Chapter XII

424.--_The Mechanical Tribrach Stage._--This important addition to the theodolite above described permits exact adjustment over a station. The upper plate of the tribrach with the movable stage is shown in Fig. 177. A dovetail slide is fitted upon the base of the stage adjustable for wear by a slip-piece with two screws at the narrow part. The slide is adjusted to position in the direction of its dovetail fitting by a large milled screw so as to move the whole instrument above it for centring in this direction. An upper slide acting in the same manner, with dovetail fitting pieces at sides moves for an equal distance for centring transverse to the lower slide by a milled head. This gives the same kind of motion of displacement that we have in the slide rest of a lathe or the mechanical stage of a microscope, except that in this case we have a kind of three-point bearing surface. The motion given to the screws permits the perfect adjustment of the theodolite over a point on the ground corresponding with the suspended plummet, after the instrument is set up to nearly its true position by movement of the tripod legs. The range of motion is from ¾ to 1 inch, a quantity quite sufficient for final adjustment, but which does not materially affect the equilibrium of the instrument upon its rigid tripod, as it has in this case a broad solid base even in the extreme positions of the slide.

This movement being _above_ the levelling screws, the adjustment of the instrument for level is not affected by its use, as in the case of all sliding arrangements _below_ the levelling screws. Suitable means are provided for taking up any wear that might occur in the slides.

425.--The above stage is supported upon three foot screws, the female fittings being specially long to give plenty of bearing surface to prevent wear; they are sawn down on one side so that they spring lightly upon the screws, and are provided with cross capstan screws for tightening up when necessary. This plan gives the screw about ¾ inch of thread, and permits adjustment for comfortable movement and for wear without any risk of shakiness. The screw in larger instruments of this class has a cap to exclude dust. The foot of the screw has a ball which rests in a slotted tube before described, Fig. 71, p. 127.

426.--This theodolite with mechanical stage is generally fitted with illuminated axis for tunnel work, art. 383. The lamp is not shown in the illustration.

The weights of this make of theodolite are about

6-inch in gun-metal 18¾ lbs., aluminium 10 lbs.
5-inch " 13¾ " " 7½ "
4-inch " 9½ " " 4½ "

427.--=8-inch Transit Theodolite.=--For ordinary surveying the smaller instruments are sufficient. For opening a survey in new countries the 8-inch instrument, Fig. 178, or a larger one, is generally used for the superior triangulation, particularly for observations at night of distant lights when greater light-grasping power is demanded of the telescope. The larger circle gives a more exact reading of the limb, which is generally divided to read clearly to ten seconds of arc, and by estimation sufficiently near to obtain five seconds reading very approximately with the verniers. When instruments exceed 8-inches, the reading is by means of microscopes, the application of which will be described further on.

428.--The 8-inch transit illustrated is of the author's model. It is in general structure similar to the 6-inch just described, except in certain specialities. The instrument does not clamp upon the vertical circle, but a similar circle is provided upon the opposite side of the axis. This answers two purposes, it balances the pressure upon the pivots and obviates disturbance of the division by the clamp. The principal bubble is supported upon the vernier frame, as special exactness is not required for the instrument to be used as a level. The base support is upon the Everest tribrach system, which will be described in the next chapter. A long compass is shown, but a telescopic compass is sometimes used. The instrument is shown with an axis-lamp and diagonal eye-piece for star observation.

429.--The vertical axis of this instrument is sometimes pierced for a look-down telescope to sight its vertical position on the ground to the centre of a peg. This will be described with geodetic instruments in the next chapter. It is an expensive refinement, seldom necessary, as the axis with plummet may easily be brought within the tenth of an inch.

Theodolites of eight inches and over are uniformly packed in two cases. The lower part is packed in a case by itself, the upper parts connected with the vertical circle and all the accessories, eye-piece, plummet, etc., forming the contents of another case, each part being sufficient for one man to carry without the tripod. The weight of the entire instrument is 29½ lbs.

430--=Quick-setting Theodolites.=--The demand for instruments with quick-setting arrangements has greatly increased of late years. They save a great deal of time in setting up, and also save wear of the levelling screws, as the instrument may be instantly set nearly level by its means, so that less than half a turn of the levelling screws will bring it to true level. An ordinary transit is shown Fig. 179, fitted with a similar arrangement to that described art. 240, p. 132.

