TIDE-MILLS. 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 Perm. 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. let, 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 the 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 classifi cation. We shall insert the description in the author's own words.
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 .arts on a plane, supposed to be drawn longitudinally and vertically through the centre of the work. k s show the respective heights of the water, on each side of the flood gate s; the &wiles, i on lie side, and k on c'i, are supposed to be open, and according to the nomination of the parts, 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 dosed, 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 1 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 mil is not used.
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 n rocky coast, and as before exposed to a surf, there should be a small tunnel excavated, (as shown by dotted lines at L) 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.