Are mills or any kind of machinery moved by the ebbing and flowing of the tide. Mills of this kind are not very common, on account of the great expense of their construction; but in situations where the tide rises to a considerable height, and where the fuel required for a steam-engine is high, and the first cost can be met, tide-mills may be very advantageously constructed.

The origin of tide-mills in this country does not appear to be recorded; but the able Belidor ascribes the invention to a master-carpenter, at Dunkirk, of the name of Perse. Mills to be worked by the rising or falling of the tide, admit of great variety in the essential parts of their construction; but this variety, Dr. Gregory observes, may be reduced to four general heads, according to the manner of action of the water-wheel. 1st, the water-wheel may turn one way when the tide rises, and the contrary when it falls. 2d, the water-wheel may be made to turn always in one direction. 3d, the water-wheel may rise and fall, as the tide ebbs and flows. 4th, the axle of the water-wheel may be so fixed as that it shall neither rise nor fall, though the rotary motion shall be given to the wheel, while at one time it is only partly, at another completely immersed in the fluid.

Some very ingenious suggestions for the construction of a tide-mill appeared some time since in a scientific journal, in which the arrangements differ in some essential respects from those apparently contemplated in Dr. Gregory's classification. We shall insert the description in the author's own words.

Fig. 1.

Tide Mills 642

In this plan, "the water is compelled to flow in and out of a basin, in such a manner, that the greatest force shall be obtained from its current, and the annexed diagrams are introduced to illustrate the following explanation."

Fig. 1 represents a perpendicular projection of the principal parts on a plane, supposed to be drawn longitudinally and vertically through the centre of the work, h v show the respective heights of the water, on each side of the floodgate s; the flushes, i on b's side, and k on c's, are supposed to be open, and according to the nomination of the part9, the water is flowing from the basin into the sea.

Fig. 2 represents a perpendicular projection of the principal parts on a plane, supposed to be drawn latitudinally and vertically through the centre of the work, h shows the level of the water when flowing from the represented side; the position of the flushes under these circumstances are shown, i being open, and k closed; v shows the level; vice versa, the position of the flushes in this case are seen, i being closed, and k open. An objection may arise from the variation of the quantity of water at the spring and neap tides. To counteract this, the flush / is introduced into the flood-gate s, which may be opened and closed according as there is a redundancy or deficiency of water: this may either be effected by centrifugal balls, or by the attention of the individual who takes care of the works. It should be entirely opened when the mill is not used.

Fig. 2.

Tide Mills 643

Although this plan is more particularly adapted for harbours and the tideway of rivers, yet there are few parts of the coast on which such a mill might not be constructed. If it be on a sandy beach, a large wooden tunnel should be laid down at the lowest tide level, in order to introduce water upon the flood-gate. The basin had better be constructed of wood, as then the sides of it may be vertical, which is of great advantage, to produce a less variable current; but it may be excavated, and lined with clay, which should be covered over with shingles, in order to prevent the clay from being washed off. If it be on a rocky coast, and as before exposed to a surf, there should be a small tunnel excavated, (as shown by dotted lines at t.) The basin in this case is easily formed; I conjecture that an excavation of the capacity of one of our first-rate ships, and about 12 feet deep, would contain water enough to two pair of stones for a grist-mill.

a a is the floating mill, of which the form of a ground plan is as Fig z. It has angular ends for two reasons; 1st, because the building may be more easily constructed in this manner, to bear the pressure on the sheaves gg; and 2dly, in order to give a proper direction to the water, b is the excavated basin; c the sea or harbour; n the soil; s the flood-gate attached to the mill, and moving up and down with the tide; j, the water-wheel; i i flushes for introducing water on the wheel; k k ditto, for letting it off; l ditto, for regulating the influx of water according to the strength of the tide; d, the channel cut in the sides of the wall, and sunk below low-water mark, according to the rise and fall of the tide above, into which are inserted small sheaves working on an iron plate placed on the flood-gate, in order to reduce the friction; f f are vertical beams of timber, supported by pieces o o, thrown across the passage horizontally and diagonally, with respect to the beam itself, for the purpose of bearing the horizontal pressure of the building on the sheaves g g, which pressure takes place on each beam alternately, with the rise and fall of the tide; e e are doors, the upper one may receive the corn or materials when the mill is at a proper level, and the lower one discharge the same when in a convenient situation; leather is placed in the joints, in such a manner as to prevent the water from getting through between the slide and flood-gate. There are two water-wheels, in order that the pressure on the grooves d d may be less partial.

A tide-mill was erected at East Greenwich, on the right bank of the Thames, under the direction of Mr. John Lloyd, an engineer of Westminster, of which the following will convey an idea -. the details are given by Dr. Gregory in his Mechanics, vol. it.

This mill is intended to grind corn, and works eight pair of stones. The side of the mill-house parallel to the course of the river, measures 40 feet within; and as the whole of this may be opened to the river by sluicegates, which are carried down to the low-water mark in the river, there is a 40 feet water-way to the mill; through the water-way the water presses during the rising tide into a large reservoir, which occupies about four acres of land; and beyond this reservoir is a smaller one, in which water is kept, for the purpose of being let out occasionally at low water to cleanse the whole works from mud and sediment, which would otherwise, in time, clog the machinery.'

The water-wheel has its axle in a position parallel to the side of the river, that is, parallel to the sluice-gates which admit water from the river; the length of this wheel is 26 feet, its diameter 11 feet, and its number of float-boards 32. These boards do not each run on in one plane from one end of the wheel to the other, but the whole length of the wheel is divided into four equal portions, and the parts of the float-boards belonging to each of these portions fall gradually one lower than another, each by one-fourth of the distance, from one board to another, measuring on the circumference of the wheel.

This contrivance is intended to equalize the action of water upon the wheel, and prevent its moving by jerks. The wheel, with its incumbent apparatus, weighs about 20 tons, the whole of which is raised by the impulse of the flowing tide, when admitted through the sluice-gates. It is placed in the middle of the water-way, leaving a passage on each side of about six feet, for the water to flow into the reservoir, besides that which, in its motion, turns the wheel round. Soon after the tide has risen to the highest, (which at this mill is often 20 feet above the low water-mark,) the water is permitted to run back again from the reservoir into the river, and by this means it gives a rotary motion to the water-wheel in a contrary direction to that with which it moved when impelled by the rising tide.