The 6-inch instrument is generally divided to 20′, but sometimes to 30′, and the vernier reads to 20″ or 30″. The figuring is from 0 to 360, right to left, taken facing the instrument.
379.—The Vernier Plate is shown in section under P′. The vernier from which it is named is shown at VV′, [Fig. 155]. The vernier plate is carried from the central axis and forms the foundation for all the superstructure. The upper and lower plates are left very free where they are brought together, the verniers being generally sprung down just to gently touch the limb. The vernier surface is let down some distance into its plate for protection. The reading of the vernier has been discussed in the last chapter.
380.—It may be particularly noted, as already stated, that the central axis and the body-piece are attached to the vernier and horizontal plates by screws. This plan might strike one as being unsound: it is not really so, the reason for this construction being that these axes are, or should be, of bell-metal, and that this metal being very hard and brittle it would not be so easily worked, or so serviceable as brass for the limb and vernier plate, neither would there be means of correcting errors which generally occur both in the workmanship and in the dividing of this delicate part. The adjustment for fixing the limb and vernier plate, technically called centring, in particular requires considerable technical skill. It is generally performed by the divider, who is a specially intelligent artisan. In the author's improved theodolite, to be described further on, the axis is in one casting with the standard; but in this case the construction is different, the axis being made larger and the whole body being in a special gun-metal which approaches bell-metal in hardness.
381.—The vernier plate carries the ball nut of the tangent screw, shown at [Fig. 153]J. The general arrangement may be seen by the section, but is more fully described art. 347. One thing is important in this screw, viz., that it should range without strain quite parallel with the plates, so as not to give the slightest tendency to elevate or depress the edge upon which it is placed during motion in any part of its thread. The clamp is sometimes placed between the plates.
382.—The Compass-box.—The general construction of this is shown, [Fig. 155], W. In the transit theodolite it is fixed firmly by screws to the vernier plate and is made to form a steadying piece to the A-frames C′ C″ which support the upper part of the instrument. For this purpose the compass-box is made as a solid casting in brass, which is much stiffened by the raised step which forms the divided circle. Four solid lugs in the same casting project from the rim of the compass, and form stiffening pieces between the lower parts of the A-frames; these are secured to the lugs by four screws, one of which is shown, [Fig. 153], at a. The lug screws hold the whole superstructure together quite independently of the vernier plate, to which it is afterwards firmly fixed. The compass needle is lifted by means of a milled head, just inside one of the standards, not shown. For a general description of the compass-box see [art. 138]. The vernier plate carries two or more verniers. The verniers are read by a pair of microscopes, Fig. 155 MM′ placed one on each end of a radial arm N having its axis of motion upon a large collar of the vertical axis. By this plan, when one microscope is set to read by the coincidence of lines upon one of the verniers, the other microscope on the other arm or arms will be set also in like position over the other vernier or verniers.
Fig. 154.—Vertical circle with clipping arm of transit theodolite.
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The verniers are adjusted ready for reading when the telescope is accurately directed upon any object of which it is desired to ascertain the angular position in relation to magnetic north, or a definite object. The vernier plate also carries a spirit level at O, [Fig. 153], which is adjustable by a pair of capstan-headed screws.
