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LEVEL ; a mathematical instrument used for drawing a line parallel to the horizon, and continuing it at pleasure, and, by this means, for finding the true level, or the difference of ascent or descent between several places, for conveying water, draining fens, placing the surfaces of floors, &c, level, and for various other purposes in architecture, agriculture, hydraulics, surveying, &c. There is a great variety of instruments for this purpose, differently constructed, constituted of different metals, according to the particular purposes to which they are applied ; as the carpenter's level, mason's level, balance level, mercurial levels, surveying and spiral levels; but, however their construction may vaiy, they may all be referred to the following three classes: 1. those in which the vertical line is determined by a suspended plumbline or a balanceweight, and the horizontal position is shown by a line perpendicular to it; 2. those which determine a level line by the surface of a fluid; 3. spirit levels, which point out the horizontal direction by a bubble of air floating in a fluid contained in a glass tube. 1. Those of the first kind, depending upon the plumbline, are very common, but not very accurate. The simplest form is that of two rulers united in the form of the letter L; they must be exactly perpendicular to each other; then, if a plumbline is suspended from the top of the vertical ruler, and the edge thereof be made to coincide with the plumbline, the other ruler must be horizontal. This, when applied to the top of a wall, a beam, or a floor, will show if they are horizontal. This is the kind of level used by artificers; sometimes it is formed like the letter A, of three rulers, the plumbline being suspended from the vertex, and the two legs set on the surface to be levelled. The line hangs opposite to a mark made on the middle of the cross ruler, when the feet are on the same level. Sometimes the horizontal piece crosses the perpendicular at its foot, and the plumb, suspended from the top of the perpendicular, is received in an opening at their junction. 2. The water level shows t\*e horizontal line by means of a surface of water or other fluid, founded on this principle, that water always places itself horizontally. The most simple kind, made of a long wooden trough, which is filled with water, shows on its surface the line of level. This is the ancient chorobates. The water level is also made with two cups fitted to the two ends of a straight pipe, an inch in diameter, and four feet long. The water communicates from one cup to the other; and this pipe being movable on its stand by a ball and socket, when the two cups are seen to be equally full of water, their two surfaces mark the line of level. This instrument, instead of cups, may also be made with two short cylinders of glass, three or four inches long, fastened to each extremity of the pipe with wax or mastich. The pipe, filled with colored water, shows itself through the cylinders, by means of which the line of level is determined ; the height of the water, with respect to the centre of the earth, being always the same in both cylinders. This level, though very simple,, is yet very commodious for levelling small distances. 3. The spirit or air level shows the exact level, by means of a bubble of air, enclosed, with some fluid, in a glass tube of an indeterminate length and thickness, and having its two ends hermetically sealed. When the bubble fixes itself at a mark in the middle of the tube, the case in which it is fixed is then level. When it is not level, the bubble will rise to one end. This glass tube may be set in another of brass, having an aperture in the middle, where the bubble may be observed. The liquor with which the tube is filled, is oil of tartar, or aqua secunda, those not being liable to freeze, as common water, nor to rarefaction and condensation, as spirit of wine is. These instances will explain the principle of the different kinds of levels. Their varieties are too numerous to be described here.