through in a second, when the water acts upon them so as to have the greatest power to turn the mill.

Divide the circumference of the wheel in feet by the velocity of its floats in feet per second, and the quotient will be the number of seconds in which the wheel turns round.

By this last number of seconds divide 60, and the quotient will be the number of turns of the wheel in a minute.

Divide 120 (the number of revolutions a millstone four feet and a half diameter ought to have in a minute) by the number of turns of the wheel in a minute, and the quotient will be the number of turns the mill-stone ought to have for one turn of the wheel.

Then, as the number of turns of the wheel in a minute is to the number of turns of the mill-stone in a minute, so must the number of staves in the trundle be to the number of cogs in the wheel, in the nearest whole numbers that can be found.

Of Clock-Work.

396. The technical terms, or terms of art, and the names of the various parts of a clock and watch will give a popular idea of the construction of either; and if you open your watch the following parts may be understood from description..

The wheels, and the rest of the work, are contained in the frame, which consists of the pillars and plates..

That which the main-spring lies in, is the spring-box: that which the spring winds about, in the middle of the springbox, is the spring-arbor; to which the spring is hooked at one end. At the top of the spring-arbor, is the endless screw, and its wheel but in spring clocks, it is a ratchet-wheel, with its click that stops it.

That which the main-spring draws, and about which the chain or string is wound, and which is commonly taper, is the fusee. In larger work, going with weights, where it is

cylindrical, it is called the barrel; the small teeth at the bottom of the fusee, or barrel, that stop it in winding up, is the ratchet. That which stops it when wound up, and is for that end driven up by the string, is the gardecut.

The parts of a wheel are, the hoop, or rim; the teeth; the cross; and the collet, or piece of brass, soldered on the arbor, or spindle, on which the wheel is riveted.

A pinion is that little wheel which plays in the teeth of the wheel; its teeth (which are commonly few in number) are called leaves, not teeth.

The ends of the spindle are called pivots; the holes in which they run, pivot holes.

The guttered wheel, with iron spikes at the bottom, in which the line of ordinary thirty-hour house clocks runs is called the pulley.

The dial-plate, the hands, screws, wedges, stops, &c. hardly need mentioning.

Thus much for general names, which are common to all parts of a movement.

Watches are all such movements as shew the parts of time; and clocks are such as publish it by striking on a bell, &c. But commonly, the name of watches is appropri ated to such as are carried in the pocket; and that of clock to the larger movements, whether they strike the hour or not. Watches which strike the hour, are called pocket-clocks, or more commonly repeating-watches.

The parts of a movement to be considered, are the watch and clock parts. The watch part of a movement is that which serves to measure the hours; in which the first thing to be noticed is the balance, whose parts are the rim, which is the circular part of it; the verge is its spindle, to which belong the two pallets, or leaves, which play in the teeth of the crown-wheel in pocket-watches, that strong stud in which the lower pivot of the verge plays, and in the middle of which one pivot of the balance-wheel plays, is called the pottance. The bottom of this is called the foot; the middle part (in which the pivot of the balance-wheel turns) is called the nose; the upper part, the shoulder of the pottance. The piece which covers the balance, and in which the upper pivot of the balance plays, is the cock. The small spring in pocket watches underneath the balance, is the regulator, or pendu+ Jum spring.

The parts of a pendulum are the verge, pallets, and cocks, as before. The ball in long pendulums, the bob in short ones, is the weight at the bottom, which is fixed to the wire or rod. The term peculiar to the royal swing, are the pads, and there are, besides these, several other terms which clock-makers

use in various sorts of pieces, as the snail or step-wheel in repeating clocks, the rack, the safe-guards, the several levers, lifters, and detents: but it would be tedious, nor is it necessary, to mention the particulars.

Of Pendulums.

397. A Pendulum is a heavy body hanging by a string or wire, moveable on a centre, and each swing is called a vibration, or oscillation. The vibrations are produced by the falling of the weight to the lowest part of the circle, and by the force acquired by the fall.

Galileo was the first who observed that all the vibrations of the same pendulum, whether great or small, are performed nearly in equal times: and the longer a pendulum, the slower are its vibrations, the squares of the times being inversely as the lengths.

