DESCRIPTION OF THE DESIGN OF EAST GREENWICH TIDE MILL
” ….. 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, (see fig 6.) is intended to equalise the action of the 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 6 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 level is often twenty 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, although that in which the water-wheel moves is changed, are so truly ingeniuous as to deserve a distinct description.
” Let, then, AB (fig 5) be a section of the water wheel, 1,2,3,4,5, &c. its floats, CD the first cog wheel upon the same axis as the water wheel: the vertical shaft PE carries the two equal wallower-wheels E and F, which are so situated on the shaft that one or other of them may, as occasion requires, be brought to be driven by the first wheel CD; and thus the first wheel acting upon F and E at points diametrically opposite, will, although its own motion is reversed, communicate the rotatory motion to the vertical shaft always in the same direction. In the figure the wheel E is shown in gear while F is clear of the cog wheel CD: and at the turn of the tide the wheel F its let into gear, and E is thrown out: this is effected by the lever G, whose fulcrum is at H, the other end being suspended by the rack K, which has hold of the pinion L on the same axis as the wheel M; into this wheel plays the pinion N, the winch O on the other end of whose axle furnishes sufficient advantage to enable a man to elevate or depress the wallower wheels as required.
“The centre of the lever may be shown more clearly by fig. 6 where a b is a section of the lever, which is composed of two strong bars of iron, as a b: there are two steel studs or pins which work in the grooves of the grooved wheel J, this wheel being fixed on the four rods surrounding the shaft, of which three only can be shown in the figures, as c, d, e; the ends of these are screwed fast by bolts to the sockets of the wallower-wheels, and they are nicely fitted on the vertical shaft so as to slide with little friction: thus the wallowers may be raised or lowered upon the upright shaft, while the gudgeon on which it turns retains the same position. When the top wallower is in gear it rests on a shoulder that prevents it from going too far down; and when the bottom one is in gear, there is a bolt that goes through the top wheel socket and shaft, which takes the weight from the lever G, at the same time that it prevents much fiction on the studs or pins of the lever which works in the grooved wheel J.
” 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. 5) 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, S, and at the same time acting between the folding gates TW, which are thus converted into very large hydrostatic bellows, buoys up the wheel and frame 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 4 feet; and the mill works with a head of 5 feet.
“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 at S and the folding gates TW should prevent this, there are strong rackworks of cast iron by which the wheel frame can 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 R 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 CD; at each of these vertical shafts turns a large horizontal wheel at a suitable distance above the wallowers, while each horizontal wheel drives 4 equal pinons placed at equal or quadrantal distances on its periphery each pinon 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 pinons giving motion to the bolting and dressing machines, and different subordinate parts of the mill.wheel CD …”.
[Extract from Mechanics]
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