contrivance to throw the magnetic needle off its centre when not in use): it is convenient to take bearings, and is not necessarily connected with the operations of levelling, but extends the use of the instrument, making it a rough circumferenter. The whole is mounted on parallel plates, and three legs, the same as a theodolite.

It is evident, from the nature of this instrument, that three adjustments are necessary. First, to place the intersection of the wires in the telescope, so that it shall coincide with the axis of the cylindrical rings, on which the telescope turns; secondly, to render the level parallel to this axis; and lastly, to set the telescope perpendicular to the vertical axis, that the level may preserve its position while the instrument is turned quite round upon the

staves.

TO ADJUST THE LINE OF COLLIMATION.

The eye-piece being drawn out to see the wires distinctly, direct the telescope to any distant object, and by the screw, A, adjust to distinct vision; * bring the intersection of the cross wires to coincide with some welldefined part of the object, then turn the telescope round on its axis, as it lies in the Ys, and observe whether the coincidence remains perfect during its revolution: if it does, the adjustment is correct; if not, the wires must be moved one-half the quantity of error, by turning the little screws near the eye-end of the telescope, one of which must be loosened before the opposite one is tightened, which, if correctly done, will perfect this adjustment.

TO SET THE LEVEL PARALLEL TO THE LINE OF
COLLIMATION.

Move the telescope till it lies in the direction of two of the parallel plate-screws, ee (the clips which confine the telescope in the Ys being laid open), and by giving motion to the screws, bring the air-bubble to the middle of the tube, shown by the two scratches on the glass. Now reverse the telescope carefully in its Ys, that is, turn it end for end; and, should the bubble not return to the centre of the level as before, it shows that it is not parallel to the optical axis, and requires correcting. The end to which the bubble retires must be noticed, and the bubble made to return one half the distance, by the parallel platescrews, and the other half by the capstan-headed screw at the end of the level; when, if the halves have been correctly estimated, the air-bubble will settle in the middle in both positions of the telescope. This, and the adjustment for the collimation, generally require repeated trials before they are completed, on account of the difficulty in estimating exactly half the quantity of deviation.

TO SET THE TELESCOPE PERPENDICULAR TO THE
VERTICAL AXIS.

Place the telescope over two of the parallel plate-screws, and move them (unscrewing one while screwing up the other) until the air-bubble of the level settles in the middle of its tube; then turn the instrument half round upon the vertical axis, so that the contrary ends of the telescope may

be over the same two screws, and if the bubble again settles in the middle, all is right in that position; if not, half the error must be corrected by turning the screw, B, and the other half by the two parallel plate-screws, over which the telescope is placed. Next turn the telescope a quarter round, that it may lie over the other two screws, and make it level by moving them; and the adjustment will be complete.

Before making observations with this instrument, the adjustments should be carefully examined and rectified, after which the screw, B, should never be touched; the parallel plate-screws alone must be used for setting the instrument level at each station; and this is done by placing the telescope over each pair alternately, and moving them until the air bubble settles in the middle. This must be repeated till the telescope can be moved quite round upon the staff-head, without any material change taking place in the bubble.

OF THE LEVELLING-STAVES.

Two mahogany station-staves generally accompany the spirit-level; they consist of two parts, capable of being drawn out when considerable length is required. They are divided into feet and hundredths, or feet, inches, and tenths; and have a sliding-vane, with a wire placed across a square hole in the centre, as shown in the annexed figure; the vane being raised or lowered by the assistant until the cross wire corresponds with the horizontal wire of the telescope, the height of the wire in the vane, noted on the staff, is the height of the apparent level above the ground at that place.