431.--=Railway Theodolite.=--There are objections made to the transit theodolite by some civil engineers that it is a large and heavy instrument, only to be accepted for its perfect convenience over lighter forms. To meet this objection the author has made a special transit theodolite, Fig. 180, which is sufficient for railway work and general surveying upon moderately level country. The transit principle is conserved by balancing the telescope on its axis to permit it to transit over the eye-end only. The vertical arc is omitted as being unnecessary for railway work. The instrument is constructed especially low and of great rigidity and solidity, with light weight. The compass is of the trough kind. The limb is covered for protection. It is extremely portable. Weight of 4-inch, 7¼ lbs.; 5-inch, 9½ lbs.; 6-inch, 12¾ lbs. Being constructed for rough, hard wear and local use it is not made in aluminium.

432.--For tunnelling underground railways the mining theodolite described further on will be found the most valuable for railway engineers.

433.--=Mountain Transit Theodolite.=--This instrument, Fig. 181, is designed for geographical exploration, and making sketch surveys. It embraces the transit principle for the convenience of taking zenith stars. It is made in 3-inch and 4-inch sizes. It has two verniers to the horizontal limb reading to minutes, and a single vernier to the vertical circle. It has been made by the author in aluminium alloy only, the total weight being 2¼ lbs. for the 3-inch and 3½ lbs. for the 4-inch. The eye-piece reads direct or diagonally. It has clamp and tangent adjustment to both circles, and a trough compass. The tripod slides up to half length, each leg being adjustable to fix to any length within the range of the slide to accommodate it to the surface of inclined rocks.

434.--_A Mountain Theodolite_ is a term applied to any very small or light theodolite. These are generally made to order, very frequently to a reduced model of a larger theodolite, 3 inches being a common size. The telescope is occasionally placed upon the side of the horizontal axis to transit. Theodolites of this class generally weigh much more than the above-described instrument, a common weight being 5 to 7 lbs.

435.--=Improved Solar Attachment.=--The reviser's improved solar attachment admits of a full vertical circle being employed; it also has a clamp and tangent to the hour circle and declination arc and quick acting clamp and fine adjustment to the solar arm. An instrument so fitted is shown on p. 260, Fig. 182.

436.--=Micrometer Reading Theodolites.=--The favor with which the smaller micrometer reading theodolites have been received and the ever increasing demand for them owing to their much greater accuracy has induced the reviser to introduce a whole range of these instruments. In many cases where greater accuracy is required for horizontal than for vertical angles, the micrometers are only fitted to the horizontal circle and the vertical circle has verniers as usual. A useful instrument for general surveying without any unnecessary elaboration is shown below at Fig. 183.

The 6-inch instrument reads to 5 seconds of arc on the horizontal and to 10 seconds of arc on the vertical circle, and the 5-inch instrument to 10 seconds of arc on the horizontal and to 20 seconds of arc on the vertical circle.

A full micrometer reading instrument is shown at Fig. 184, the 6-inch reading to 5 seconds of arc on both circles and the 5-inch reading to 10 seconds of arc on both circles.

A specially light form of micrometer reading transit (Fig. 185) has recently been designed by the reviser which has met with much favour. It has a 4½-inch horizontal circle reading by micrometers to 20 seconds of arc which may be approximately read by mental subdivision to 5 seconds, and a 4-inch vertical circle reading by verniers to single minutes. It is also made micrometer reading to 20 seconds of arc to both circles. The compass is of circular form reading by microscope to ¼-degrees, which may easily be estimated to a third of this.

An example of a more refined micrometer transit is shown at Fig. 186. This instrument was designed by Dr. E. Deville, LL.D., Surveyor-General of Canada, and is arranged for latitude determination by Talcott's System, and general geodetic work.

It has a 6-inch horizontal circle reading by micrometers to 5 seconds of arc, and a 4-inch vertical circle reading by vernier to 1 minute, with zenith spirit level graduated to 2 seconds of arc, detachable and interchangeable with a small level for ordinary use. The telescope is 14 inches solar focus with large object glass so that observations may be taken of stars up to the sixth magnitude; it has a revolving micrometer diaphragm and electric illumination to micrometers, to diaphragm both front and back, and to zenith spirit level, it has also a rheostat for regulating the amount of light, and a dynamo generator for supplying the current.

The universal transit designed by the reviser is shown at Fig. 187.

This is an 8-inch instrument, reading by micrometers on both circles to 2 seconds of arc, and is fitted with all the necessary arrangements for universal work.

A description of the larger geodetic instruments is given in a subsequent chapter.