To test the correctness of the centering, revolve the telescope, and observe whether it appears to shift the position of an object. Should any movement be perceived, the centering is not perfectly effected. It may here be repeated, that in all telescopes the position and adjustment of the line of collimation depends upon that of the object-glass; and, therefore, that the movement of the eye-piece does not affect the adjustment of the wires in any respect. When the centering has been once effected, it remains permanent, the cover being screwed on again to conceal and protect it from derangement at the hands of the curious or inexperienced operator. 182. To Adjust the Level-Bubble.-Clamp the instrument over either pair of leveling-screws, and bring the bubble into the center of the tube. Now turn the telescope in the wyes, so as to bring the level-tube on either side of the center of the bar. Should the bubble run to the end, it would show that the vertical plane, passing through the center of the bubble, was not parallel to that drawn through the axis of the telescope rings. To correct the error, bring the bubble entirely back, with the capstan-head screws, which are set in either side of the level-holder, placed usually at the object end of the tube. Again bring the level-tube over the center of the bar, and the bubble to the center, turn the level to either side, and, if necessary, repeat the correction until the bubble will keep its position, when the tube is turned half an inch or more, to either side of the center of the bar. The necessity for this operation arises from the fact that when the telescope is reversed end for end in the wyes in the other and principal adjustment of the bubble, we are not certain of placing the level-tube in the same vertical plane; and therefore it would be almost impossible to effect the adjustment without a lateral correction. Having now, in great measure, removed the preparatory difficulties, we proceed to make the level-tube parallel with the bearings of the Y rings. To do this, bring the bubble into the center with the leveling-screws, and then, without jarring the instrument, take the telescope out of the wyes and reverse it end for end. Should the bubble run to either end, lower that end, or what is equivalent, raise the other by turning the small adjusting nuts, on one end of the level, until by estimation half the correction is made; again bring the bubble into the center and repeat the whole operation, until the reversion can be made without causing any change in the bubble. It would be well to test the lateral adjustment, and make such correction as may be necessary in that, before the horizontal adjustment is entirely completed. 183. To Adjust the Wyes.-Having effected the previous adjustments, it remains now to describe that of the wyes, or, more precisely, that which brings the level into position at right angles to the vertical axis, so that the bubble will remain in the center during an entire revolution of the instrument. To do this, bring the level-tube directly over the center of the bar, and clamp the telescope firmly in the wyes, placing it as before, over two of the leveling-screws, unclamp the socket, level the bubble, and turn the instrument half-way around, so that the level-bar may occupy the same position with respect to the leveling-screws beneath. Should the bubble run to either end, bring it half-way back by the Y nuts on either end of the bar; now move the telescope over the other set of leveling-screws, bring the bubble again into the center, and proceed precisely as above described, changing to each pair of screws, successively, until the adjustment is very nearly perfected, when it may be completed over a single pair. The object of this approximate adjustment is to bring the upper parallel plate of the tripod-head into a position as nearly horizontal as possible, in order that no essential error may arise, in case the level. when reversed, is not brought precisely to its former situation. When the level has been thus completely adjusted, if the instrument is properly made, and the sockets well fitted to each other and to the tripod-head, the bubble will reverse over each pair of screws in any position. Should the engineer be unable to make it perform correctly, he should examine the outside socket carefully to see that it sets securely in the main socket, and also notice that the clamp does not bear upon the ring which it encircles. When these are correct, and the error is still manifested, it will, probably, be in the imperfection of the interior spindle. After the adjustments of the level have been effected, and the bubble remains in the center, in any position of the socket, the engineer should turn the telescope in the wyes until the pin on the clip of the wye will enter the little recess in the ring to which it is fitted, and by which is ensured the vertical position of the spirit-level and cross-wire. When the pin is in its place, the vertical wire may be applied to the edge of a building, and in case it should not be parallel with it, two of the cross-wire screws that are at right angles with each other may be loosened, and by the screws outside, the cross-wire ring turned until the wire is vertical. The line of collimation must then be corrected again, and the adjustments of the level will be complete. 184. To Use the Level.-Set the legs firmly into the ground. The bubble should then be brought over each pair of leveling-screws in succession and leveled in each position until the bubble will remain in the center of the tube as the telescope is turned upon its axis in any direction. Bring the wires precisely in focus and the object distinctly in view, so that all errors of parallax may be avoided. This error is seen when the eye is moved to either side of the center of the eye-piece of a telescope in which the foci of the object and eye-glasses are not brought precisely upon the cross-wires and object; in such a case the wires will appear to move over the surface, and the observation will be liable to inaccuracy. In all instances, the wires and object should be brought into view so perfectly, that the cross-wires will appear to be fastened to the surface, and will remain in that position however the eye is moved. 185. Leveling Rods.-The various leveling rods used by American engineers are made in two or more parts, which slide from each other as they are extended in use. They are of different patterns, distinguished as the New York, Philadelphia, and Boston or Yankee rods. The New York and Boston rods are divided into feet, tenths and hundredths, and read by verniers to thousandths. 186. The New York Rod shown in the engraving as cut in two, so that the ends may be exhibited, is made of maple, in two pieces like the former, but sliding one from the other, the same end being always held on the ground, and the graduations starting from that point. The graduations are made to tenths and hundredths of a foot, the tenth figures being black, and the feet marked with a large red figure. The front surface, on which the target moves, reads to about six and a half feet; when a greater hight is required, the horizontal line of the target is fixed at the highest graduation, and the upper half of the rod, carrying the target, is moved out of the lower, the reading being now obtained by a vernier on the graduated side, up to an elevation of twelve feet. The target is round, made of thick sheet brass, having, to strengthen it still more, a raised rim, which also protects the paint from being defaced. The target moves easily on the rod, being kept in any position by the friction of the two flat plates of brass which are pressed against two alternate sides, by small spiral springs, working in little thimbles attached to the band which surrounds the rod. There is also a clamp-screw on the back, by which it may be securely fastened to any part of the rod. The face of the target is divided into quadrants, by horizontal and vertical diameters, which are also the boundaries of the alternate colors with which it is painted. The colors usually preferred are white and red; sometimes white and black. The opening in the face of the target is a little more than a tenth of a foot long, so that in any position a tenth, or a foot figure, can be seen on the surface of the rod. The right edge of the opening is chamfered, and divided into ten equal spaces, corresponding with nine hundredths on the rod; the divisions start from the horizontal line which separates the colors of the face, |