Fig. 64.—Section of one turn-up leg of the same.
Fig. 65.—Section of tripod.
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216.—Tripods, or Stands.—This matter was deferred when describing the Y-level. The same form of tripod is used for both Y-level and dumpy. In this country the tripod is generally made of straight-grained, well-seasoned Honduras mahogany, which stands better than any other wood. When the tripod is folded up for carrying or for putting by it forms a cylindrical pole which is bellied out at about one-third its length from the top, and diminishes downwards and upwards from this point. For a 14-inch Y-level or dumpy the dimensions of the tripod are about 3½ inches at its greatest diameter when closed, tapering off to 2½ inches at both the top and the bottom ends. For a 12-inch level the section is somewhat less. Each leg of the tripod takes an equal section of the cylinder, the inner angle meeting in the axis being at an angle of 120°, as shown in section Fig. 65. Shovel-pieces are shown in [Fig. 59] AA′ (p. 110), attached to the top of each leg by four screws passing from the brass to the wood. There should be also two screws from a brass plate inside the leg to the shovel-piece, making connection brass to brass: this is important, as fixings from the brass to the wood only become loose and shaky by shrinkage. The shovel-piece is formed into a strong tenon at its upper end, through which a bolt passes connecting the book-pieces together. The book-pieces are plates cut to an angle of 120°, so as to fall true on the tenons of the shovel-pieces. Where hand-work is used for making the tripod head, the book-pieces are attached by three screws; where machinery is used, the head is made in the shaping machine out of a solid casting, which is much better. The tripod head carries a screw about 1½ inches diameter with coarse thread, which fits into a socket on the lower parallel plate of the level, whether Y or dumpy. There should always be a plain piece, technically a lead, above the screw. This holds the instrument steady before it is screwed down, and also leads the screw directly to its corresponding thread, thus saving risk of crossing the thread. A common defect in tripod heads is the thinness of the tenon, so that the leg, if twisted, is felt to be rickety. This tenon is better made wide, as shown in the staff head in [Fig. 70A], seq.
217.—There is a little difference of opinion as to the form of the woodwork of the tripod for 14-inch levels, some preferring an open framed stand in place of the solid form shown in section Fig. 65. These open framed stands are not so compact to carry, and, as the author thinks, unnecessary for levels of 12 inches and under where the tripod head is solidly made. They are well adapted for larger levels and for theodolites, therefore the description of a framed tripod will be deferred to the discussion of these instruments further on.
A few engineers prefer yellow pine for the tripods instead of mahogany: this is much lighter for its relative stiffness, but it is rather soft for the fixing to the shovel-pieces, and therefore scarcely so reliable as mahogany for durability. Where lightness is important the author employs cedar, which is as light as pine but harder.
218.—The lower points of the legs, technically toes, are pointed to an angle of about 60°, and are shod on the insides with steel plates to bite the surface upon which the tripod stands when the legs are extended for use. Two brass rings slip over and bind the legs together when the tripod is out of use.
219.—Many years ago the author introduced the plan of having one of the legs to turn up at about 1 foot distance from the toe. This is shown Fig. 63 at A, and in detail section Fig. 64. The joint is made perfectly firm by a winged screw at S, which screws from a boss cast on the hinge J to a solid metal shoe P. When the leg is turned up, the screw fixes it in the female screw S. This plan is very convenient for use in mountainous districts, as it enables the level to be set up fairly well without an uncomfortable angle to any of the legs, or risk of the instrument toppling over. This plan is now nearly superseded by a ball joint as a part of the setting-up adjustment.
The tripod head shown under the level of [Fig. 59] is by no means the best, but it is the easiest made therefore, it is the general trade form in use, both for the level and theodolite. Some very superior forms will be discussed further on in description of the instruments to which they are attached.
220.—The adjustments of the Dumpy Level.—As this instrument does not possess the means of revolving the telescope upon its axis as with the Y-level, the adjustments are somewhat more complicated, and are performed in an entirely different manner when they are to be made by the civil engineer. The differences are not so great in the hands of the optician, as he generally possesses a movable pair of Y's upon which he can adjust the telescope conveniently for collimation within his own works, by supporting the telescope tube in Y's at a position exterior to the bands which surround it. The tools for this adjustment the author has occasionally supplied upon demand with the dumpy level. But what is necessary here will be to give the mode of adjustment which the civil engineer can accomplish at any time without supplementary apparatus.