Fig. 193.—Simple theodolite.
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448.—Simple Theodolite.—The plain theodolite being of the cheapest construction may be stripped of its superior functions, which are used for testing its adjustments, and be made into a simple angle measurer for laying out or plotting small parcels of ground, small estates in building ground, local sewage, gas and water works, and many other cases of small surveys, for which purpose it will be found sufficient, with a saving of about half the cost of a perfect theodolite. The instrument shown above, Fig. 193, was designed by the author to meet the above cases. In this instrument there is no vertical arc. The telescope has a socket axis carried upon a single standard. The axis cannot be seen in the figure from interference of the telescope placed in front of it. The telescope is arranged to be fixed in a level position by means of the loose pin being pressed in a pair of holes. It may then be used as a level by means of the spirit level shown on the vernier plate. The horizontal circle is divided to read with a single vernier to 3′ of arc by means of a hand magnifier which is placed in the case with the instrument. There are internal and external axes, each provided with clamp and tangent motions to the horizontal circle, as with the plain theodolite. It is supported on a tribrach, the legs of which are upon the plan, [art. 249].
If it be made with two verniers and divided upon silver it becomes a useful light instrument for filling in details of superior triangulation. Weight, about 4¾ lbs.
449.—Examination and Adjustment of the Theodolite.—The description given of a transit theodolite, [arts. 369] to 389, will show that this instrument is provided with means of adjustment in every requisite direction. Larger transit instruments possess the same means of adjustment, but in some parts these have greater refinement. Plain theodolites have the like methods, except in the case of the transverse axis, which is adjusted once for all by the maker. It will be necessary, therefore, to limit our space to a discussion of the examination and adjustments of the transit theodolite only, of which we have given a full description, [arts. 369] to 389, noting only where variations occur from partial differences between this and others.
450.—A theodolite as it comes from a respectable maker is usually carefully adjusted in all its parts. If it has travelled a long journey it is, however, well for an experienced surveyor to put it through its various adjustments. The corrections, if any are required, will be generally very small, and these in all probability will be of the same kind as will occur in the use of the instrument and in the accidental conditions to which it may be subjected during conveyance from place to place upon a survey; therefore it is well to be familiar with them.
When a new instrument is received from the maker, it is necessary to observe attentively the manner of its packing as it lies in its case. It is well at first to lift the parts a few times gently out of the case and replace them, so that this may be done at any future time with certainty and without any risk of strain upon the instrument, remembering always that an instrument in conveyance is much more liable to be thrown out of adjustment by carelessly replacing it in its case than from its ordinary use, [art. 42].
451.—For examination or adjustment of the theodolite the tripod stand should be at first firmly fixed with legs extended to an angle of about 70° to the ground, which should be solid and hard. As the telescope has to be brought to the height of the observer's eye, it is well to mention his stature in ordering an instrument. The tripods that are made for tall men are often awkward and unsteady if the legs are extended to bring the telescope down to the height of a short person. They may always be cut down and refinished by the maker. When the tripod is set up the toes should be each separately pressed down, so that future slips are impossible. This being done the instrument is taken from its case and grasped firmly by the body part under the horizontal circle, and placed on the tripod at once, then screwed firmly but not too tightly down upon its bearing surface. With a 6-inch transit theodolite the upper part is sometimes detached and packed separately in its case. Where this is so, after the body part is fixed on the tripod, the cleats on the top of the standards must be opened out, and the upper part of the instrument, lifted by its telescope, be slowly lowered into its bearings, being particular at the same time that the clips under the telescope embrace their stay-piece on the standard. The cleats must then be closed over the pivots. The instrument being set up to position, all levels may be adjusted to the centres of their runs, and every part clamped sufficiently to make the instrument firm, but in no case using violence to produce a strain in any part. The clamps or other fittings are afterwards separately released as they are required for examination or adjustment of the parts to which they relate.
452.—Examination for Coincidence of Exterior and Interior Vertical Axes.—The theodolite being set up solidly, and all clamps fixed as above described, unclamp the lower or exterior axis clamp and set the vernier plate levels parallel with opposite pairs of parallel plate screws if the instrument adjusts on the parallel plate system, [art. 193], or one level parallel with one pair of foot screws if it is made on the tribrach system, [art. 234]. Now adjust both levels. Turn the instrument half round (180°) and observe if the levels keep the centres of their runs. If they do so they are in adjustment to the exterior axis. If found imperfect, the adjustment by the capstan heads of the levels is set, by means of the tommy or pin which is provided in the instrument case, for half the error as it appears by the bubble, the other half being given by readjustment of the parallel plate or tribrach screws. In these adjustments it is necessary to be particular always to observe the bubbles after the hands have left the instrument, not during the adjustment, which produces strain upon the instrument. Now clamp the lower clamp and note if this clamping has disturbed the levels. If the levels are very sensitive it will do so in a slight degree, but the disturbance should be very small if the clamp is perfect. Now unclamp the vernier plate and note again if this clamp disturbs the levels: this should also affect them very little. Now observe the levels if they stand exactly as they did when the exterior axis was unclamped at their present position, and also at right angles to this. If they remain as before the axes are truly concentric. If they do not, there is no remedy except at the hands of the maker. The vertical axis to which the above examination applies is considered the most important part of the instrument, and the work should be thoroughly well done; nevertheless, if the levels are very sensitive, which they seldom are, such minute faults may be detected, that a small allowance may be made for imperfection of work, and the instrument still be considered a sound one. In the use of the instrument it is always well, after the circle is set either by the magnetic compass or by sighting a distant point for direction, to clamp the lower clamp and readjust the levels to the vernier plate. In this way the axis that will afterwards be used for triangulation will be vertical, and small errors due to want of coincidence of axes be eliminated.