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 preuure on the sheaves g g ; and 2dly, in order to give a proper direction to the water. b is the excavated basin ; c the sea or harbour; a the soil ; a 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; I 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-wheeb, in order that the pressure an the grooves d d may be leas 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 I the details are given by Dr. Gregory in his Mechanics, vol. ii.
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 sluice gates, 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.