Heat expands, and consequently lengthens, pendulams; and cold contracts, and shortens them. A pendulum, to vibrate seconds, must be shorter at the equator than at the poles.

Methods have been used for correcting the irregularity arising from expansion and contraction; one of these is the gridiron pendulum. Deal is the best substance for pendulumrods, as it is very little affected by heat and cold.

Of Wheel-Carriages.

398. The wheels of carriages turn round, on account of the friction they sustain in contact with the roads; and large wheels are more advantageous than small ones.

In four-wheeled carriages, the fore-wheels are made smaller than the hind ones, for the conveniency of turning ; otherwise they would be better of the same size. Broad wheels are better for heavy carriages-such as waggons—because they press and harden, instead of cutting up the roads, as small wheels do.

The Steam-Engine.

399. The steam-engine, one of the noblest monuments of human ingenuity, was originally invented by the Marquis of Worcester, in the reign

of Charles II. And various persons improved on the Marquis's idea, till 1762, when the late Mr. Watt began to turn his attention to this machine, which he brought to perfection.

In the old engines, where the working stroke was only downwards, the piston-rod was attached to the beam by chains, which bent round an arch on the end of the beam, to make the piston-rod move in a perpendicular direction. In Watt's engines, where the working-stroke is doubled, that is, both upwards and downwards, chains could not answer this purpose, as, when the piston was forced upwards, they would slacken, and would not communicate the motion to the beam. It was necessary, therefore, that the piston-rod should be fastened to the beam with inflexible bars; but that the stroke might be perpendicular, a contrivance, called the parallel-joint was invented, which answers the intended purpose. In order to make the engine itself open and shut the steam and eduction-valves, long levers are attached to them, moved by the piston-rod of the air-pump. This part of the apparatus is called the working-geer, and is so contrived, that the valves may be worked either by hand or by the perpendicular rod. By shutting these valves, the engine may be stopped in an instant.

To communicate a rotatory motion to any machinery by the motion of the beam of the steam-engine, Watt made use of a large fly-wheel; on the axis of which, is a small concentric-toothed wheel. A similar toothed-wheel, is fastened by straps to a rod coming from the end of the beam, so that it cannot turn round on its axis, but must rise and fall with the motion of the great beam.

A bar of iron connects the centres of these two smalltoothed-wheels, so that they cannot quit each other. This mode of moving the fly, is preferable to a crank; as it goes with twice the velocity; it is called the sun and planet wheel, from the resemblance of the motion to that of those luminaries. The mode of operation in Mr. Watt's engine, will be best understood by inspecting one of them at work.

The actual performance of some of these engines, as they have been ascertained by experience, is as follows:

An engine, having a cylinder of 31 inches in diameter, and making 17 double strokes per minute, performs the work of 40 horses, working night and day (for which three relays, or 120 horses must be kept), and burns 11,000 pounds weight of Staffordshire coal per day. A cylinder of 19 inches making 25 strokes, of 4 feet each, per minute, performs the work o

12 horses working constantly, and burns 3,700 pounds per day. A cylinder of 24 inches, making 22 strokes of 5 feet, burns 5,500 pounds of coals and is equivalent to the work of 20 horses.

400. Watt's Steam-Engine at Work.

Illus. A is the boiler, to which Mr. Watt has paid very great attention. It is generally of an oblong form; and the flame, after striking on its concave bottom, circulates round the sides, and sometimes returns in a pipe through the body of the water, before it is suffered to go up into the chimney. In his engines there are commonly two of these boilers, so that one of them may work while the other is repairing. B is the steam pipe which conveys the steam to the cylinder C, which is cased, and closed at top by a plate, having a collar of leather, through which the piston-rod D works. a and c are the steam-valves, through which the steam enters into the cylinder: it is admitted through a, when it is to press the piston downwards, and through c when it presses it upwards. b and d are the eduction valves, through which the steam passes from the cylinder into the condenser e, which is a separate vessel placed in a cistern of cold water, and which has a jet of cold water, continually playing up in the inside of it: f is the air-pump, which extracts the air and water from the condenser. It is worked by the great beam or leaver, and the water brought by it from the condenser, after being