For the transit theodolite, adjustment by means of a collimator, [art. 229], is much more convenient and exact, as lateral and vertical errors in the position of the webs can be detected in one operation. When a Y-level is at hand, this may be used as a collimator if it is first set to solar focus.
462.—Examination for Perpendicularity of Transit Axis and Telescope.—The whole of the lower part of the instrument retaining its position with all clamps firm, open the cleats upon the top of the standards so as to release the transit axis. Now release one of the clip screws and gently lift the upper part of the instrument out of its bearings. Turn the telescope the reverse way upwards, which will be in this case bubble downwards. Release the clamp and turn the clips to the reverse position of the telescope, and reverse the position of the pivots in their bearings. If the telescope be now directed to the same point as before, and the webs still fall upon it, the telescope adjustment is at right angles to the transit axis.
463.—Examination of the Magnetic Needle.—If the needle be placed in a circular box, as shown in the engraving, [Fig. 30], it admits of no adjustment. If it is placed in a trough, [Fig. 161], it admits of adjustment generally by lateral screws to a portion of its division. If the needle is used for a survey, it is set to the zero of the horizontal circle by clamping the vernier plate and bringing the northern vernier to zero, then releasing the exterior axis and bringing the needle by the motion of the lower tangent screw to the zero of its circle. The corrections of the needle for giving true north have been discussed, [art. 132]. It is difficult to read an ordinary edge-bar needle correctly, it is also difficult to mount it perfectly true. It may be read at both ends, and if the 0° and 180° points cut the line fairly it is considered correct; if not, the mean of the difference may be taken. In some instruments a microscope is mounted over the needle point that the needle may be adjusted to a web; but British surveyors seldom feel confident of surveys by the magnet, and for triangulation generally prefer to employ a certain number of distant fixed points, the bearings of which are at first as accurately ascertained as possible, for referring objects, rather than to refer frequently to the magnet. When the needle is out of use it should always remain lifted off its centre. When the instrument is put by for a long period it is better to place it in a vertical position and free the needle, so that it rests in the magnetic meridian, in order to preserve its magnetism as much as possible.
464—Use of the Theodolite.—In setting up a theodolite, place the tripod nearly over the position in which it is to be used. This is frequently the socket hole formed in the earth by the removal of a ranging pole or picket, to be described Chapter XVII. Then, after it is set up, suspend the plummet from the hook, which will be found inside the head of the tripod. If the ground be solid and level, then by shifting the toes of the tripod slightly, and firmly pressing them down one by one, the centre of the plummet may be brought easily within about ·25 of an inch of its true position. The theodolite is then placed on its tripod, observing that the telescope is in a position easy to be used. The centre of the picket-hole, when this is used for a station, is generally taken by guess-work, which is considered near enough. It may be taken with a little more refinement by placing, in the same hole, a short false picket of nine inches or so in length, but of the same diameter as the ordinary picket, the top of which is cut off smooth and polished, and has lines sawn across its centre inlaid with ebony, described in Chapter XVII. The false picket is carried about with the theodolite. With Everest's and many other forms of theodolites the hook is fixed under the axis of the instrument. In this case it is usual to set the theodolite before adjusting it to the station, as there is no separate hook to the tripod, which also occurs with all framed stands.
465.—Where there is no hook to the tripod an excellent plan is to have a false centre, which may be a piece of turned wood with a hole through it, to fix on the top of the tripod head. The plummet cord adjusts through the hole. This false centre is also convenient where the axis adjusts to position by a mechanical stage. Fig, 194 shows a false centre formed of a piece of ivory with two slots to permit the cord to loop over and yet hang centrally.
466.—It may be observed that if the tripod be set up out of level, which it must necessarily be in many cases, the hook, if attached to the stand and following its inclination, will not hold the cord at a truly vertical position to the axis. Surveyors commonly allow a little for this inclination. It is much more accurate to have the cord suspended directly from the axis of the instrument when it is constructed to admit this. Then if a false centre be used the plummet should be suspended a second time from the axis hook. With the kind of runner shown in the figure this need take little time, as it is instantly detached and replaced.
Fig. 194.—False centre for a tripod head.
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467.—After the tripod is fixed with the theodolite upon it, the readers are set to exact focus. The horizontal circle is then brought to zero by the vernier plate clamp and tangent, and the compass brought to magnetic north, if all angles are taken in reference to this as a check, by means of the lower clamp and tangent. The vernier plate clamp is then released. The eye-piece is correctly focussed upon the webs, lines or points against the northern sky, or upon a piece of white paper held obliquely if this is preferred. The telescope is then ready to be directed towards a picket or other station mark to be observed, and set correctly to focus this, after which the eye is moved to the right and left, to the extent of clear vision in the eye-piece, to see that the object appears to remain fixed upon the intersection of the upper V of the webs, or does not dance, as it is sometimes termed. The observation, if of a picket, should be taken as near the ground as possible, as it may not be set quite upright. If the telescope is directed to objects where the sun's rays would enter it, the ray shade should be pulled out sufficiently to quite shield the object-glass. The initial reading to be recorded is always taken on the face of the instrument, in which position the upper tangent screw is always on the right-hand side. When the observation is clear and satisfactory, it is recorded in the field-book. If the sight lines taken are to be measured by the chain, the amount of inclination is taken by the vertical circle reading to the top of the picket if this is the 6-feet ordinary length, or to a marked band if this is longer. The inclination may be taken exactly to angle by vernier, or roughly by scale of difference of hypotenuse and base if this is engraved on the vertical circle, or by both of these—the one as a check upon the other. It is common to take the upper inclination as a plus (+) and the lower as a minus (-). Inclination observation is recorded at the same time as the horizontal position. Other observations of the various positions or pickets are taken in a similar manner at the same time. When a stadia or tacheometrical diaphragm is used the angle is recorded and the stadia reads the distance. It is thought well when the theodolite is in position to take as many exact observations as possible in all directions of intended stations. It is also convenient to take a number of observations, which from the circumstances present may be inexact, such for instance as the angles subtended by trees, gates, rough buildings, or even sometimes the corners of fields, as from such observations these objects may be placed nearly enough for ordinary plotting by the angles they subtend from this and another station upon the plan. In any case they form a check to positions if taken with pickets afterwards more definitely. These may be marked in the field-book inx. for inexact.