces of tubing made of tile. The water finds its way readily into these through the crevices. In some cases, water can be kept from flowing over lands and forming marshes, by embankments. These are more especially useful in land lying below the level of rivers and lakes; immense tracts are thus preserved in Holland. When these embankments are very extensive, there is a constant filtration of the water through them; drains are therefore constructed to collect it into certain spots, and mills erected to pump it over the banks. These are usually small windmills, working a large wooden screw in a cylindrical barrel, causing the water to rise up the cylinder into a trough, from which it is poured over the embankment;