LEVEL

An instrument which shows the direction of a straight line parallel to the plane of the horizon. The plane of the sensible horizon is indicated in two ways: by the direction of the plummet or plumb-line, to which it is perpendicular; and by the surface of a fluid at rest. Accordingly, levels are formed either by means of the plumb-line, or by the agency of a fluid applied in some particular manner. They all depend upon the same principle, namely, the action of terrestrial gravity. Levels in which the plumb-line forms the e ential part are those most usually employed for the common purposes required by bricklayers, masons, carpenters, . They are constructed under many different forms; but the general principle is as follows: — A frame or board is prepared, having one edge perfectly straight, and a straight line is drawn on the frame at right angles to the straight edge. To some jjoint of this straight line a thread carrying a plummet is attached; consequently, when the frame is placed in such a position that the thread of the plummet, hanging freely, coincides with the straight line, the straight edge of the frame, which is perpendicular to it, must be horizontal. See Plummet. The Artillery Foot Level, and the Gunner's Level, besides the line and plummet, have a scale for showing the inclination of a straight line to the horizon. The former has two equal legs or branches placed at right angles; and from their point of junction a thread and plummet hangs, and plays over a quadrant divided into twice 45° from the middle. The plane or line on which the two ends rest is horizontal when the thread falls over the zero point of the scale; and when it falls over any other point, the degree marked on the scale indicates the inclination of the line to the horizon. The gunner's level is on the same principle, though differently constructed; the thread or plummet being replaced by a solid piece of bra , loaded at the lower end, and the legs, or rather the edges, of the bra plate making an angle of 45°. It is used in the same manner as the former. Spirit Level By far the most convenient and also the most accurate level is the spirit level, represented in the annexed figure; " which is nothing more than a gla tube nearly filled with a liquid (spirit of wine being now generally used, on account of its mobility and not being liable to freeze), the bubble in which, when the tube is placed horizontally, would rest indifferently in any part if the tube could be made mathematically straight; but that being impo ible to execute, and every tube having some slight curvature, if the convex side be placed upwards the bubble will occupy the higher part, as in the figure (where the curvature is purposely exaggerated). Suppose such a tube as A B firmly fastened on a stmight bar C D, and marked at a b, two points dis-' tant by the length of the bubble; then, if the instrument be so placed that the bubble shall occupy this interval, it is clear that C D can have no other than one definite inclination to the horizon; because, were it ever so little moved one way or other, the bubble would shift its place, and run towards the elevated side. Suppose now that we would ascertain whether any given line P Q be horizontal; let the base of the level C D be set upon it, and note the points a b, between which the bubble is exactly contained; then turn the level end for end, so that C shall rest on Q, and D on P: if then a. ^ the bubble continue to occupy the same place between a and b, it is evident that P Q can be no otherwise than horizontal; if not, the side towards which the bubble runs is highest, and must be lowered. Astronomical levels are furnished with a divided scale, by which the places of the ends of the bubble can be nicely marked." {Herschers Astronomy, p. 92.) The accuracy of the indications of the level depends in a considerable degree on the regularity of the interior surfaces of the tube. They are commonly made of gla tubes in the same state as they are obtained at the gla house; but when very great accuracy is required, as in astronomical observations, the interior surfaces are sometimes ground so as to give them a regular cylindrical, or rather spindle form, with a slight spherical curvature. The tube and bubble must be of considerable length. The larger the bubble the more freely it moves, and consequently the more sensible is the level to a small inclination. Witlr proper care they can be executed, it is said, with such delicacy as to ir»dicate a single second of angular deviation from exact horizontality. [s. 669]