Fig. 143, 144.—Sections of clamp and tangent in two directions.
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346.—Clamp and Tangent Adjustment.—The vernier reading to the circle, when this was adjusted by the hand, was scarcely practicable at nearly its full value until the discovery of the clamp and tangent screw motion was made. This useful invention is due to Helvetius, the celebrated astronomer of Danzig (about 1650). By this mechanical arrangement the circle or arc is left quite free to move about its axis until the clamp is screwed down, which then fixes it firmly. The fixing arrangement of the clamp is attached to a solid part of the instrument, but is so constructed that when it is clamped it may yet be moved without unclamping, in relation to the fixed part of the instrument, by the tangent screw which, as its name indicates, is placed in a direction tangential to the circle or arc. This arrangement may take many forms in detail, two of which, the most general and especially adapted to surveying instruments, will be described.
Fig. 145.—Elevation and part section of clamp and tangent.
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347.—The above illustrations, Figs. 143, 144, represent a clamp and tangent motion in two sections at right angles to each other. This form is common to vertical circles and arcs generally, of a theodolite, arc of sextant, circles upon some mining-dials, protractors, and many other instruments. Fig. 145 is partly a front elevation of the same, but with part of the clamp screw A cut off. The stem of the tangent screw is shown in section at E. In all the figures L is the limb of the circle or arc. This has a groove at its under side at G, into which a fillet of the clamping piece C is inserted to make the clamp slide freely about the periphery of the circle when the clamping screw A is loose. A spring is sometimes inserted to open the clamp between the sliding piece K and the clamp C. FF, Figs. 143, 144 is the tangent nut to E. This nut is sawn down and has a cross screw to keep sufficient tightness to prevent loss of time, and yet to allow the tangent screw to work pleasantly at the same time that it holds the circle and vernier quite dead to the position to which it is adjusted by the screw. The tangent nut F has to move to the direction horizontal to the plane of the tangent screw; therefore it has an axis vertical to the plane of the clamp. This is shown at K. The axis is held down firmly by a nut and a washer fitted with a square hole, to prevent the nut unscrewing. The tangent screw has a collar fitting or shank at the tangent boss B, which is turned down from the full-sized metal of the screw. The fellow collar on the outer side of the boss is formed by the shank of the milled head of the tangent screw D. The hole through the milled head is made square, so that it can be adjusted up to the boss without risk of after unscrewing by friction by the screw E. This is tightened up by means of a screw-driver applied at E. The boss B has a vertical axis N, similar to the tangent nut, and is attached to a solid part of the instrument by the washer and nut shown at 0.
348.—The above construction is solid and good, and will bear considerable wear; but there is a little delicacy of touch required to adjust the collars to the boss and to give pleasant tightness to the screw; a better plan is to dispense with the split in the tangent nut and the inner collar turned on the tangent screw, and place a spiral spring over the tangent screw which follows the adjustment, or in some cases a long bow spring may be conveniently used in place of the spiral. These plans answer very well: one of them will be presently described for axis clamping. In place of the groove at G the clamp is sometimes constructed to move on an arm direct from the axis of the circle. This is on the average a pleasanter motion, but in complex instruments it would often interfere with the motion of other necessary parts.
349.—Axis Clamp and Tangent.—This is generally used to bring the horizontal axis of an instrument to bearing, and is made independent of the circle and vernier. The ordinary form, which is very effective when properly constructed, is shown Fig. 146. This form is used for clamping the vertical axis of a theodolite, mining-dial, Y-level, and some other instruments. The clamp C surrounds the axis as a collar, from which two lugs in the same casting are projected at a. These are brought tight upon the outer axis socket B by means of the screw W, which has a wing-nut head to give good purchase. In the construction of this form of clamp the collar should be fitted and ground to its bearings with the lug in the solid, and the cut at a be sawn through afterwards.
