to the transverse axis of the telescope, this point will be in the prolongation of BA. In order to ascertain whether this is the case, loosen either clamp, turn the instrument in azimuth through 180°, set the clamp and, by means of the tangent

screw, direct the line of sight again to B, and plunge the telescope again. If the line of sight strikes the same point as before, it is perpendicular to the transverse axis, and no adjustment is

necessary.

But, suppose that the point set after the first plunging is at D, and that the point set after the second plunging is at E, to one side of D. This will show that the line of sight must be adjusted. In order to make this adjustment, measure the distance DE (the points D and E are set at the same distance from A, as nearly as can be estimated by the eye), and set a mark at F, making the distance EF equal to one-fourth DE. Move the cross-hairs by means of the capstan-headed screws until the vertical hair exactly covers the mark at F, being careful to move them in the opposite direction to that in which it would appear they should move. In order to move the cross-hairs, loosen the screw on the side of the telescope tube away from which they are to be moved, and then tighten the screw on the opposite side. Bring the screws to a firm bearing, but do not turn them so tight as to cause any strain. The cross-hairs having been thus moved and the telescope plunged back, the line of sight will not fall on the point B, but on a point G, at a distance from B equal to EF. By means of either tangent screw, bring the line of sight again on the point B, then plunge the telescope. If the adjustment is perfect, the line of sight will strike the point C which is in the prolongation of the line BA and midway between D and E. The adjustment should be tested by reversing the instrument again in azimuth, then plunging the telescope and sighting forwards as before. It may be necessary to

repeat the operation several times in order to obtain an exact adjustment.

Third Adjustment.-To make the transverse axis of the telescope perpendicular to the vertical axis of the instrument, so that when the instrument is leveled the transverse axis of the telescope will be horizontal.

Suspend a fine, smooth plumb-line from a rigid support at as high an elevation as convenient and at a distance from the instrument not exceeding the length of the line. The weight should be suspended in a pail of water, care being exercised that it does not touch the bottom of the pail and that the line is not exposed to wind. With both plate bubbles in the middle of their tubes, direct the line of sight to the upper end of the plumb-line; then, turning the telescope slowly downwards, notice whether the intersection of the cross-hairs exactly follows the line throughout its length. If it does follow it, the line of collimation revolves in a vertical plane. The plumb-line will usually vibrate slightly, but its mean position can be estimated by the eye. If the intersection of the crosshairs does not coincide with the plumb-line throughout its length, but diverges to one side as it approaches the bottom of the line, the error must be corrected by raising or lowering one end of the transverse axis of the telescope, which is adjustable by means of screws placed in one of the standards. If the intersection of the cross-hairs diverges on the side of the plumb-line toward the adjustable end of the transverse axis, this end is to be lowered; if on the opposite side, it is to be raised. This adjustment can also be tested and made in the following manner: Level the instrument, and direct the telescope to some well-defined point on a church spire or other high object, as the point A, Fig. 6. Having set both the upper and the lower clamp, depress the object end of the telescope and set a point in the line of sight on the ground at the base of the object; loosen the upper clamp, reverse the instrument in azimuth, plunge the telescope, sight again on the high point, again turn the telescope downwards, and notice whether or not the line

B

C

FIG. 6

of sight strikes the same point as before. If it does, the transverse axis of the telescope is horizontal. If the point first set is the point B, and the second line of sight passes through D, instead of B, the transverse axis is not horizontal, and must be adjusted. The adjustment is made by raising or lowering one end of the transverse axis (in this case the righthand end would have to be lowered), and again repeating the test, until the points B and D coincide; that is, until the line of sight, when the telescope is depressed, strikes the same point, as C, both before and after reversal.

Fourth Adjustment.-To make the bubble of the telescope level stand in the middle of its tube when the line of sight is horizontal.

This adjustment makes the transit adapted to leveling work. It is the same as that of a regular level, and is described in connection with the level.

Fifth Adjustment.-To make the vernier of the vertical arc or circle read zero when the line of sight is horizontal.

