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line, telescope, tube, centre, inches, board and sight

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LEVEL, an instrument constructed for the purpose of ascertaining the exact level of any fluid, building, &c. Of these there are two distinct kinds, viz. the hori zontal and the perpendicular : the first sort, which comprises spirit and air levels, is chiefly in use among surveyors; the latter is ordinarily employed by artifi cers, and depends for exactness on a plumb line.

The instruments used by persons tak ing the levels of lands, waters, &c. where by to ascertain the comparative heights of different spots, or tracts, are simple in the extreme, being generally made with a telescope of about fifteen inches long, fixed above a circular opening in a brass plate, so as to show a compass that tra verses immediately below its centre, and gives not only the number of points, i. e. thirty-two, according to the mariner's di vision, but by means of a neat brass rim, graduated with three hundred and sixty degrees, divided into thirty-six portions of ten degrees each, and numbered, shows the exact angle made between any two sights taken by the telescope, which traverses on two legs, supported in grooves on the outer edge of the brass plate, and allowing it to move round in a direction perfectly parallel thereto. The plane thus described by the circular mo tion of the telescope is made to corres pond with that of the horizon by the aid of a small brass tube, about six qr eight inches in length, fixed exactly parallel with the line of sight through the teles cope, and screwed to its cylinder in such manner as to remain firm. This little tube has on its upper side, or surface, an opening, into which a piece of clear glass, corresponding with the cylindrical curve of the tube, is fitted and properly ce mented. This piece of glass being per fectly centrical, serves to show how the fluid, generally alcohol (or pure spirits), with which the tube is filled, with the ex ception of a very minute portion, stand in respect of inclination with, or from the horizon. When the bubble of air left in the tube floats exactly centrical in that portion which is covered with glass, the tube itself must be level ; and as it is af fixed at an exact parallel with the line of sight, which passes through the axis, or centre of the telescope, from the eye to the crossing of two hairs, at right angles, within the telescope, the instrument itself must then be level, and that part of any object, however distinct, which is cut or indicated by the line of sight, is ascertain ed by the centre of the cross made by the hairs being on a rectilinear level with the line of sight. But in consequence of the

curvature of the earth's surface, the hori zontal level will be different from the rec tilinear level, and will describe an arc parallel with the surface of the earth. This curvature amounts to about eight inches in every mile ; or, in more minute parts, may be taken at four and a half lines for every hundred yards.

The usual mode of taking a level is by means of a painted board, about a foot square, having a broad white stripe drawn horizontally across its centre. This board slides up and down a long pole, which being held perpendicularly by an assistant, at any appointed spot of which the level is to be ascertained, the instrument is brought to the exact direc tion in which the pole is situated; so that the latter may coincide, or as it is techni cally called, "be in one" with that basis which is vertical within the tube. The legs on which the level is supported, (ge nerally the same as in theodolites, &c.) are spread so as to be firm ; after bringing the compass as nearly as may be practi cable to a level : by means of four screws, which serve to raise the different sides of the plate at pleasure, the utmost preci sion is attainable. The board is then moved up or down on the pole, which is marked all the way up in feet, inches, halves, and quarters, until the centre painted line " is in one" with the horizon tal hair within the telescope. The height of the telescope above the surface on which it stands must be deducted from the number of feet and inches, at which the line on the board stands above the spot where the pole is fixed: the residue shows how much that is below the place where the instrument stands. But if the height of the line on the board be less than that at which the line of sight in the level stands from the ground, then the difference between those two heights will exhibit how much the former is above the latter.

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