SPIRIT-LEVEL, is a tube of glass nearly filled with spirit of wine or distilled water, and hermetically sealed at both ends, so that when held with its axis in a horizontal posi tion, the air which occupies the part not filled with the spirit or water places itself con tiguously to the upper surface. The tube being supposed to be perfectly cylindrical, the exact horizontality of its axis is ascertained by the extremities of the air bubble being at equal distances from the middle point in the length of the glass. The spirit-tube is placed within a brass case having a long opening on the side which is to be uppermost, and is attached to a telescope ; the telescope and tube are then fitted to a frame, or cradle, of brass, which is supported on three legs.
The use of the spirit-level in accurate sur veying requires great care in the adjustment of the tube and the attached telescope.
The Levelling-Staff till lately in general use for finding the relative heights of ground is a rod consisting of two parts, each six feet long, which, by being made to slide on one another, will indicate differences of level nearly as great as twelve feet. The face of the rod
is divided into feet, inches, and tenths, or into feet with centesimal subdivisions ; and a vane, or cross-piece of wood, perforated through the middle, is moved up or down upon the rod by an assistant till a chamfered edge at the perforation is seen by the ob server at the spirit-level to coincide with tho horizontal wire in the telescope. The rod proposed by Mr. Gravat is divided into hun dredths of a foot by stripes which are alter; nately black and white, and are numbered at every foot in the usual way with figures large enough to be seen on looking through th: telescope; the tenths of a foot are indicate& by lines longer than the others. A similar staff has been proposed by Ur. Sopwith and Mr. Barlow.