SPIRIT-LEVEL, 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 position, the air which occupies a part not filled with the spirit or water places itself contiguously 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 spiritstube is used in determining the relative heights of ground at two or more stations, and in order to render it available for this pur and tube are then fitted to a frame, or cradle, of brass, which is sup ported on three legs. In the interior of the telescope, at the common focus of the object-glass and eye-glasses, are fixed, generally, two wires, at right angles to each other, their intersection being in the lino of collimation, or that which joins the centresof all the lenses.
The case containing the spirit-tube is made to turn on a joint at one extremity, as a, by the revolutions of a screw, b, at the opposite extre mity ; and the telescope rests, near each end, within two arms at the top of a small pillar, A or a, the pillar and its arms resembling the letter Y, and the interior sides of the arms being tangents to the tube of the telescope. One of these pillars is made capable of a small movement in a vertical direction by turning a screw, c, at its base, for the purpose of elevating or depressing one end of the telescope and spirit-tube ; and in the more perfectly constructed instruments, both the pillars may be so moved. The pillars are at the extremities of a strong brass plate, a r, the under side of which is connected with the tripod-stand, which supports the whole instrument; and a compass box, a, is attached immediately to the plate, as in the figure, or is raised above the telescope by means of four small pillars. A hollow conical socket, cc, of brass is screwed to the under side of the plate, and is intended to receive a piece of bell-metal of a corresponding fofm, which constitutes the upper part of the stand. This piece serves as a vertical axis, upon which the telescope, the spirit-level, and the com pass are to turn round horizontally : sometimes, however, the conical pivot projects from the under part of the plate, E F, and the socket is on the stand.
The three legs which are to support the instrument are firmly fixed to a circular plate, x, perforated at its centre, and having about the perforation a hollow spherical zone, resembling a small inverted cup. In the simpler kinds of spirit-levels a circular plate, L, of the same .
dimensions as the last, carries above it the pivot before mentioned; and from below it projects a stem, terminating in a ball, which fits the inverted cup or socket. By means of four screws which pass through one of these two plates (the upper plata in the cut), nearly at the extremities of two diameters at right angles to one another, the upper plate is made parallel to the horizon, and consequently the conical pivot which it carries is brought to a vertical position.
The above is a general description, which will serve nearly for every spirit-level at present in use, whatever be its form ; and the following is the usual manner of performing the adjustments, prepa ratory to the instrument being employed on the ground:— The telescope should, by a proper opening of the legs of the stand, be at first rendered as nearly level as can be estimated by the eye; then, being turned so as to lie vertically above the line joining two opposite screws in what are called the parallel plates (a and L), the spirit-tube is brought to horizontal position by relaxing the screw nearest to its higher end, and tightening that which is opposite to it : the like operation is to be performed with the other pair of screws, after placing the telescope vertically above them. In order to render the tube parallel to the axis of the telescope, after the bubble of air has been made to occupy the middle place by the process just mentioned, let the telescope be reversed in the arms (the Ys as they are called); then if the bubble does not still occupy the middle, it must be made to do so by successive trials, endeavouring to correct half the error by means of the screw b, and the other half by the screw C.