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.

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.

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.

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.

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.

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