Fig. 146.—Clamp and tangent to a vertical axis.
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350.—The tangent screw adjustment is shown at T, moved by the milled head M, the boss E being fixed to the instrument. This part of the arrangement is just the same as that described above for a vernier tangent. Objection has sometimes been made to this form of clamp, that it tends to become weak after a time from the constant clamping and releasing, which causes loss of elasticity in the metal. When this occurs it is no doubt due to the metal of the clamp not being good gun-metal; or, if brass, not thoroughly pressed or hammered before the piece is made up. A plan, in not uncommon use in Germany, of avoiding this supposed source of weakness is to bring up a tumbling piece direct on the axis by a screw. This is shown in Fig 147, screw W; tumbling piece A. This produces a direct clamp upon the axis socket B′. The clamp ring CC′ is made loose on its socket.
351.—In practice it is found impossible to clamp the axis of a theodolite without disturbing the centre more or less. In some experiments the author made he found the direct or tumbling piece clamp Fig. 147, although it holds firmly, disturbs the centre much more than the clasping clamp Fig. 146. Therefore when the former is used the clamp should be upon a strong flange. This increases weight, and it can scarcely be so well for a portable instrument. In all cases, in the construction of the instrument, clamps should be fitted and screwed down before the centre is ground and finished. This ensures the centre being made correct in its clamped position, in which it will afterwards be used.
Fig. 147.—Clamp and tangent to vertical axis, German plan—Hunäus.
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The arrangement Fig. 147 shows also a spring S falling upon a stud at E, fixed upon a part of the instrument upon which it acts as a fulcrum. The spring should be of hard rolled German silver. In this case the tangent screw needs no split or other adjustment to make it tight, as all loss of time is taken up by the spring.[14] The plan is found practically to answer fairly; but unless this is very carefully made there is a want of solidity in the movement which a well-fitted, direct-acting tangent screw possesses.
352.—The French generally in all their superior instruments clamp upon a flange carried out from the lower rim of the socket, with the screw placed longitudinally to the axis. When this plan is very carefully carried out, so that the clamping has neither tendency to raise or lower the socket-piece, it is no doubt very good. In large instruments, where weight is no object and the flange may be made large, it is certainly the best plan. In such cases the clamp may be released as a free fitting to prevent the possibility of strain. Fig. 148 shows the French plan attached to a tribrach: S socket, F flange, C clamping screw, T tangent screw. The tangent in this arrangement acts against a spiral spring contained in a tube A, which gives a very steady motion to the instrument.
