CHAPTER V.
LEVELLING STAVES—CONSTRUCTION—VARIOUS READINGS DISCUSSED—SOPWITH'S—FIELD'S—STRANGE'S—STANLEY'S NEW—METRICAL—SIMPLE CONSTRUCTION MINING STAFF—PAPERED LEVELLING STAFF—PRESERVATION—PACKING PADS—STAFF PLATE—STAFF LEVEL—PRACTICE OF LEVELLING—INDEX OF BUBBLE—LAMP—CURVATURE CORRECTIONS—STATION PEGS—REFINEMENT OF LEVELLING—LEVELLING BOOKS—INK BOTTLE, ETC.
258.—Levelling Staves.—Since great improvements have been made in the telescopes used as part of all modern surveyors' levels, particularly by increasing their light-receiving capacity, all systems of vanes which were formerly made to be seen distinctly at a distance have disappeared from use by British surveyors; it is now found that the plain reading of a divided staff can be taken by means of the telescope at a sufficient distance from the observer for all practical purposes. In this country one construction of staff is now generally adopted; and the only variations that are made in this are found occasionally in the readings. The construction of the level staff in common use is that invented by the late Thomas Sopwith,[7] called the telescopic staff, the face view of which is shown Fig. 96. For ordinary open field work this is made 14, 16, or 18 feet in its extended length; but generally, except for levelling on mountainous land, the 14 feet is used. This staff when closed is about the same length as the tripod, 5 feet 4 inches, and may be conveniently stowed away under the seat of a railway carriage. Sopwith's staff, as it was formerly made, consisted of two square parallel tubes and one inner solid parallel slide. Made in this manner it was liable to be rather shaky when extended, besides which it frequently got jammed in the telescopic boxes if put away damp from rain: this tended at first to limit its use. It is now usual to make the boxes slightly conical, that is, diminished towards the upper part, so that they are rigid when opened out but are very free when closed, which quite remedies the defects just mentioned.
Figs. 93, 94.—Section of Sopwith's staff.
Larger image
259.—The ordinary construction of Sopwith's staff and the best mode of manufacture is shown, with the joints grooved together, in section Fig. 93. The outer tube or case A, which in the 14-feet staff is 5 feet in length, is made of mahogany 5/16 inch thick, the front being ¼ inch. The outer dimensions of the section are 3-1/8 inches by 2 inches. The second tube B is 5 feet 1 inch long, of outer dimensions 2-3/8 inches by 1¼ inches. The inner slide C is solid, 5 feet 2 inches long, 1¾ inches by ¾ inch. All the slides are sunk on the face about 1/16 inch to prevent the divisions being rubbed by exposure in sliding together. The slides have each a brass shoe and cap. They are held when extended by a spring catch, the detail of which is shown in Fig. 94, section y to z of Fig. 93—S spring of T form screwed firmly to the edges of the box. The catch is made at A over the edge of the brass cap A′. The spring should be of very hard rolled brass. It is well to have one or two brass bands round the body of the outer casing to secure this as far as possible from being split by accident.
260.—The most important consideration in the manufacture is that the telescopic work should fit well, and that the boxes should be glued up quite square and out of winding. The boxes should, after the glue is quite set, be screwed with brass screws at distances of about 6 inches apart, to secure the joints which may afterwards in use be exposed to long-continued rain. The fittings should be carefully made, so that when the staff is extended there should be no shakiness sufficient to cause serious vibration when it is used in windy weather. The interior of the slides when finished should be thoroughly oiled with raw linseed oil, and the outer surfaces be well soaked in shellac dissolved in spirit, and then French polished over this. The brass work should be well lacquered.
