exertion will be saved to the individual, which might better be converted into that of going his rounds twice, where he now only goes once. Warnings to the nightly thief of timely attack or retreat will likewise be taken away; and if instead of an open, the watchman was to carry a dark, lanthorn, the robber would have no security whatever in calculating the moment of his depredation, and might be detected in the very outset of his attack, as the slightest sound would alarm the watchman walking in silence, and not drowning distant noise by that of his own voice. The same machine will answer in custom-houses, ware-houses, banking-houses, manufactories, bleaching-grounds, and every place where watching or other attendance, to be useful, must be exact: even sentinels on military duty might be required to leave tokens as memorials of their vigilance.

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Mr. Day has, we understand, obtained the usual patent for securing to himself the right of making and selling this instrument; yet surely not to the exclusion of others invented for the same purpose for the late Marquis of Exeter informed the public more than three years ago, through the medium of Nicholson's Philosophical Journal, that a clock for a similar purpose had been invented by Messrs. Boulton and Watt of Birmingham, which costs no more than thirty shillings. His lordship had then had two of them at Burleigh-Hall more than four years; and he gives the following description of them: They go eight days, and have a face like a clock, but do not strike. The dial goes round and the hour-finger is fixed : round the edge of the dial are moveable iron pins, corresponding with the quarters in each hour. A small hammer placed behind the hour-finger, when moved downwards, pushes into the dial one of the pins which happens to be under it at the time, which pin remains so abased until the dial nearly returns to the same place, when by an inclosed plane the pin is raised up into its first position. This gives time to have the machine examined in the morning, to see how many pins have been struck, and at what time they were pushed downwards. The hammer is moved by the pulling of a chain with a handle, like house-door bells, which, by cranks and wires, is attached to it. I have one in my library, the handle is out of doors. The other machine is placed in a building at the other end of my premises."

WATER-MILLS, the general term by which all kinds of mills which have a stream of water for their first mover, are designated. The term is also sometimes applied to machines driven by wind, for the purpose of draining water out of fen lands; but it is with more propriety confined to the preceding acceptation.

It is not our intention in the present article to enter minutely into the description of the various kinds of machinery driven by water as an active power, but to confine ourselves to a few general remarks upon the construction of that part only which is essential to water-mills, the water-wheel: for the axis of this wheel may be employed to transmit the force impressed upon it to any species of machinery, A concise view of the theory of water-wheels, together with a tolerably copious statement of the experiments and results of Smeaton, have been laid before the reader in book iv. of our first volume; we propose now to present some observations on their shape, magnitude, and velocity. The most general division of water-wheels is into two kinds, resulting from the manner in which the fluid is made to act: when water is made to act by its weight, it is delivered from the spout as high on the wheel as possible, that it may continue long to press it down; but when it is made to strike the wheel, it is delivered as low as possible, that it may have previously acquired a great velocity: thus are the wheels said to be overshot or undershot. The four kinds of wheels mentioned in art. 467. vol. i. belong, in fact, to one or other of these general divisions.

1. An overshot-wheel is nothing but a frame of open buckets so disposed round the rim of a wheel as to receive the water delivered from a spout in such a manner that one side of the wheel is loaded with water while the other is empty: of consequence the loaded side must descend; and by this motion the Aluid runs out of the lower buckets, while the empty buckets of the rising side of the wheel, in their turn, come under the spout, and are filled with water. A slight inspection of the figure of an overshot water-wheel, in plate XVIII. of our first volume, will convince the student of the impossibility of constructing the buckets so as to remain completely filled with water till they reach the bottom of the wheel: indeed, if the buckets are formed by partitions directed to the axis of the wheel, the whole water must be run out by the time that they have descended to the level of the axis; and, of consequence, there must be a great diminution in the mechanical effect of the wheel. Millwrights have, therefore, turned their chief attention to the determination of a form for the buckets which shall enable them to retain the water along a great portion of the circumference of the wheel. It would require much more room than we can assign to this article, to describe half the contrivances which have been proposed: we shall therefore only mention one or two of the best, as described by Dr. Robison in the Encyclopedia Britannica.

In fig. 11. pl. XXXII. AM represents part of the shrouding or ring of buckets of an overshot-wheel which has 40 buckets.

The form of one of these buckets is shewn by GOFABCD; in which the sole of one bucket AF should be of AE the depth of the shrouding, and the shoulder AB of the bucket should be one half of AE. The arm BC of the bucket must be so inclined to the shoulder AB that HC perpendicular to AHF at H may be of AE; and the wrest, (or probably wrist) of the bucket CD must be so inclined to Bcn, the direction of the arm, that D may be about of En.

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From this construction it follows, that the area HABC is very nearly equal to DABC: so that the water which will fill the space HABC will all be contained in the bucket when it shall come into such a position that AD is a horizontal line; and the line AB will then make an angle of nearly 35° with the vertical, or the bucket will be 35° from the perpendicular passing through the axis of motion. If the bucket descend so much lower that one-half of the water runs out, the line AB will make an angle of about 24 with the vertical. Therefore the wheel, filled to the degree now mentioned, will begin to lose water at about of the diameter from the bottom, and half of the water will be discharged from the lowest bucket about of the diameter further down. Had a greater proportion of the buckets been filled with water when they were under the spout, the discharge would have begun at a greater height from the bottom, and a greater portion of the whole fall of water would be lost. The loss by the preceding construction is less thanth (supposing the water to be delivered into the wheel quite at its top), and may be estimated at about th; for the loss is as the versed sine of the angle which the radius of the bucket makes with the vertical. The versed sine of 25° is 18085, nearly 4th of the radius, or Tth of the diameter. Had only of this water been supplied to each bucket as it passes the spout, it would have been retained for 10° more of a revolution, and the loss of fall would have only been about th.