To perform this adjustment, level the instrument and turn the telescope on its transverse axis until the bubble in the attached level is nearly in the middle of its tube; clamp the telescope, and center the bubble of the attached level exactly by means of the gradienter screw g, Fig. 1. If the vernier of the vertical limb does not read zero, set it so that it will read zero by means of the capstan-headed screws that control it.

This adjustment is not strictly necessary, provided the reading of the vernier when the telescope is horizontal is observed and noted. This reading is called the index error of the vertical circle or vernier and should be allowed for in reading vertical angles.

Adjustment of the Cross-Hairs.-For convenience in directing the telescope to a signal, it is desirable that the vertical cross-hair should be truly vertical, and the other truly horizontal. The two cross-hairs are attached to an adjustable diaphragm exactly at right angles to each other, so that when one is vertical the other is horizontal. In order to test the vertical cross-hair, sight on any sharply defined point, focusing the telescope perfectly and bringing the point exactly in range with either end of the vertical cross-hair. Then turn the telescope

on its transverse axis slowly and notice whether the point sighted remains on the cross-hair throughout the motion. Should any deviation be discernible, loosen the capstan-headed screws that control the cross-hairs, and by the pressure of the hand, or by tapping lightly against the heads of the screws outside the telescope tube, rotate the cross-hairs very carefully in the direction opposite that in which they should apparently be rotated, until the point sighted remains on the cross-hair throughout the motion of the telescope. Then tighten the screws sufficiently to bring them to a firm bearing without straining them, and repeat the test.

This test should be applied before performing the third adjustment for the line of collimation. If the plate levels are in perfect adjustment, it can also be made by sighting at a plumb-line suspended at a suitable height and distance, with the plate levels centered perfectly, and observing whether the vertical cross-hair coincides exactly with the plumb-line.

TRANSIT FIELD WORK

To Prolong a Straight Line.-Let AB, Fig. 7, be a straight line whose position on the ground is fixed by stakes set at A and B, and let it be required to prolong the line to C. This

4

B

C

FIG. 7

can be done in two ways; namely, by foresight only, or by backsight and foresight, the latter method being commonly called backsight.

By Foresight.-The transit is set over the point at A, and the line of sight directed to a flag held at B; if the point C is to be set at a given distance from B, the chainmen measure the required distance, the head chainman being kept in line by the transit man. When the required distance has been measured, the point C, which evidently lies in the prolongation of AB, is marked by a stake or otherwise.

By Backsight.-Set the transit over the point at B and sight on a flag held at A. Plunge the telescope, which will then be directed along the prolongation of AB. Any required

distance BC may then be measured from B in the direction indicated by the line of sight.

Measurement of Horizontal Angles.-The horizontal circle of the transit, like that of the compass, measures only horizontal angles; that is, angles between the horizontal projections of the lines of sight. Let AB and AC, Fig. 8, be two lines on the ground the angle between which it is desired to measure with the transit. Set up the instrument precisely over the vertex A, level it carefully, loosen the upper clamp, and turn the upper plate until the zero of the vernier to be read (say vernier A) nearly coincides with the zero of the graduated circle. Clamp the plates, and by turning the upper tangent screw bring the zero of the vernier exactly in line with that of the limb. This

operation is called setting the vernier at zero. Loosen the lower clamp (if it is not already loosened), and direct the telescope to a flag held at B. Next, loosen the upper clamp, and direct the telescope to a flag held at C. The arc of the graduated circle traversed by the zero point of the vernier will measure the angle BAC, whose value can be determined by reading the instrument; that is, by adding the reading of the vernier to that of the limb.

It is not always necessary nor convenient to set the vernier at zero before measuring an angle. The upper clamp being set, whatever the position of the vernier may be, the telescope is directed to B, as explained, and the reading of the instrument taken. The upper clamp is then loosened, the telescope directed to C, and the instrument read again. The difference between the two readings is the value of the angle.