CHAPTER XI.

TIDAL ACTION.

THE natural phenomena of the rise and fall of a tide, due originally to the attractive power of the moon and sun in relative proportions of 24 to 1, is complicated by a variety of disturbing circumstances, such as the position of the two bodies mentioned relatively to one another, the parallax of the moon, and the inertia of the mass of water. There are further, the local conditions of contour of the bed, and of the fringe or margin of the ocean, the friction on its bottom and edges, the action of wind, currents, temperature, and barometric pressure.

All these have now been so far discounted by observation and record that it is possible in any given neighbourhood to ascertain the average and maximum tidal motion, and thereby determine the possibility of its use as a motive force.

The available rise and fall is, as already remarked, very greatly affected by local causes, and therefore very little alike in different places. In deep indents of the shore open in the direction of the tidal motion, and of a gradually contracting shape, the convergence of water in motion causes a very great rise and fall. Hence those very high tides of the Bristol Channel, the bays of Fundy and of St. Malo, in which tides up to 60 and even 100 feet rise, are known.

On the contrary, at certain places on the opposite Irish coast the movement is reduced to about 3 feet, though a little distance away it is 12 feet.

In mid-Pacific the range is but 2 to 3 feet, in London it averages 22 feet, at Liverpool 15.5 feet, at Portsmouth 12.5 feet, at Plymouth 12.5 feet, at Bristol 33 feet, while at Southampton, owing to the double inlet round the Isle of Wight, it has a double high tide, first falling 18 inches and then rising again to flood level. Barometric influences have curious effects. At Brest the tide rises 8 inches for a fall of 1⁄2 inch in the barometer, at Liverpool 1 inch to of an inch fall, and in London about of an inch to each of an inch fall. So that with a low barometer a high tide may be anticipated, speaking generally.

Naturally, the tide is also disturbed by winds, either accelerating their action or adding to their volume, and vice

versa.

Estuaries are, by this means, sometimes drained entirely dry, while those in an opposite direction, receiving the force of the wind, are overflooded. From these facts it will be evident that the adaptation of tidal action to power is not so simple as it appears, yet where it exists in a marked degree, it is by no means to be rejected in these days of storage of power. It is, in some measure, the very immensity of the phenomenon that prevents its practical usage.

The destructive action of the waves prevents the construction of buildings for machinery on the margin of the sea, except in protected bays, estuaries, and creeks. But these exist in numbers sufficient to warrant a much wider use in future of this enormous daily energy offered to man by nature.

At Walton-on-the-Naze will be seen a very good example of the use of tide energy. In a protected, almost land-locked estuary, a dam has been run across the channel, impounding at high tide a large acreage of water. The receding tide leaves a gradually increasing fall or drop for this mass of water, which in escaping operates

the wheels of two flour-mills situated at a convenient point.

Wherever the construction of such a dam does not demand too serious an expense, and where nature has provided protection from the direct action of the sea-waves upon the works, such a construction might be advantageously considered.

The result obtainable will be compounded of two factors: I. The area of water-reservoir to be enclosed and its contents

=

(acre I foot deep 43,560 cubic feet.)

2. The mean tidal movement:

It will be obvious that the acreage should be considerably in excess of the total requirements, as the effect of the outflowing water is only equal to a mean of about one-half the total tide-fall.

Practically speaking, unless the acreage impounded is enormously in excess of the turbine or water-wheel's capacity, it would not be advisable to use the water during the fall of the first half of the tide, nor during its final half-rise again, so that the wheel would only be in use during 1⁄2 fall and 1⁄2 rise of a tide of say 5 hours, plus the slack or low water. The mean fall would then be 3/4 of the full fall, and the power obtainable would equal

(Outflow in cubic feet × 62) × (fall of tide in feet × .75) 300 minutes × 33,000

This assumes the use of a turbine or water-wheel giving out 75 per cent. of the power of the water applied through it. The use of tidal movement as a source of power is thus limited practically to 10 hours' work per day of 24 hours,

in two stretches of about 5 hours each (or 300 minutes), at constantly varying periods of the day and night. To employ a number of workmen upon such irregular hours of labour would manifestly be inadvisable, and therefore, except for nearly automatic machinery, mere tidal machinery becomes an impossibility without storage of power.

Its complexion is quite altered, by the aid of electric machinery and accumulators, and its use may not only be made entirely continuous, but its force may be conveyed a convenient distance away from tidal effects.

Practical Possibilities and Results. horse-power calculated above, deduct

From the effective

for loss in produc

ing an electric energy, and about 2 per cent. for every 100 yards the current is to be conveyed. From this result deduct a further for losses in reconverting that energy into mechanical work, by means of a motor.

Thus a 10 effective horse-power turbine drives a dynamo, from which a current equal to 8 effective horse-power issues. This current loses by carriage or conveyance over 250 yards to the accumulators a further 5 per cent., leaving at the accumulator 7.6 effective horse-power. By losses in accumulators and in an electric motor we lose a further 20 per cent., leaving a force upon the mill belt of 6.08 effective horse-power.

By a proper arrangement of accumulators such a motor would be able to run continuously 10 hours at a time, the water-mill running two irregular spells of 5 hours each at any part of the day or night.

The relative dynamos, wires, accumulators, and motors will be found fully detailed for driving by this or any other means in the last section of Chapter XXXV. of this book, but for the sake of convenience and clearness, I have tabulated such plants as would be required to make use of water-powers from 6 horse-power upwards.