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Tide-wheel.

A wheel turned by the ebb and flow of the tide, and employed as a motor for driving machinery, etc.

The most remarkable variations in the tide are at Chepstow, where the rise of spring tides is about 60 feet; at Bristol it is 40 feet: in Mount St. Michael's Bay it is 46 feet; in the Bay of Fundy and on the coast of Nova Scotia it is about 60 feet; whilst in the Northern Atlantic it is on the average from 10 to 12 feet; at St. Helena only 3 feet; and on the shores of the islands of the Pacific it is barely perceptible

Where the rise is so extreme, it is produced by the contraction of the sides of the river or estuary, as the Bristol Channel, for instance, or a convergence to one point of wide stretches of coast, as at the Bay of Fundy.

In some cases the phenomenon of the bore is produced, which is defined by Colonel Emery as being a peculiar undulation, which announces the arrival of the flood tide in many rivers. It consists of two, three, or sometimes four waves, very short, and succeeding one another rapidly, which bar the whole river, and ascend it to a great distance; they often break upon the crown, and upset everything they meet in their course, and are accompanied by a fearful noise. In the Severn, the bore is stated to be of almost daily occurrence, and sometimes even to attain a hight of 9 feet; in the Dordogne it rises from 5 to 6 feet, and travels at the rate of about 5 miles in 34 minutes; in the Seine it does not exceed 3 feet; in the Thames it only exists in a rudimentary state; whilst in the Hoogly, at Calcutta, it rises about 5 feet, and is transmitted at the rate of about 17 1/2 miles per hour; and in the Menga the rise is said to be 12 feet.

The tide-wheel at East Greenwich on the Thames is, or was, a breast-wheel raised and lowered with the tide, so as to always have a submergence of four feet water. The buckets are divided into four steps, so as to prevent any jerking or irregular motion. The wheel turns both with the flowing and ebbing of the tide, having a sluice-gate and tail-gate on each side, one pair being opened when the other pair is closed.

Dryden's is an undershot-wheel, each float being set at an equal angle with the radius drawn from it to the center. The bottoms of the buckets have narrow openings to admit air as they rise from the water, so that they may not have to overcome atmospheric pressure.

A species of wheel like the sails of a windmill may be employed, having a vane or float so arranged that it will always turn the same face toward the current, and its axis always rotate in the same direction. See current-wheel.

The turbine is also employed, but is adapted to work with one flow only.

Gwynne's double-acting balanced pressure-wheel is intended to produce a continuous movement both with the ebb and flow; the buckets are arranged to present a direct surface to the passage of the water in either direction, and the wheel turns on a vertical spindle in a casing through which the water flows

1877.

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