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

Fig. 160.—Ring plummet, Shortt's Patent.

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Of later years, however, the demand for four-screw levelling instruments has been maintained, especially from Canada, owing to the influence of the American school of teaching, and in consequence all the author's improved theodolites are fitted with either three- or four-screw levelling, whichever is desired. It is a strange fact, however, that with all the American makers, although they list all their ordinary instruments with four-screw levelling, their refined ones, which they term "precision" instruments, will be found with three-screw levelling.

392.—Detached parts of a Theodolite.—The Plummet supplied with the theodolite is made to hang from a hook under the centre of the axis of the instrument, the cord, which is of soft silk, being looped or knotted to hold in the hook. The lower end of the plummet is brought to a point which, when in use, falls directly under the vertical centre of the instrument upon the surface of the ground. In Fig. 157 the screw and plummet are shown detached. The cord C is attached to the plummet by passing it through a hole in the milled-head screw S at the top of the plummet, and by making a knot K in the cord. Fig. 160 shows an ingenious ring plummet recently invented and patented by Mr. W. H. Shortt, A.M. I.C.E. The chief object was the production of a plumb bob whose plumbing point should be situated at, or very close to, the centre of oscillation in order that the position of the point might be unaffected by oscillation of the bob itself, apart from any swing which it might have about the point of suspension of the string. A further object was to shape the bob so that a person holding the string, or standing close to it when attached to an instrument and looking down at the bob, should be able to see readily the exact position of the plumbing point.

These objects have been attained by making the bob in the form of a ring, so that the centre of oscillation which lies in the centre of the ring can itself be used as the plumbing point, since it can be readily seen and indicated by the extremities of pointers projecting towards the centre from the inside of the ring.

A great advantage of this bob is that when plumbing on to a flat surface it does not fall over when lowered, but may be allowed to actually lie on the surface while the position of the point is being marked. Also it can best be steadied by lowering into contact with the ground and raising again.

The plumbing pointers are largely protected from injury when the bob is in use, and when not in use the suspension string can be wound diametrically across the bob, in recesses provided for the purpose, thus completely protecting the points.

393.—The Loop.—It is somewhat difficult in the ordinary way to adjust the plummet to the station mark on the ground or on a peg. The cord is sometimes placed in an ivory runner fixed to the top of the cord, [Fig. 159]. This gives friction on the cord and permits extension and contraction of the loop for adjustment. Where the plummet has to be suspended from the instrument as well as from a hook inside the stand, which is sometimes convenient, it is better to have the runner cut out on one side. This permits easy change and it is just as firm.

394.—Messrs. Gurley Bros. of Troy, N.Y., have a good plan for shortening the plummet line. This is effected by making a reel in the plummet, which is wound by a milled head at the top of it, [Fig. 158].