Rising Tide Watermills

In order to explain the nature of this species of tide-mills, we shall describe one which has lately been erected on the right bank of the Thames, at East-Greenwich, under the direction of Mr. John Lloyd, an ingenious engineer of Brewer's-green, Westminster.

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 waterway to the mill : through the waterway 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.

Axle Position

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 lowertlmn 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 waterway, 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 rotatory motion to the water-wheel, in a contrary direction to that with which it moved when impelled by the rising tide : the contrivance by which the wheel is raised and depressed, and that by which the whole interior motions of the mill are preserved in the same direction.

When the tide is flowing, after the mill has stopped a sufficient time to gain a moderate head of water, the fluid is suffered to enter and fall upon the wheel at the sluice Q, (fig. 105,) and the tail water to run out at the sluice R.

The hydrostatic pressure of the head of water acting against the bottom of the wheel-frame, and at the same time acting between the folding-gates, which are thus converted into very large hydrostatic bellows, buoys up the wheel and frame, (though weighing, as before observed, nearly 20 tons,) and makes them gradually to rise higher and higher, so that the wheel is never, as the workmen express it, drowned in the flowing water ; nor can the water escape under the wheel- frame, being prevented by the folding-gates, which pass from one end to the other of the wheel. In this way the wheel and frame are buoyed up by a head of four feet ; and the mill works with a head of 5 or 5 1/2 feet.

Ebb Tide

When the tide is ebbing, and the water from the reservoir running back again into the river, it might, perhaps, be expected that in consequence of the gradual subsiding of the water, the water-wheel should as gradually lower ; but lest any of the water confined between the wheel-frame, and the folding-gates, should prevent this, there are strung rackworks of cast-iron, by which the wheel-frame cau be either suspended at any altitude, or gradually let down so as to give the water returning from the reservoir an advantageous head upon the wheel ; then the sluice is shut, and V opened As well as X, the water entering at X to act upon the wheel, and flowing out at R. The upper surface of the wheel-frame is quadrangular, and at each angle is a strong cast-iron bar, which slides up and down in a proper groove, that admits of the vertical motion, but prevents all such lateral deviation as might be occasioned by the impulsion of the stream.

At each end of the water-wheel there is a vertical shaft, with wallowers and a first cog-wheel, as F E, and ? D ; and each of these vertical shafts turns a large horizontal wheel at a suitable distance above the wallowers, while each horizontal wheel drives four equal pinions placed at equal or quadrantal distances on its periphery, each pinion having a vertical spindle, on the upper part of which the upper millstone of its respective pair is fixed. Other wheels, driven by one or other of these pinions, giving motion to the bolting and dressing machines, and different subordinate parts of the mill.

Although the vertical shaft at each end of the water-wheel rises and falls with that wheel, yet the large horizontal wheel turning with such shaft does not likewise rise and fall, but remains always in the same horizontal plane, and in contact with the four pinions it drives. The contrivance for this purpose is very simple, but very efficacious; each great horizontal wheel has a nave, which runs upon friction-rollers, and has a square aperture passing through it vertically, just large enough to allow the shaft P to slide freely up and down in it, bat not to turn round without communicating its rotatory motion to the wheel; thus the weight of the wheel causes it to press upon the friction-rollers, and retain the same horizontal planes, and the action of the angles of the vertical shaft upon the corresponding parts of the square orifice in the nave causes it to partake of the rotatory motion, such motion being always in one direction, in consequence of the contrivance by which one or other of the wallowers E F is brought into contact with the opposite points of the first cogwheel CD.

Several of the subordinate parts of this mill are admirably constructed ; but we can only notice here the means by which the direction of the motion in the dressing and bolting machines may be varied at pleasure. On a vertical shaft are fixed, at the distance of about 15 or 18 inches, two equal cog-wheels, and another toothed-wheel, attached to a horizontal axle, is made so as to be movable up and down by a screw, and thus brought into contact with either the upper or lower of the two cog-wheels on the vertical shaft; thus, it is manifest, the motion is reversed with great faci- fity by changing the position of the horizontal axle so that the wheel upon it may be driven by the two cog-wheels alternately. A wheel and pinion working at the other end of the horizontal axle will communicate the motion to the dressing machines.

We should have been glad to see adopted in this well- constructed mill, a contrivance, recommended and pursued by the American mill-wrights, for raising the ground corn to the cooling-boxes or beaches from which it is to be conveyed into the bolting-machine. In this mill, as in all we have seen, the corn is put into bags at the troughs below the mill-stones, and thence raised to the top of the mill-house by a rope folding upon barrels turned by some of the interior machinery of the mill. In the American method, a large screw is placed horizontally in the trough which receives the flour from the mill-stones.

The thread or spiral line of the screw is composed of pieces of wood about two inches broad and three long, fixed into a wooden cylinder seven or eight feet in length, which forms the axis of the screw. When the screw is turned round this axis, it forces the meal from one end of the trough to the other, where it falls into another trough, from which it is raised to the top of the mill- house by means of elevators, a piece of machinery similar to the chain-pump. These elevators consist of a chain of buckets, or concave vessels, like large tea-cups, fixed at proper distances upon a leathern band, which goes round two wheels, one of which is placed at the top of the mill-house, and the other at the bottom, in the meal-trough. When tlie wheels are put in motion, the band revolves, and the buckets, dipping into the meal-trough, convey the flour to the upper story, where they discharge their contents. The band of buckets is enclosed in two square boxes, in order to keep them clean, and preserve them from injury.