and consequently the whole machine, is put in motion. The bridgetree a b is elevated or depressed by turning the nut c at the end of the lever cb. In order to understand how this motion is produced, let us suppose both the apertures shut, and the tube TT filled with water up to T. The apertures A, B, which are shut up, will be pressed outwards by a force equal to the weight of a column of water whose height is TT, and whose area is the area of the apertures. Every part of the tube AB sustains a similar pressure; but as these pressures are balanced by equal and opposite pressures, the arm A B is at rest. By opening the aperture at A, however, the pressure at that place is removed, and consequently the arm is carried round by a pressure equal to that of a column TT, acting upon an area equal to that of the aperture A. The same thing happens on the arm TB; and these two pressures drive the arm AB round in the same direction. This machine may evidently be applied to drive any kind of machinery, by fixing a wheel upon the vertical axis CD. In the preceding form of Barker's mill, the length of the axis CD must always exceed the height of the fall ND, and therefore when the fall is very high, the difficulty of erecting such a machine would be great. In order to remove this difficulty, M. Mathon de la Cour proposes to introduce the water from the millcourse into the horizontal arms A, B, which are fixed to an upright spindle CT, but without any tube TT. The water will obviously issue from the apertures A, B, in the same manner as if it had been introduced at the top of a tube TT as high as the fall. Hence the spindle CD may be made as short as we please. The practical difficulty which attends this form of the machine, is to give the arms A, B, a motion round the mouth of the feeding pipe, which enters the arm at D, without any great friction, or any considerable loss of water. This form of the mill is shown in fig. 103, where F is the reservoir, K the millstones, KD the vertical axis, FEC the feeding pipe, the mouth of which enters the horizontal arm at C. In a machine of this kind which M. Mathon de la Cour saw at Bourg Argental, AB was 92 inches, and its diameter three inches; the diameter of each orifice was 1 inch, FG was 21 feet; the internal diameter of D was two inches, and it was fitted into C by grinding. This machine made 115 turns in a minute when it was unloaded, and emitted water by one hole only. The machine, when empty, weighed 80 pounds, and it was half supported by the upward pressure of the water. This improvement, which was first given by M. Mathon de la Cour, in the Journal de Physique, 1775, appeared twenty years afterwards in the American Philosophical Transactions, as the invention of a Mr. Ramsey; and Mr. Waring, who inserted the account, contrary to every other philosopher, makes the effect of the machine only equal to that of a good undershot wheel, moved with the same quantity of water falling through the same height. Dr. Gregory, in his Mechanics, vol. ii. has given this paper with some corrections, and recommends it as the best theory. The following rules, deduced from his calculus, may be of use to those who wish to make experiments on the effect of this interesting machine. 1. Make each arm of the horizontal rotatory tube or arm of any convenient length, from the centre of motion to the centre of the apertures, but not less than one-third (oneninth, according to Mr. Gregory) of the perpendicular height of the water's surface above their centres. 2. Multiply the length of the arm in feet by .6136, and take the square root of the product for the proper time of a revolution in seconds, and adapt the other parts of the machinery to this velocity; or if the required time of a revolution be given, multiply the square of this time by 1.629 for the proportional length of the arm in feet. 3. Multiply together the breadth, depth, and velocity per second, of the race, and divide the last product by 18.47 times (14.27, according to Mr. Gregory) the square root of the height, for the area of either aperture. 4. Multiply the area of either aperture by the height of the fall of water, and the product by 41 pounds (55.775, according to Mr. Gregory) for the moving force, estimated at the centres of the apertures in pounds avoirdupois. 5. The power and velocity at the aperture may be easily reduced to any part of the machinery by the simplest mechanical rules. TIDE-MILLS. TIDE-MILLS, as their name imports, are such as employ for their first mover the flowing and ebbing tide, either in the sea or a river. Mills of this kind have not often, we believe, been erected in England, though several of our rivers, and particularly the Thames, the Humber, and the Severn, in which the tide rises to a great height, furnish a very powerful mover to drive any kind of machinery, and would allow of tide-mills being very advantageously constructed upon their banks. The erection of such mills is not to be recommended universally, as they are attended with a considerable original expense; beside that, some of their parts will require frequent repairs: but in some places, where coal is very dear, they may, on the whole, be found less expensive than steamengines to perform the same work, and may, on that account, be preferred even to them. We have not been able to ascertain who was the first contriver of a tide-mill in this country, nor at what time one was first erected. The French have not been so negligent respecting the origin of this important invention, as to let it drop into obscurity; but have taken care to inform us that such mills were used in France early in the last century. Belidor mentions the name of the inventor, at the same time that he states some peculiar advantages of this species of machine. "L'on en attribue," says he, "la première invention à un nommé Perse, maître charpentier de Dunkerque, que mérite assurément beaucoup d'éloge, n'y ayant point de gloire plus digne d'un bon citoyen, que celle de produire quelqu'invention utile à la société. En effet, combien n'y a-t'-il point de choses essentielles à la vie, dont on ne connoît le prix que quand on en est privé: les moulins en général sont dans ce cas-là. On doit sçavoir bon gré à ceux qui nous ont mis en état d'en construire partout: par exemple, à Calais, comme il n'y serpente point de rivières, on n'y a point fait jusqu'ici de moulins à eau, et ceux qui vont par le vent chômant une partie de l'année, il y a des tems où cette ville se trouve sans farine, et j'ai vu la garnison en 1730, obligé de faire venir du pain de Saint-Omer, au lieu qu'en se servant du flux et reflux de la mer, on pourrait construire autant de moulins à eau que l'on voudroit : il y a d'autres villes dans le voisinage de la mer sujettes au même inconvénient, parcequ'apparemment elles ignorent le moyen d'y remédier.' Mills to be worked by the rising and falling of the tide, admit of great variety in the essential parts of their construction; but this variety may perhaps be reduced to four general heads, according to the manner of action of the water-wheel. 1. The water-wheel may turn one way when the tide rises, and the contrary when it falls. 2. The waterwheel may be made to turn always in one direction. 3. The water-wheel may fall and rise as the tide ebbs and flows. 4. The axle of the water-wheel may be so fixed as that it shall neither rise nor fall, though the rotatory motion shall be given to the wheel, while at one time it is only partly, at another completely, immersed in the fluid. In the mills we have examined, says Dr. Gregory, the first and third of these divisions have been usually exemplified in one machine; and the second and fourth may readily be united in another: we shall, therefore, speak of them under two divisions on y. 1. When the water-wheel rises and falls, and turns one way with the rising tide, and the contrary when it ebbs. 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. 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, which will be better understood by referring to fig. 104, 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, although that in which the water-wheel moves is changed, are so truly ingenious as to deserve a distinct description, illustrated by diagrams. Let, then, AB (fig 105) be a section of the water-wheel, 1, 2, 3, 4, 5, &c. its floats; CD the first cogwheel upon the same axis as the water-wheel; the vertical shaft FE 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 geer, while F is clear of the cogwheel CD; and at the turn of the tide the wheel F is let into geer, 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 axle 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 wallowerwheels, as required. The centre of the lever may be shown more clearly by fig. 104, 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 I, this wheel being fixed on the four rods surrounding the shaft, of which three only can be shown in the figures, as cde; 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 geer, it rests on a shoulder that prevents it from going too far down; and when the bottom one is in geer, 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 friction on H |