TIDE MILL. A mill driven by the water which Is affected by the rise and fall of the tides is called a tide mill, whether the wheel be affixed to a building on the land, or to a yeasel floating In the river. 1n the former case the water sometimes acts upon an utelerehot wheel in escaping from a reservoir, which has been filled during the flood tide; but the working of the mill in this case can only take place when there is a sufficient difference of level above, and below, the wheel, to produce the necessary head, and the sluices must be arranged in such a manner as to regulate the dynamical effort exercised upon the wheel during the period of working, which is usually about four hours in each ebb. At other times, the motion is given by a species of horizontal reaction wheel, made to revolve horizontally hy the advancing or receding tide ; in this case oleo the action of the wheel cannot take place con tinuously, for at the turn of the tide in either direction there is period during which it neither advanced nor recedes; but directly the current assumes a marked velocity, either of ebb or flow, the wheel will be set in motion, in alternate directions. As the velocity of the tidal current is always greatest about the half tides, it is necessary to provide some machinery for the working parts of the mill, if they should 'happen to be of a nature to require uniformity of motion ; this remark of course applies to both the undershot, and to the horizontal wheel.
The tidal wheels attached to boats on rivers are, in fact, vertical float wheels working in unlimited water; that is to say, the stream alone acts upon the wheel by its horizontal velocity, without being ia any way confined or directed by a race or channel. These wheels rarely
exceed from 12 to 17 feet in diameter, and the floats vary from 12 to 24 in number ; the depth of the floats never exceeds of the radius, but is usually only about of that dimension, and the whole of the floats are immerged; the width of the face of these wheels varies be tween 8 and 18 feet. In tidal rivers, where the current alternates in its direction, the floats are placed upon the radii of the wheel; though the wheels working in unlimited water, flowing in one direction, ere found to produce a greater useful effect when they are made with floats inclined to the line of flow. Boat mills of this description, it may be added, should never be tolerated on rivers capable of receiving an active navigation ; and under all circumstances their action is irregular, and of very little practical value. Their working effect may be represented by the formula ew=20 in which PW = the effective power trans mitted by the working shaft ; a = the sectional area of the float ; the velocity of the current. The working effect of the other descrip tione of tidal wheels is to be calculated upon the ordinary principles applied to that class of machinery, when the head of water and the velocity of the current are known. [WATER Witzer..] (Consult D'Aubuisson, Hydraulique ; 'Fobre, Eased sur la construction des Roues Ilydrauliqua ; Navier, AiThiteciure Ilvdraulique ; Bossut, Ilecherches e.eperim.tntalu ear !'Eau et le Vent,&c.; Gregory, Treatise on Alechanies,)