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.

by William Ford Stanley

Figs. 162, 163, 164.—Striding level.

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400.—Striding Level.—For the adjustment of the transverse axis of a theodolite a very sensitive spirit level is used. This is mounted upon a bed, which may be formed of brass tubing, from the two ends of which adjustable legs descend, the ends of which are forked, the hollows of the forks forming V bearing surfaces. The V's rest upon the pivot of the axis. By reversing the striding level on the pivots the transverse axis of the telescope, or transit axis, can be readily adjusted truly perpendicular to the vertical axis. In the construction of the striding level, shown in detail in Fig. 162, the two striding standards SS are carried down from the ends of the casing tube B of the spirit level. These are adjustable: one, Fig. 164, by raising or lowering the end of the level tube by the capstan screws CC′, and the other, Fig. 163, by a lateral adjustment of the capstan screws PP′ that act upon the stud S, which is fixed upon an arm centred upon the axis of the tube. This connection is shown by dotted lines. By these two motions the standards are brought to perfect parallelism with each other for their bearing surfaces and adjustment of the crown of the bubble tube.

Fig. 165.—Wallis' shifting centre for theodolites.

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401.—Adjustment of the Axis for Setting it up over a Point.—Every surveyor experiences an amount of difficulty in getting the plummet to fall from the axis of the instrument exactly over a point upon the ground, or a mark upon a rock, or still more so upon a point in street paving in a town, which is necessary for exact work. It is easily set near the point, that is, within half an inch or so, by pressing or shifting the legs; but the difficulty increases as the exact point is approached, so that the setting has generally to be left at a certain state of approximation. There are a great number of schemes in use for moving the axis by adjustment of the instrument the small quantity required, without disturbing the legs of the tripod when they are firmly set down nearly correct to position. One of these would no doubt be generally applied to the theodolite, except for the reason that every means yet devised adds to its weight, and also to the expense of the instrument. A moderately simple plan, which is especially adapted to the parallel plate adjustment, is to make the lower flange of the theodolite, upon which it stands when set down off its tripod, somewhat larger and thinner. This flange, instead of being screwed directly down upon the tripod head, is placed between two ring plates, which are clamped together when the theodolite is set in position. The large hole in the centre of the ring permits movement of the lower plate of about 1 inch. Fig. 165 is an arrangement of this kind by Mr. J. Wallis. This is made entirely independent of the theodolite, and may be used or not as required. I is a screw that corresponds with the head of the tripod which takes the theodolite; T similar female screw to take the tripod head when the shifting centre is used; CC′ a box formed by screwing two tray-pieces firmly together; S clamping flange; HH′ clamp screwed into the top of box C. This has two handles by which the screw is moved to clamp when the instrument is in position. The weight of this additional part is about 3 lbs. The arrangement is particularly adapted to parallel plate adjustments.

402.—In an American plan of a transit by Messrs. Heller & Brightly, the flange is lifted by the parallel plate screws, which tighten it at the same time.[18] Messrs. Troughton & Simms have a plan of shifting the axis by means of a pair of eccentric plates, which carry the instrument in two directions nearly at right angles to each other. By this arrangement an amount of leverage is secured which produces an easier motion than that of shifting the weight of the instrument on the plans mentioned above. The author's schemes will be described as a part of his new theodolites a few pages on.

403.—Stadia Webs or Lines used for taking subtense angles by the telescope for measuring distances, which are frequently applied to theodolites, will be fully described, Chapter XII., in treating of subtense instruments generally.