Surveying and Levelling Instruments, Theoretically and Practically Described. / For construction, qualities, selection, preservation, adjustments, and uses; with other apparatus and appliances used by civil engineers and surveyors in the field.

by William Ford Stanley

Fig. 155.—Cross section of the upper part of a transit theodolite.

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383.—The Standards or A-Frames, shown C′ C″ Fig. 155, are solid castings in brass of about 7 inches in height. They are set up upon the vernier plate, to which they are attached by four stout screws, as also by cross screws to the compass as stated. This renders the superstructure of the transit as firm as may be in a built-up construction. Upon the front of one of the standards a spirit level, Figs. [153], 155, I, is placed adjustable by two capstan screws. This level, and one shown [Fig. 153] at 0 on the vernier plate are used entirely in setting up the instrument; and being placed at right angles to each other, are a means of making the vernier plate quite level. Upon the inside of each of the standards, at about 2 inches from the vernier plate, a clip-piece, Figs. 154, 155, P is secured by two screws. This takes the clipping screws, Fig. 154 HH′ to be described. At the top of the standards two V's are formed, upon which the transit axis rests. One of these is cut out of the solid casting. The other as shown in half section Fig. 155 c is formed as a parallel sliding piece with the V at the top placed in a vertical slot formed in the standard. This sliding piece has a screwed stem continued from its lower surface that passes through a vertical hole at the top of the A-frame, which is formed here as a cross-piece. Upon the screw two capstan nuts are placed, one on each side of the cross-piece, Fig. 155 xx′; these permit the adjustment of this in height so as to get the transit axis perfectly horizontal when the vertical axis is perfectly perpendicular to the horizon. The sliding piece is covered by plates back and front to render it firm in its position. The transit axis in practice is adjusted with a striding level which will be described presently.

With the author's theodolites from 6 inches downwards the old-fashioned adjustment to one upright for levelling the horizontal axis has been dispensed with for many years, and is only fitted if specially ordered, as it has been found to be a frequent source of error. Long experience has proved beyond doubt that the fewer adjustments there are, and the more parts that can be fashioned from the solid metal correctly, the longer will the instrument keep in adjustment. Should there ever be any wear on either of the V's a few strokes with a piece of very fine emery paper upon the opposite one will put it right in half the time that it could be corrected with the old-fashioned adjustable V, and no amount of vibration can alter it as with the adjusting screws.

An axis cover cap bb′ is placed on the top of each standard. The cap is screwed down at one end with a cut screw and collar. The screw is used for adjustment to gentle pressure on the axis. The second screw is a milled head EE′. Under this screw the cap is slotted out to one side, and turns on the cut screw as an axis to open the cap without removing its milled-head screw, so that the telescope can be lifted out to turn its face to the opposite side of the instrument. In the under side of the centre of the cap a cell is bored out, into which a small cork is fitted, which produces, when the cap is clamped down, a soft elastic pressure on the axis.

384.—The Transit Axis which supports the telescope rests at its ends upon two trunnions, Figs. [154], [155] AA′, technically called pivots, in the V's of the standards already described. The pivots are turned as true as possible, and afterwards ground to exactly equal size in a collar, so that they may be reversed end for end in their bearings without changing the linear direction of the transit axis, except by the little difference of pressure that one end of the axis imposes by the weight of the vertical circle and its attachments being eccentric. In larger instruments this difference of weight is counterbalanced, as shown in dotted lines at p, [Fig. 155]. The centre of the transit axis is formed into a collar e of about 1¼ inches in width, which exactly fits the outer tube of the telescope, and to which it is fixed with soft solder. The collar is directly connected with and supports a flange f. Upon this flange the vertical circle FF is fixed by three or four screws.

385.—In front of the vertical circle a flanged collar-piece carries the vertical vernier frame VV′, [Fig. 154], centred upon it. The vernier frame is attached by three screws to the clipping arm to be described, and in front of this the vertical microscope arms are centred. These carry two readers U, [Fig. 155], exactly similar to those which read upon the horizontal circle, and they are similarly centred, so that by setting one, the other is set at exactly 180° from it. In front of the centre of the microscope arms on the transit axis, an axis collar-piece j is attached by three screws cut directly into the axis. This collar and one at the other end of the axis A′, turned out of the solid, are nicely fitted to the opening between the standards to prevent lateral displacement of the axis.

386.—The Clips.—The clipping arm, which is centred on the transit axis and attached to the verniers, is shown [Fig. 154] BB′B′. It is fitted to move freely on its axis at A, so as to permit unrestrained motion of the telescope. A milled-head clamping screw with clamp, [Fig. 155], K, and the same partly cut away to show the slot in which it works, are shown at K′ [Fig. 154]. This is used to fix the verniers stationary on the circle, except for the adjustment by the tangent screw G′, which has its collar attached to the clipping arm, and its ball nut attached to the clamp at D when using the telescope for levelling. This clamp and tangent sets the vernier to zero on the circle. It is also used in setting the telescope before angles of altitude or depression can be measured. The clipping screws HH′ are used to bring the principal bubble B, [Fig. 153], on the top of the telescope to the centre of its run after the verniers have been brought to zero by means of the clamp and tangent screws. The clipping screws hold the clips, [Fig. 155], P or P′ to the one standard or the other. The whole of the vertical adjustment is exactly equivalent to that already described for the horizontal motion, except that it is placed in the vertical plane.

387.—The Vertical Circle, Figs. [154], [155], F is carried by four arms from a central boss attached firmly by screws to the transit axis. It is grooved at the edge to take the clamp-piece. The silver is inlaid in this circle in the manner shown [Fig. 117]. The vernier is read upon the circle on the plan shown [Fig. 127]. The circle is divided generally to half degrees or 20′, and is figured 0 to the horizontal with 90° upwards and downwards. The zero lines are made directly coincident with the optical axis of the telescope when it is level. The vernier reads to half minutes or 20″, in either direction, the rising arc above the level datum being considered as plus, the falling arc as minus.

388.—On the outer edge of the circle or at the back a scale of difference of hypotenuse and base reads to a line on a fiducial edge upon a part of the clip BB′, [Fig. 154], at N. This scale is calculated for decimal quantities, and gives the percentage number of links, feet, or metres to be deducted from the chain measurements upon the ground line to give the horizontal distance corresponding to the angle of inclination at which the telescope is set for observation.