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 de pend upon the- same principle, namely, the action of terrestrial gravity.
Levels in which the plumb-line forms the essential part are those most usually employed for the common purposes re quired by bricklayers, masons, carpen ters, &c. They are constructed under 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 point of this straight line a thread carrying a plummet is at tached ; 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. Sec PLUMMET.
The Artillery Foot Level, and the Gun ner's Level, besides the line and plum met, have a scale for showing the incli nation of a straight line to the horizon. The former has two equal legs or branches placed at right angles ; and from their point of junc tion a thread and plummet hangs, and plays over a quadrant divided into twice 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 incli nation 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 brass, loaded at the lower end, and the legs, or rather the edges, of the brass plate making an angle of 45°.
It is used in the same manner as the former.
Spirit Level.—By far the most ient and also the most accurate level is the spirit level, represented in the nexed figure ; "which is nothing more than a glass tube nearly filled with a liquid (spirit of wine being now generally used, on count of its mobility, and not being liable to freeze,) the bubble in which, when the tube is placed horizontally, would rest in differently in any part if the tube could be made mathematically straight ; but that being impossible 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 pur posely exaggerated.) The accuracy of the indications of the i level depends in a considerable degree on the regularity of the interior surfaces of the tube. They are commonly made of glass tubes in the same state as they are obtained at the glass-house ; but when very great accuracy is required, as in as tronomical observations, the interior sur faces are sometimes ground so as to give them a regular cylindrical, or rather spindle form, with a slight gplierical cur vature. The tube and bubble must be of considerable length. The larger the bub ble, the more freely it moves, and conse quently, the more sensible is the level to a small inclination. With proper care they can be executed, it is said, with such delicacy as to indicate a single second of angular deviation from exact horizon tality.