842

THEORY AND PRACTICE OF ENGINEERING.

Rook II.

taken. This instrument, by means of a screw, is fixed on three legs ; beneath the centreof the start’ head is a hook, to which is attached a plummet, that very much aids the observerin placing the instrument in a horizontal position.

To adjust the theodolite properly is highly important, and the first point to be attendedto is, to draw out the tube of the eye-piece till the cross wires are clearly and distinctlyseen: the next adjustment is that of the line of collimation , the term given to the linepassing through the point of intersection of the cross wires, fixed in the focus of theobject and eye-glasses, and the centre of those glasses; this, when properly adjusted,should coincide with the axes of the cylindrical rings, in which the telescope turns ;this is performed by making the cross wires coincide with a well-defined part of someobject in the distance, and turning the telescope half round till the level is atthe top; when on looking through it, the same point should be covered by thecentre of the wires, which, if it is not, must be acquired by moving the centre halfthe amount of the deviation, by means of the diaphragm screws, and correcting theother half by elevating or depressing the telescope: when the wires and the objectremain perfect in both positions of the telescope, the line of collimation in altitude ordepression is correct. This process must also be resorted to, to adjust the line of collima-tion in the vertical plane. After this, the level attached to the telescope is fixed in aposition parallel to the rectified line of collimation, or longitudinal axis of the telescope:to effect this, open the clips that retain the telescope, and bring the vertical arc at or nearzero; then turning the tangent screw, bring the air-bubble of the level to the centre ofthe tube; then reverse the telescope, and if the bubble does not return to the middle, bringit there, one half by turning the screw placed at one end of the tube, to elevate or depressthat end of the level, and the other half by the tangent screw that acts on the vertical arc.The circular plates must also be adjusted until they are perfectly horizontal, as any devia-tion would produce in the measurement of an angle a proportionate error. 'The bubbleof the level under the telescope must be brought to the middle of the tube by the tangentscrew of the vertical arc; then turn the upper plate 180 degrees from its former position.Should the bubble not return to the middle, half the difference is to be corrected by theparallel plate screws, and half by elevating or depressing the telescope by means of thetangent screw : this operation is repeated over the other pair of parallel plate screws,until the air bubble of the spirit level attached to the telescope remains permanently in thecentre of the tube, in whatever position it is turned. The two small levels on the vernierplate are then to be adjusted by the screws adapted for the purpose. The vernier of thevertical arc is next to be looked to, and should point to zero after all the other adjustmentsare effected, and any deviation from that point must be rectified by the screws which areattached to it: should the deviations, however, be small, note the amount, and apply it byadding or subtracting from each vertical angle observed. This deviation may be readilydiscovered by repeating the observation of the altitude or depression in the reversedpositions both of the telescope and vernier plates ; the two readings having equal andopposite errors, half their difference will be the index errors. When the theodolite hasbeen thoroughly and properly adjusted, it is placed exactly over the station from whencethe angles are to be taken; and this is done by observing the direction of the plumb linewhich is suspended to it: it is then set level by the parallel plate screws bringing thetelescope with the vertical arc clamped at zero over each pair alternately. Clamp thelower horizontal limb in any position, and direct the telescope to the object, moving ituntil the cross wires correspond with it; then clamp the upper limb, and by the tangentscrew make the intersection of the wires exactly bisect the object; then read off the twoverniers, noting the degrees, minutes, and seconds of both, and take the mean of the twoobservations. Then release the upper plate, and move it round, until the telescope isdirected to the second object, whose angle is required, from the first already observed,and by the clamp and tangent screws make the cross wires bisect the object. Then readoff the verniers, and the difference between their mean and the mean of the first reading willbe the angle required.

To measure angles of elevation or depression, unclamp the vertical arc, and direct theintersection of the cross wires of the telescope to the object: note the reading of thevertical arc, and rppeat the operation with the telescope turned half round in its Y’s with thelevel uppermost; the mean of the two readings will correct the error in the line of colli-mation. The magnetic bearing of an object is taken by simply reading the angle made bythe needle with the common compass, which is ordinarily attached to the theodolite, andusually four or five inches in diameter: the angle cannot be obtained with any truthnearer than by degrees. The telescope shows the objects seen through it in an invertedposition ; the reason for this inversion is that objects are seen more clearly by the omissionof some of the glasses : a second eye-piece is usually provided, which, when applied, showsthe objects in their true and natural position.

The Circumferenter is sometimes used where great accuracy is not required in the survey:it is a simple instrument, and consists of a flat bar of brass, 13 B, about 15 inches in length,