VI. TIDAL INSTRUMENTS AND TIDAL PREDICTION
35. General Remarks
Practical tidal work is divisible into the three stages of observation, reduction of observations, and prediction.
The simplest observation is that of the height of water on a graduated staff fixed in the sea, with such allowance as is possible made for wave-motion. It is far better, however, to sink a tube into the sea, into which the water penetrates through small holes. The wave-motion is thus annulled. In this calm water there lies a float, to which is attached a compassing over a pulley and counterpoised at the end. The motion of the counterpoise against a scale is observed. In either case the observations may be made every hour, which is preferable, or the times and heights of high and low water may be noted. We have explained in § 34 the methods of reducing the latter kind of observation. Although more appropriate for rough observations, this method is susceptible of great accuracy when carefully used. It has been largely superseded by the harmonic method, but is still adhered to by the British Admiralty. In more careful observations than those of which we are speaking the tidal record is automatic and continuous; the reduction may he, and probably at some future time will be, mechanical; and the prediction is so already. We shall therefore devote some space to general descriptions of the three classes of instrument. The harmonic reductions are at present (1887) actually done numerically, and in chapter iv. we have indicated the nature of the arithmetical processes.
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