mills, water, upper, stone, inches, tidal, feet and shaft

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Tidal their name indicates, tidal mills are those which employ for power the flowing and ebbing tides. At Venice and in some other places there were constructed mills adapted to take advantage of the tide by a change every six hours of the position of the wheels. Such mills are said to have been in operation at the middle of the twelfth century.

Spanish Tidal Mills. —The tidal flour-mills of Andalusia are little known beyond that locality; they are interesting not only on account of their prim itive simplicity, but also because they present the earliest form of the tur bine-wheel, which has come down in its present mode of construction from Moorish times, if not from an earlier date. These mills, which are numer ous on the salt marshes bordering the lower portions of the Andalusian rivers within the tidal range, are built across the creeks, whose natural capacity is so increased by excavations as to retain at spring-tides from half a million to a million cubic feet of tidal water. The head, which rarely exceeds 7 feet, will, with a run of three or four stones, work a mill about six hours, though the number of stones that can be worked simul taneously is regulated by the height of the tide; the mills are all stopped at neap-tides.

The mill-houses are of one story, with upper structure built as light as possible; at one side there is an opening 7 or S feet wide, through which the tide-water flows, and which is covered by a strong wooden shutter opening upward as the water enters and closing when the cur rent of the water begins to return to the sea. The foundations of these mills are rudely placed on broad flat stones, which are widely scarped and heavily buttressed. The chambers enclosing the wheels are built in cul verts formed of brick underneath the mill, and to these chambers the water is admitted through square holes, which are covered and uncovered by means of a rough plank. Between the wheel and the brick chamber there is considerable space, through which much water escapes unutilized.

The wheel, which is made of the hardest wood obtainable, is horizontal, and partakes more of the nature of a dash-wheel than of that of a turbine, because it is actuated more by the current of the water than by its pressure. Its diameter is about 5;4 feet, and it contains twenty-seven blades, formed of quartered logs roughly hewn and keyed radially into a vertical shaft like the spokes of a cart-wheel; they are about 4 inches wide by 3 inches deep at the periphery, and are curved and rounded at the hack and hol lowed out on the upper side. To the extent of S inches above and below

the wheel the shaft has a diameter of about to inches, and from this diam eter it tapers under the stone to 4 inches. It communicates motion directly to the upper stone by an iron gudgeon 1/ inches square, which is secured in the axis of the shaft by iron rings tightened with wedges; it passes through the centre of the lower stone, having a hearing composed of brass or a wooden bush; its top end extends and fits into a cross-bar sunk into the face of the upper stone. The lower end of the shaft is piv oted on an iron spike resting in a plate nailed to the bridge-beam, one end of which rests in a hole in the wall, and to the other end of which a wooden bar is secured, which passes through the floor to a place convenient for raising and lowering the stone.

The millstones, taken from quarries near Jerez, are about 4 feet in diameter, and when new are 13 inches thick. For convenience, they are turned over for dressing. The corn is placed in an inverted pyramidal box holding about 1;4 bushels, and escapes through its apex, by means of a small wooden shoot, to the central opening of the upper stone. In its passage thence it is shaken by a stick tied to the shoot, one end resting upon the rough upper surface of the stone, whose revolutions keep it con stantly in motion. These mills are easily managed, and each will grind about ix, bushels per hour.

Barker's Mill (pi. 3, fig. 6) is an apparatus which is driven by the reac tive force of water. A vertical axis (C, D) moving on a pivot (D) carries the upper millstone (m) after passing through a packed central opening in the fixed millstone (C). Around and upon this axis is secured a vertical tube (T, T) communicating with a horizontal tube, at whose extremities, upon its opposite sides (A, B), are two apertures. Water from the fore-bay (.11, A') enters the tube (7, T), and its efflux through the apertures of the tube acts by counter-pressure upon the interior sides of the arms (A, B), oppo site to the apertures; consequently, the whole machine is put in motion. The bridge-tree (a, b) is elevated or depressed by at the end of the lever (a, b), the nut (r), by which the distance apart of the stones is regulated to the desired fineness of the grinding. The grain is deposited iu the hopper (II), from whose lower orifice it drops into the eye of the upper millstone.

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