The bucket has been much improved in its construction by Mr. Robert Burns, at the Cotton Mills of Houston, Burns, and Co., at Cartside, Renfrewshire. This ingenious millwright divides the bucket by a partition вm, concentric with the sole and rim, and of such an altitude, as to make the inner and outer portions of the bucket of nearly equal capacity. It is justly observed by Dr. Robison, that Mr. Burn's principle is susceptible of considerable extension, and, when the practice of making the water-wheel of iron is adopted, he recommends the use of two or more partitions; for such a series of partitions, though each should be very thin, will contribute much to the general firmness of the whole wheel. In consequence of this contrivance the fluid is longer retained in the descending buckets: and

when the supply of water is very scanty, a proper adjustment of the apparatus which regulates the position of the spout will direct nearly all the water into the exterior buckets, and thus, by placing it at a greater distance from the axis, sensibly augment its mechanical energy. The doctor suggests also that the breadth of the buckets, or the rim of the wheel, should be tolerably large, that the quantity of water received from the spout may not nearly fill the bucket: and in order to prevent the air from impeding the rising buckets, or as the watermen term it, the buckets from sucking up the tail-water, he advises that the shoulder or sturt AB of each bucket be perforated with a few holes. As to the spout which conveys the water, it should be considerably narrower than the breadth of the wheel; and, as Dr. Brewster justly remarks, this distance of the spout from the receiving bucket, should in general be two, three, or four inches, that the water may be delivered with a velocity a little exceeding that of the rim of the wheel; otherwise the wheel will be retarded by the impulse of the buckets against the stream, and the dashing of the water over would occasion a diminution of power.

With respect to variations in the fall of water, since the active pressure is measured by the pillar of water reaching from the horizontal plane where it is delivered on the wheel, to the horizontal plane where it is spilled by the wheel, it has been concluded that the pressure must be proportional to the wheel; and therefore the water must be delivered as high, and retained as long as possible. This maxim, however, is subject to limitations, and is not perhaps strictly consistent with sound theory. When the fall is exceedingly great, a wheel of an equal diameter becomes enormously big, and extremely expensive. In cases like this, where we are unwilling to lose any part of the force of a fall-stream, the best form of a bucket-wheel is an inverted chain pump.

The velocity of an overshot-wheel is a matter deserving of great care and attention; and different authors have arrived at very opposite conclusions respecting it. The most accurate seems to be that an overshot-wheel does the more work as it moves slower the popular reasoning adduced to prove this, has been of the following kind. Suppose that a certain wheel has 30 buckets, and that 6 cubic feet of water are delivered in a second on the top of the wheel, and discharged without any loss by the way, at a certain height from the bottom of the wheel. Let this be the case whatever is the rate of the wheel's motion the buckets being of a sufficient capacity to hold all the water which falls into them. Suppose this wheel employed to raise a weight of any kind, water, for instance, in a chain of 30

buckets, to the same altitude and with the same velocity. Suppose, farther, that when the load on the rising side of the machine is one half of that on the wheel, the wheel makes 4 revolutions in a minute, or one turn in 15 seconds. During this time, 90 cubic feet of water will have flowed into the 30 buckets, and each have received 3 cubic feet. In that case, each of the rising buckets contains 14 feet; and 45 cubic feet are delivered into the upper cistern, during one turn of the wheel, and 180 cubic feet in one minute.

Now, suppose the machine so loaded, by making the rising buckets more capacious, that it makes only 2 turns in a minute, or 1 turn in 30 seconds. Then each descending bucket must contain 6 cubic feet of water. If each bucket of the rising side contained 3 cubic feet the motion of the machine would be the same as before. This is a point none will controvert. When two pounds are suspended to one end of a string which passes over a pulley, and one pound to the other end, the velocity of descent of the two pounds, will be the same with that of a four pound weight, which is employed in the same manner to draw up two pounds. Our machine would therefore continue to make four turns in a minute, and would deliver 90 cubic feet during each turn, and 360 in a minute. But, by supposition, it is making only two turns in a minute; which must proceed from a greater load than 3 cubic feet of rising water in each rising bucket. The machine must therefore be raising more than 90 feet of water during one turn of the wheel, and more than 180 in a minute.

Thus it appears, that if the machine is turning twice as slow as before, there is more than twice the former quantity in the rising buckets; and more will be raised in a minute by the same expenditure of power. In like manner, if the machine go three times as slow, there must be more than three times the former quantity in the rising buckets, and more work will be done.

But farther we may assert, that the more we retard the machine to a certain practical extent, by loading it with more work of a similar kind, the greater will be its performance; and the truth of the assertion may be thus demonstrated: Let us call the first quantity of water in the rising bucket, 9; the water raised by four turns in a minute, will be 4 x 30 x 0 = 120 g. The quantity in this bucket, when the machine goes twice as slow, has been shewn to be greater than 20; call it 29+; the water raised by two turns in a minute will then be 2 x 30 x (2 g + x) = 120 g + 60 x. Suppose, next, the machine to go 4 times as slow, making but one turn in a minute; the rising bucket must now contain more than twice