An instrument employed for obtaining a line or plane parallel to the plane of the horizon. One principal use of the level is to find the difference of elevation of two or more planes, for the purposes of conveying water, constructing roads, etc. from one place to another. Among the various contrivances employed for constructing instruments for finding the level, the following are some of the principal. The water level, the horizontal line by the surface of the fluid: - The most simple kind is nothing more than a long wooden trough, filled with water, such as is described by Vitruvius, under the name of the chombales. Another level of this kind consists of two cups, fitted to the ends of a straight cylindrical tube, of an inch in diameter, and three or four feet long, by which the water freely communicates from one cup to the other; the tube is movable on its stand by means of a ball and socket, and thus the surfaces of the water, when the cups are equally full, show the line of level. Two glass cylinders, of three or four inches in length, may be substituted instead of the cups, being fastened with wax or mastic to the ends of the tube: this machine should be filled with coloured water.

A very simple quicksilver level, invented we believe by Mr. Parker, of Sweeny, is delineated in the annexed cut, which it is said is much used in the north of England for irrigation, draining, etc. and does not cost as many shillings as the usual instrument employed colts of pounds, a a are two funnels or basins cast in iron from the same mould, their lower ends b b are made cylindrical, and bored so as to be of exactly equal diameters; they are screwed down by flanges to the tube f, which is flat at top, and through which the mercury flows, c c are two floats also exactly equal to each other in length, weight, and lower surface. A hole, one-eighth of an inch in diameter, is drilled on one side of the funnels for introducing the quicksilver, and afterwards closely stopped with a fine cork; d is a mahogany top to the instrument, to which the funnels, etc. are screwed; e e are collars or guides for the floats, made of lignum-vitae, and to prevent the escape of the quicksilver; g is the stand and pivot on which it turns.

Between all the junctures leather washers are introduced, to keep them tight.

Level 42

In using this instrument, an observation is made on the tops of the floats, which, when they exactly coincide or project equally, shows the ground to be a true horizontal plane; on the contrary, when they differ in height, the ground is not level. This mode of taking an observation appears to us defective, as the most distant of two surfaces, when upon the same plane, cannot be distinctly seen. It would be better to have a circular hole through the tops of the floats, and to look at them from a fixed point on either side: when the two circles coincided, they would appear to the eye as only one of a true figure, and denote a true level; any deviation would be clearly denoted by the circles intersecting each other, and producing a curved figure, with two pointed ends, which would express exactly the extent of the alteration required. The simplicity and strength of the instrument permit of its being thrown down, and rolled about without sustaining injury, and any rough unlettered man may use it with effect It may be had, we are informed, of Mr. Batt, Seedsman, 412, Strand, London.

In the air level, invented by M. Thevenot, the level is determined by means of an air-bubble, inclosed with some fluid in a glass tube, hermetically sealed at both its ends; the case or ruler in which the tube is fixed will be exactly level when the bubble remains at a marked point at the middle of the tube; the bubble being on either side of this mark shows the variation. The glass tube is sometimes enclosed in another of brass, the centre of which has a hole sufficiently large to observe the place of the bubble. The liquor employed should be such as will not readily freeze, rarefy, or condense, such as oil of tartar, aqua secunda, etc. The instrument last described has received many successive improvements, by the addition of sights and other apparatus. M. Huygens contrived a level, carrying a telescope instead of plain sights, which possesses some advantages above the common sort; and his invention has been improved upon in a variety of ways by the instrument makers. A machine of this kind, containing the principles both of the barometer and thermometer, was proposed by the late Dr. Desaguliers; but though in theory it seemed good, yet in practice it was found very inaccurate. {Philosophical Transactions, Vol. XXXHI. p. 65.) Amongst the various spirit levels that have been proposed, that contrived by Mr. Hadley deserves to be noticed; it is adapted to a quadrant for taking the meridian altitude at sea when the horizon is not visible.

A description and drawing of this useful instrument may be seen in Philosophical Transactions, Vol. XXXVIII. The reflecting level is the next to be mentioned: it represents the object as reflected upon a long surface of water, in an inverted position, and was invented by Marriotti. Another kind, which we owe to Cassini, consists of a polished metal mirror, placed at a small distance before the object-glass of a telescope, suspended perpendicularly; this mirror being set at an angle of 45°, the perpendicular line of. the telescope will become a horizontal line, that is, a line of level. The plumb or pendulum level, ascribed to Picard, shows the horizontal line by means of a line cutting the plummet line perpendicularly; it consists of two legs, joined together at right angles, with a telescopic sight, and other apparatus; the whole is fixed on a stand by means of a ball and socket. The balance level is an instrument suspended by a ring; and when in equilibrio, two sights, properly fitted to the instrument, will show the line of level.

The carpenter's, bricklayer s, or paviour's level, consists of a long narrow board, to the middle of which is fixed, perpendicularly, another board, broader and shorter than the former, and having a line drawn through its middle, cutting the first mentioned board at right angles.

A plummet being suspended from the top of the upright piece shows that the base is horizontal when its line, and the line drawn from the point of suspension, exactly coincide. The masons level is composed of three pieces, framed together in the form of an isosceles triangle. From the vertex of this a plummet is suspended, which, when it hangs directly over the mark in the centre of the base, indicates that the base is exactly in the line of level.