CHAPTER II.

THE HANDLING OF INSTRUMENTS

BEFORE attempting the simplest kind of survey with his instrument, the absolute tyro must handle it for a while in order to familiarise himself with the functions of its various screws and fittings. (See fig. 5.)

Adoption of Routine.

Every beginner is recommended, from the inception of his instrumental work, to adopt a routine method of handling his tools, and to adhere to it. This will tend to the avoidance of accidents and mistakes which may otherwise arise out of a moment's inattention. Many valuable instruments have been broken by picking them up without noticing that the screws or caps, which hold the telescope in the V supports, or the whole instrument to the stand, had been left unfastened. Probably as many more have been damaged by incorrect repacking in the box, with subsequent attempts to force the lid to close. When a new, or unfamiliar, instrument arrives, it is advisable to write in pencil the names of a few of the principal fittings against the position in the box where they lie, before taking anything out.

When handling the various screws, etc., no force must be used. A very light pressure suffices; unnecessary force will result in damage, or in moving the whole instrument on its stand, thus rendering accurate work impossible. The

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A. Vertical circle. B, B, B. Adjusting screws to diaphragm. C, C, C. Capstanheaded nuts for adjusting main plate bubble. C',C'. Capstan-headed nuts for adjusting small plate bubble. D, D, D, D. Capstan-headed nuts for adjusting azimuth bubble. E. Clipping screw for adjustment of verniers to vertical circle. E'. Locknut to clipping screw. F. Quick-release opposing spring to clipping screw. G. Clamping screw of upper limb. G'. Slow motion (tangent) screw to upper limb. H. Clamping screw of lower limb. H'. Slow motion (tangent) screw to lower limb. I. Clamping screw of vertical circle. I'. Slow motion (tangent) screw to vertical circle. J, J, J. Levelling screws (footscrews). K, K. Screws for vertical adjustment of horizontal axis. L. Level-bubble on telescope. L', L'. Capstan-headed nuts for adjusting level-bubble. M, M, M, M. Magnifiers for reading verniers. N, N. Screws for taking up wear of footscrews in tribrach arms. P. Horizontal circle. Q, Q. Caps to trunnions. R. Securing screws of caps to trunnions. S. Trough compass attachment. T. Screw which moves eyepiece across diaphragm. U. Azimuth bubble. V, V. Verniers to vertical circle. V'. Vernier to horizontal circle. W. Main plate bubble. X. Screw focussing eyepiece. Y. Spring locking plate. Z. Locking plate clamping screw.

fingers should be kept away from all graduated surfaces and lenses, nor should the instrument be touched with perspiring hands if it can be avoided.

Setting up Tacheometer or Theodolite.

The tripod must be firmly planted on the ground and the points pushed into the soil until a firm base is obtained. The lower part of the instrument, after noting the position of its various parts, as packed, so as to facilitate replacement, is then taken from its case and placed upon the tripod stand, without the telescope, which is usually packed separately. In order to fasten it securely to the stand, the clamp screw Z (fig. 5) is slackened, and the locking plate Y pushed to one side with the thumb so as to permit the rounded ends of the footscrews J, J, J to slide into the recesses provided for them. Thereafter the plate Y is allowed to spring back into place, and the screw Z is tightened, leaving the instrument securely clamped to the tripod. At this stage the instrument can be very roughly levelled by moving the tripod legs while referring to the horizontal plate bubbles.

The caps Q, Q are now turned back, and the telescope with its attachments is taken from the case. The vertical arc clamp screw I is slackened, and the arm turned downwards approximately at right angles. The telescope is then carefully lowered into position in the V-shaped bearings, in such a way that the projection on the cross bar of one of the standards comes between the clipping screw E and its opposing spring F. A partial turn of F releases the spring which presses against the projecting stop, and holds the arm perfectly rigid. The caps Q, Q are returned over the ends of the axis and fastened there with the screws R, R; the trough compass is placed in position within the slide S, and the plumb-bob is hooked to the ring beneath the vertical axis of the instrument, which can now be safely moved from place to place. The vertical circle should, if possible, be in the

position it occupied when the instrument was adjusted, as its weight and that of its fittings is not always properly counterbalanced except in the best instruments. If the circle stood over the vernier marked A when the level-bubbles were adjusted, it is best to work with it over the same vernier, as changing it to the other side may throw the bubbles slightly out of truth. (See Chapter VIII.)

The next step is to place the horizontal and vertical axes of the instrument exactly horizontal and vertical.

The Level-Bubbles.

In the theodolite, which has no level-bubble on the telescope, this is done by reference to the bubbles (fig. 5) upon the horizontal plate. But in the tacheometer a more delicate level L is fixed to the telescope, and it is better to use this bubble for the purpose. The plate bubbles, owing to partial shade thrown by the A-shaped supports, differences of temperature, or want of exactitude in adjustment, may not be in perfect agreement with it. In the course of a day's work, especially in hot climates or on sunny days, some variation between the bubbles will probably be found; it is therefore recommended that the large telescope bubble should invariably be the criterion in tacheometric work, or, in the event of this bubble being broken, then the bubble against the vertical arc should be used, except for making the preliminary and approximate adjustments when levelling up the instrument, for which the plate bubbles are sufficiently accurate.

The Vertical Circle Vernier Zeros.

It must not be forgotten that, when putting the telescope axis into the trunnions, one of the clipscrews E, or rather the spring F which replaces one of them in modern instruments, was moved. On the exact adjustment of the clipping arm depends that of the zero lines on the vertical arc verniers,

and although a spring clamp is used instead of an opposing screw, reliance cannot be placed upon its returning exactly to the correct position on every occasion.

The first time an instrument is levelled up after being taken from its box, it is therefore necessary to examine this adjustment in addition to levelling up the instrument. Thereafter it will not (or should not) require attention unless the telescope is removed from the trunnions to repack it in the box, or should the wrong screw be moved by mistake, while it can be disregarded altogether if no vertical angles are to be read.

The so-called permanent adjustment of the level-bubbles, diaphragm, and other parts of the instrument which may become deranged by rough handling or through other causes, should not be attempted by an inexperienced person. It is, of course, desirable that the surveyor should, as soon as possible and under proper supervision, learn to keep his instrument in perfect adjustment, without which he can have no certainty as to the accuracy of his work. A full description of the method is given in Chapter VIII.

Levelling the Instrument.

Presuming the permanent adjustments to be correct, the process of levelling the instrument, after setting it up, is as follows:

Release the clamp screw H of the lower limb and turn the instrument so that the longer level-bubble W on the horizontal plate lies in the position a-a' (fig. 6), parallel with two of the footscrews of the instrument. Bring this bubble to the centre of its run, either by turning these two footscrews in opposite directions, one with each hand, if it is far out, or by turning one only if it is nearly in the middle. Next turn the same bubble so that it lies over the other footscrew, in the position b, b', and with this screw only bring the bubble again to the centre of its run. Return to the