The telescope and its attachments, the clamp and tangent, the vertical circle, the level, and the sights, give to this instrument a great advantage over the ordinary compass.

The cross wires, two fine fibers of spider's web, extending across the tube at right angles, intersect in a point which, when the wires are adjusted, determines the optical axis or line of collimation of the telescope, and enables the surveyor to fix it upon an object with great precision.

The clamp and tangent screw consist of a ring encircling the axis of the telescope, having two projecting arms the one above, slit through the middle, holding the clamp screw; the other, longer, connected below with the tangent screw.

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The ring is brought firmly around the axis by means of the clamp screw, and the telescope can be moved up or down by turning the tangent screw.

The vertical circle, graduated to half-degrees, is attached to the axis of the telescope, and, in connection with the vernier, gives the means of measuring vertical angles to minutes with great facility.

The level attached to the telescope enables the surveyor to run horizontal lines, or to find the difference. of level between two points.

Sights on the telescope are useful in taking backsights without turning the telescope, and in sighting through bushes or woods.

Sights for right angles attached to the plate of the instrument, or to the standards supporting the telescope, afford the means of laying off right angles, or running out offsets without changing the position of the instrument.

SURVEYOR'S TRANSIT.

240. Adjustments.

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1. The levels are adjusted in the same manner those of the compass, and when adjusted should keep their position if the two plates are clamped together and turned on a common socket.

2. The needle is adjusted as in the compass.

3. The line of the collimation is adjusted by bringing the intersection of the wires into the optical axis of the telescope, which is accomplished as follows:

Set the instrument firmly on the ground and level it carefully, then, having brought the wires into the focus of the eye-piece, adjust the object glass on some well defined object, as the edge of a chimney, at a distance of from two to five hundred feet. Determine whether the vertical wire is plumb by clamping the instrument firmly to the spindle, and applying the wire to the vertical edge of a building, or observing if it will move parallel to a point a little to one side; if it does not, loosen the cross-wire screws, and, by the pressure of the hand on the head outside the tube, move the ring within the tube, to which the wires. are attached, gently around till the error is corrected.

The wires being thus made respectively horizontal and vertical, fix their point of intersection on the object selected, clamp the instrument to the spindle, and, having revolved the telescope, find or place some object in the opposite direction, at about the same distance from the instrument as the first object.

Great care should be taken in turning the telescope not to disturb the position of the instrument upon the spindle.

Having found an object which the vertical wire bisects, unclamp the instrument, turn it half-way round,

and direct the telescope to the first object selected, and having bisected this with the wires, again clamp the instrument, revolve the telescope and note if the vertical wire bisects the second object observed; if so, the wires are adjusted, and the points bisected are, with the center of the instrument, in the same straight line.

If the vertical wire does not bisect the second point, the space which separates this wire from that point is double the distance of that point from a straight line drawn through the first point and the center of the instrument, as is shown thus:

D

C E

Let A represent the center of the instrument, BC the line on whose extremities, B and C, the line of collimation is to be adjusted, B the first object, and D the point which the wires bisected after the telescope was made to revolve on its axis. The side of the telescope which was up when the object glass was directed to B, is down when the object glass is turned toward D. When the telescope is unclamped from its spindle and turned half-way round its vertical axis, and again directed to B, the side of its tube which was down when the object glass was first directed to B will now be up. Then clamping the instrument, and revolving the telescope about its axis, and directing it toward D, the side of its tube which was down when the object glass was first turned toward D will now be up, or the telescope will virtually have revolved about its optical axis, and the vertical wire will appear at E as far on one side of C as D is on the other side.

To move the vertical wire to its true position, turn the capstan head screws on the sides of the telescope, remembering that the eye-piece inverts the position of the wire, and, therefore, that in loosening one of the screws and in tightening the other the operator must proceed as if to increase the error. Having moved back the vertical wire, as nearly as can be judged, so as to bisect the space ED, unclamp the instrument, direct the telescope as at first, so that the cross wires bisect B, proceed as before, and continue the operation till the two points D and E coincide at C.

4. The standards must be of the same height, in order that the wires may trace a vertical line when the telescope is turned up or down. To ascertain this, and to make the correction, proceed as follows:

Having the line of collimation previously adjusted, set the instrument in a position where points of observation, such as the point and base of a lofty spire, can be selected, giving a long range in a vertical direction.

Level the instrument, fix the wires on the top of the object, and clamp to the spindle; then bring the telescope down till the wires bisect some good point, either found or marked at the base; turn the instrument half around, fix the wires on the lower point, clamp to the spindle, and raise the telescope to the highest object, and if the wires bisect it, the vertical adjustment is effected.

If the wires are thrown to one side, the standard opposite that side is higher than the other.

The correction is made by turning a screw underneath the sliding piece of the bearing of the movable axis.