Surveying and Levelling Instruments, Theoretically and Practically Described. / For construction, qualities, selection, preservation, adjustments, and uses; with other apparatus and appliances used by civil engineers and surveyors in the field.

CHAPTER VIII.

SPECIALITIES IN MODERN AND IMPROVED FORMS OF TRANSIT THEODOLITES FOR SURVEYING—RAILWAY WORK—EXPLORING.

416.—The description given in the last chapter of a 6-inch transit theodolite gives all particulars of the original Old English form, which in a general way comprises the constructive principles of all others. When we consider modern instruments the details are found to vary greatly, but most particularly in the direction of uniting in solid castings many parts that may be shaped out by machinery in a manner impossible by hand-work, which avoids the instability of the work being screwed together in many pieces, and makes it at the same time lighter, more rigid, and less liable to jar out of adjustment. This direction of construction is also followed in the best modern work on the Continent and in America. It would extend this work beyond convenient limits to offer details of the wide variations employed in practice, but as the author has made this subject a life study, and has embraced, modified, and endeavoured to improve this class of work in all its details, freely adopting any improvement he has observed, his own instruments will represent largely his present ideas of the best forms, with the economy of having engravings for illustration to hand. Transit theodolites of portable form will be considered here, leaving larger stationary instruments to another chapter.

Fig. 169.—Stanley's patent new model theodolite.

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417.—New Model Transit Theodolite.—In this instrument the principles of construction are the same as in the ordinary transit theodolite fully described in the last chapter, but the distribution of materials and details are very different. The general arrangement of a 5-inch instrument is shown in Fig. 169. One important difference, as before mentioned, is that the work is not built up so much in separate castings and pieces as is usual, but every possible casting is shaped out of the solid to the finished form. The vertical axis is of nearly the same construction as the ordinary transit, except that the central axis is about double as strong, being of once and a half the ordinary diameter. It is made in one casting with the upper framework. The vernier plate is formed of thin hard hammered gun-metal, which is screwed upon the axis. This plate has not in this construction to support the superstructure as in an ordinary theodolite, but has only to hold the two axis bubbles, which are thereby brought distinctly in view, and the clamp and tangent motion, which is also placed conveniently for use upon this upper plate, in a position where there is less risk of accident than when it is placed upon the outer edge of the limb.

418.—The Readers to the horizontal limb are jointed to turn up against the standards and adjust for reflection, as shown Fig. 131. In this manner the readers do not need detachment to place the instrument in its case.

Fig. 170.—Section of standards of new model theodolite.

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419.—The Central Axis and the standards are made in one casting in hard gun-metal. The standards are of light cylindrical and ribbed section. This construction, although of only about one-half the weight of the A-frame arrangement with its attachments, described in the last chapter, was found upon testing to have more than double the rigidity in resisting deflection, with perfect certainty of avoiding the accidental occurrence of imperfect fitting of parts, or of screws jarring loose, Fig. 170. The making of the vertical axis and the standards in one piece was in a certain sense an experiment. It has been found in practice of many years now to give much greater resistance to all ordinary strains and jars, and ensure the instrument keeping in order and adjustment when jolted by carrying over the shoulder, just as the same principle acts in the dumpy level; but at the same time, in cases of violent accident, such as the fall of the instrument from a height, it renders repairs somewhat more expensive, as this entire part might have to be reinstated instead of the axis only, the axis of the theodolite being generally made very weak that it may go first, often indeed with a slight jar. Many details are the same as the transit theodolite before described, adopting what is thought to be the soundest principle in all cases.

420.—The Compass-box in this instrument is attached under the limb. It is of the trough form shown Fig. 32, page 74. The magnetic north is set to zero. The tribrach is of the form described for levels, illustrated Figs. 72 and 73, p. 128.

421.—The weights of transit theodolites of this construction are about

6-inch	in gun metal	14	lbs.,	aluminium,	8	lbs.
5-inch	"	11	"	"	6	"
4-inch	"	7¾	"	"	4½	"

This pattern embodies all the essential features of a thoroughly reliable and convenient instrument for all-round general surveying. It has no unnecessary elaborations and is a strong, light and compact instrument suitable for continuous hard wear. It has fewer pieces than any other design and is packed in its case complete in one piece ready to screw upon its stand upon being taken out of its case.

Fig. 171.—Stanley's simple sliding stage for tribrach theodolite.

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422.—The author has devised a special arrangement for displacement of axis for this theodolite, which does not interfere with its valuable quality of standing the tribrach on a wall or flat surface, Fig. 171. In this scheme the arms of the tribrach are slightly elevated by the foot screws. A flange is formed on the top of the head with a leading tube through it to the upper surface of the lower tribrach plate; upon this tube an upper flange is screwed, so that the plate comes between the two flanges, where it may be fixed by means of rotation of the flange by a thumb-piece. The engraving shows the arrangement with the axis displaced to its extreme point, about ¾ of an inch from the centre.

Fig. 172.—Stanley's 4-screw sliding stage.

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Fig. 173.—Stanley's solid round form tripod with sliding head.

Fig. 174.—Stanley's telescopic tripod with sliding head.

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A somewhat similar arrangement is made for four-screw levelling instruments shown on page 250 at Fig. 172, but in this the sliding motion is fixed to position by the action of the levelling screws. It is sometimes preferred to have the sliding adjustment upon the tripod head instead of upon the instrument, and in some cases for getting a greater range of movement, on both, and for this purpose the reviser has designed the two tripods shown at Figs. 173 and 174, the former being of the round solid pattern, and the latter having adjustable sliding legs. A somewhat similar arrangement is made for a sliding head to a framed tripod as shown below, Fig. 175.

Fig. 175.—Stanley's framed stand with sliding head.

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423.—Improved Transit with Adjustable Axis.—This instrument, Fig. 176, in general, resembles that last described, except that it has a larger telescope and it is mounted on a sliding stage with screw adjustments, which is particularly described below. It is frequently provided with a tacheometrical eye-piece for giving horizontal distances by subtense taken on the incline, which will be described in Chapter XII.

Fig. 176.—Stanley's patent new model theodolite, with mechanical stage.

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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.

Fig. 177.—Stanley's patent tribrach mechanical stage.

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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.

Fig. 178.—8-inch transit theodolite.

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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 [255]
[256]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.

Fig. 179.—Stanley's quick-setting transit theodolite.

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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.

Fig. 180.—Railway theodolite.

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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.

Fig. 181.—Stanley's mountain transit theodolite.

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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.

Fig. 182.—Stanley's solar attachment.

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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.

Fig. 183.—Stanley's micrometer transit theodolite.

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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.

Fig. 184.—Stanley's full micrometer transit theodolite.

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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.

Fig. 185.—Stanley's light micrometer transit.

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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.

Fig. 186.—Dr. Deville's transit.

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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.

Fig. 187.—Stanley's patent universal transit theodolite.

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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.

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