This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1883 Excerpt: ... energy of the water which issues Trom taps, often with considerable velocity, and partly in the shape of work done against friction in the pipes. When there is a continual drawing off from various points of a main, the height to which the water will rise in the houses which it supplies is least in those which are most distant from the reservoir. 179. Water-level.--The instrument called the water-level is another illustration of the same principle. It consists of a metal tube bb, bent at right angles at its extremities. These carry two glass tubes aa, very narrow at the top, and of the same diameter. The tube rests on a tripod stand, at the top of which is a joint that enables the observer to turn the apparatus and set it in any direction. The tube is placed in a position nearly horizontal, and water, generally coloured a little, is poured in until it stands at about three-fourths of the height of each of the glass tubes. By the principle of equilibrium in vessels communicating with each other, the surfaces of the liquid in the two branches are in the same horizontal plane, so that if the line of the observer''s sight just grazes the two surfaces it will be horizontal. This is the principle of the operation called levelling, the object of which is to determine the difference of vertical height, or difference of level, between two given points. Suppose A and B to be the two points (Fig. 89). At each of these points is fixed a levelling-staff, that is, an upright rod divided into parts of equal length, on which slides a small square board whose centre serves as a mark for the observer. The level being placed at an intermediate station, the observer directs the line of sight towards each levelling-staff, and the mark is raised or lowered till the line of sight ...