VI. TIDAL INSTRUMENTS AND TIDAL PREDICTION (cont.)
36. The Tide Gauge
The site for the erection of a tide-gauge depends on local circumstances. It should be placed so as to present a fair representation of the tidal oscillations of the surrounding area. A tank is generally provided, communicating by a channel with the sea at about 10 feet (more or less according to the prevalent surf) below the lowest low-water mark. In many cases on open coasts and frequently in estuaries the tank may be dispensed with. At any rate we suppose that water is provided rising and falling with the tide, without much wave-motion. The nature of the installation depends entirely on the circumstances of the case. A vertical pipe is fixed in the water in such a way as to admit it only through holes small enough to annul wave-motion and large enough to make no sensible retardation of its rise and fall in the pipe. The diameter of the pipe differs greatly in different instruments: sometimes that which we have described as the tank serves as the pipe, and sometimes the pipe alone dips into the sea. A cylindrical float, usually a hollow metallic box or a block of green-heart wood, hangs and floats in the pipe, and is of such density as just to sink without support. In Sir W. Thomsons gauge the float hangs by a fine platinum wire, in Newmans (used in India) by a metallic ribbon. In the latter a chain hangs at the bottom of the float of such weight that, whether the water be high or low, there is the same upward force on the float. It is necessary that the pull on the float should be constant, otherwise a systematic error is introduced between rising and falling water. The suspension wire is wrapped round a wheel, and imparts to it rotation proportional to the rise and fall of tide. By a simple gearing this wheel drives another, by which the range is reduced to any convenient extent. A fine wire wound on the final wheel of the train drags a pencil or pen up and down or to and fro proportionately to the tidal oscillations. The pencil is lightly pressed against a drum, which is driven by clockwork so as to make one revolution per day. The pen leaves its trace or tide-curve on paper wrapped round the drum. Generally, however, the paper is fixed to the drum, and the record of a fortnight may be taken without change of paper. An example of a tide-curve for Apollo Bunder, Bombay, from 1st to 15th January 1884, is shown in fig. 3. Sometimes the paper is in a long band, which the drum picks off from one coil and delivers on to another. The contact of the pen must be such that the work done in dragging it over the paper is small, otherwise a varying tension is thrown on to the float wire. Hence, if the friction is considerable, the float must be large.
The conditions necessary for a good tide-gauge appear to be better satisfied by Sir W. Thomsons than by any other; but, as his instrument is recent, other forms have been much more extensively used, and have worked well. The peculiarity of Thomsons tide-gauge is that, by giving the drum an inclination to the vertical, the pressure of the pen on the paper and on its guides is very delicately regulated to the minimum necessary for effecting the purpose. In other gauges the drum has been either vertical or horizontal, and the amount of friction has necessarily been considerably greater. [Footnote 371-1]
371-1 For further details concerning the establishment of tide-gauges, see Major Braids Manual of Tidal Observation, London, 1887, and Sir W. Thomson, "On Tidal Instruments," in Inst. Civ. Eng., vol. lxv, p. 10.
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