Fig. 153.—6-inch transit theodolite—back view, with sections.
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371.—The Lower Parallel Plate N.—This has a large boss-piece taken up from its central part, which forms a dome of a hollow globular section, technically termed the socket, shown at X. In the interior of the lower part N a coarse female screw is cut, of about fourteen threads to the inch, which is used to attach the instrument to its tripod.
372.—The Upper Parallel Plate is constructed as a flange from a solid boss L. This piece is generally made in gun-metal of a form as solid as possible, to resist the straining action of the parallel plate screws. The boss is prolonged downwards by a stem-piece, upon the lowest part of which a ball collar of globular section is firmly screwed. The screw is turned by means of two opposite holes, into which a powerful forked screw-driver is inserted, until it is jambed up too tightly against its shoulder to ever become loose by the ordinary use of the instrument. The ball collar fits into the socket carried up from the lower parallel plate. The whole of this globular arrangement is termed the ball and socket. The boss L of the upper parallel plate, with its stem, has a hollow conical hole through its axis, into which the body-piece, to be described, fits accurately. Upon its outer upper part an inset collar is formed which acts as a guide to the clamp K. At the outer edge of the parallel plate M′ four vertical, conical holes are made, which take socket-pieces, which are tapped as nuts to the parallel plate screws M. These socket-pieces are jambed into their holes tight home to their shoulders. The socket-pieces are made separate, both to give a greater length of female screws than the thickness of the plates, and that they may be easily restored at any time if worn loose in the threads by the action of the plate screws.
373.—The Parallel Plate Screws.—One in elevation is shown at M, with its point dotted, and one in section at M′. The four parallel plate screws are in opposite pairs, placed exactly at right angles to each other in a line passing through the vertical axis of the instrument. These are made of gun-metal about 3/8 inch in diameter, with a deep thread of about thirty-two to the inch. They require cutting on a nice steady screw-cutting lathe. The lower points of the screws are slightly domed, sufficiently only for the amount of rocking they have to take, so as to impress the lower parallel plate as little as possible. The milled heads M are placed between the parallel plates, not above, as previously described for levels. There being a constant strain upon these screws in use and by intrusion of grit from flying dust they soon become worn. After wear the threads may be recut deeper, and new socket-pieces fitted to the upper parallel plate. To prevent wear the upper parts of these screws are sometimes encased in tubes—a plan very generally adopted in America. At the foot of one of the parallel plate screws a stay-piece is fixed to the lower parallel plate, which forms a kind of ring round the screw. This prevents the parallel plates from shifting upon the axis at the ball and socket. The parallel plate screws should be without any shake or what is technically termed loss of time. They should move firmly but softly. They should support the instrument against the ball and socket upon which the whole rocks to position by their aid, but not be screwed down too tightly, as this has a tendency to disturb the axis of the instrument however solidly it may be made. Makers often have instruments in their hands for repairs in which the parallel plate screws have been deeply indented into the lower parallel plate, with the centre of the instrument permanently strained more or less.
374.—The Body-piece.—The only outward part seen in elevation of this is shown at T: it is shown in section T′. This piece carries the limb of the instrument SS′ by a centred collar to which it is attached by screws. About the centre of the body-piece an inset collar is formed to take the clamp KK which bites upon it. The lower outer part of the body-piece forms a conical fitting in the boss of the upper parallel plate L. The interior is a hollow conical axis. The body-piece is generally made of hard gun-metal. The greatest possible care is required in its manufacture, [art. 21]. The interior and exterior should be perfectly concentric at every part. Much of the value of the instrument depends upon the perfection of the work in this piece.
375.—Axis Collar Clamp K has been already described, [art. 349], and is illustrated in [Fig. 146], which is taken from a theodolite, so that only specialities in relation to the instrument [Fig. 153] need be noted. This clamp surrounds the body-piece and clamps it by means of the screw K shown on the left hand. The clamp is connected with the upper parallel plate through the tangent screw, the head of which is shown at P, so that when the screw K is tightened the parts L and T are fixed together, except that a slow motion can be given to these parts by the tangent screw P. By this clamp and tangent arrangement the whole of the upper part of the instrument is rendered free to revolve, to bring the instrument to bearing when the clamp is loosened, the final adjustment being secured after clamping by the tangent screw. It is this part of the instrument which is used after setting it up to bring the magnetic needle true to magnetic north, or otherwise to direct the telescope to any established distant mark, object, or star that may be fixed for the zero or other index point of the horizontal circle, to which all readings from its position are referred.
376.—The Central Vertical Axis is shown only in half section at Z. This is made uniformly of bell-metal, in the form of a truncated cone, extending from the horizontal circle plate S to the interior of the socket N. Its fitting surfaces are at the two ends of the cone, extending about half an inch, the central part being chambered back. At the upper part a pin-piece centres the vernier plate, to which it is attached by a wide collar with three or four screws. A square shoulder rests with weight only just sufficient to support the instrument upon the body-piece. This part has to be so adjusted that the axis perfectly fits and yet moves freely. A square-hole collar and screw are fixed on the lower end of the axis, just to touch the socket of the body-piece, so as to secure the axis in its position when the instrument is lifted. An eye or a hook is fixed into the screw at the lower end to take the cord of the plummet used for fixing the instrument over a definite point on the ground. This is not shown in the engraving.
377.—The axis of an ordinary theodolite is made the weakest part. It is generally considered in the trade right for it to be so, as in case of accident no other part of the vertical axis system is likely to be deranged; and this is the easiest part to replace, being, as it were, independent of other fittings. Whether this should be taken cum grano salis is a question; at any rate with the axis weak it is not policy to load the upper part of the instrument with metal—which in places at least, is generally made ten times as strong as the axis—when the instrument has to be carried about by a person over his shoulder. Some suggestions will be made on this point hereafter.
378.—The Horizontal or Lower Plate or Limb.—Sometimes the whole of the piece SS′ is termed the limb, but more generally this word is applied to the divided part only. This plate is of brass, and is attached to the body-piece by screws. The outer rim, which is somewhat triangular section, is undercut upon the inner side of its lower surface to support the clamp-piece, the outer edge being turned to a fillet to take the clamp which is rebated to fit it. The upper surface of the rim, or the limb proper, is turned to the frustum of a cone of about 45°. This part is covered with silver, which is beaten out to the conical form and soldered down upon it, and afterwards turned to true form. The dividing has been discussed in the last chapter.