in the art of war, a number of men posted at any passage, or a company of the guards who go on the patrol.

WATCH and CLOCK WORK.

A clock is a machine constructed in such a manner, and regulated by such uniform movements, as to measure time and all its subdivisions with great exactness. The same definition comprehends watches of all kinds; and indeed they are both made upon the same principles. We shall therefore give a view of the construction of both these machines under this article.

Of the Mechanism of Clocks, and how they measure Time.

The first figure of Plate CLX is a profile of a clock; P is a weight suspended by a rope that winds about the cylinder or barrel C, which is fixed upon the axis a a; the pivots b b go into holes made in the plates TS, TS, in which they turn freely. These plates are made of brass or iron, and are connected by means of four pillars ZZ; and the whole together is called the frame.

The weight P, if not restrained, would necessarily turn the barrel C with an uniform accelerated motion, in the same manner as if the weight was falling freely from a height. But the barrel is furnished with a ratchet wheel K K, the right side of whose teeth strikes against the click, which is fixed with a screw to the wheel D D, as represented in fig. 2. So that the action of the weight is communicated to the wheel D D, the teeth of which act upon the teeth of the small wheel d which turns upon the pivots c c. This communication of the teeth of one wheel with another is called engrenage or pitching; and a small wheel, like d, is called a pinion.

The wheel E E is fixed upon the axis of the pinion d; and the motion communicated to the wheel D D by the weight is transmitted to the pinion d, consequently to the wheel E E, as likewise to the pinion e, and wheel FF, which moves the pinion f, upon the axis of which the crown or balance wheel G H is fixed. The pivots of the pinion f play in holes of the plates L M, which are fixed horizontally to the plates T S. In a word, the motion begun by the weight is transmitted from the wheel G H to the palettes I K, which communicates its motion by means of the fork U X riveted on the palettes, to the pendulum A B, which is suspended upon the hook A. The pendulum A B describes, round the point A, an arc of a circle, alternately going and returning. If then the pendulum be once put in motion by a push of the hand, the weight of the pendulum at B will make it return upon itself, and it will continue to go alternately backward and forward till the resistance of the air upon the pendulum, and the friction at the point of suspension at A, destroys the original impressed force. But as, at every vibration of the pendulum, the teeth of the balance-wheel G H act so upon the palettes I K, (the pivots upon the axis of these palettes play in two holes of the potence s t,) that after one tooth H has communicated motion to the palette K, that tooth escapes; then the opposite tooth G acts upon the palette I, and escapes in the same manner; and thus each tooth of the wheel escapes the palettes I, K, after having communicated their motion to the palettes in such a manner that the pendulum, instead of being stopped, continues to move.

The wheel E E revolves in an hour; the pivot c of this wheel passes through the plate, and is continued to r; upon the pivot is a wheel N N with a long socket fastened in the centre; upon the extremity of this socket r the minute-hand is fixed. The wheel N N acts upon the wheel O; the pinion of which, p, acts upon the wheel g g, fixed upon a socket which turns along with the wheel N. This wheel g g makes its revolution in 12 hours, upon the barrel of which the hour-hand is fixed.

From the above description it is easy to see, 1. That the weight p turns all the wheels, and at the same time continues the motion of the pendulum. 2. That the quickness of the motion of the wheels is determined by that of the pendulum. 3. That the wheels point out the parts of time divided by the uniform motion of the pendulum.

When the cord upon which the weight is suspended is entirely run down from off the barrel, it is wound up again by means of a key, which goes on the square end of the arbor at Q, by turning it in a contrary direction from that in which the weight descends. For this purpose, the inclined side of the teeth of the wheel R (fig. 2) removes the click C, so that the ratchet-wheel R turns while the wheel D is at rest. But as soon as the cord is wound up, the click falls in between the teeth of the wheel D and the right side of the teeth again act upon the end of the click, which obliges the the wheel D to turn along with the barrel; and the spring A keeps the crank between the teeth of the ratchet-wheel R.

We shall now explain how time is measured by the motion of the pendulum; and how the wheel E, upon the axis of which the minute-hand is fixed, makes but one precise revolution in an hour. The vibrations of a pendulum are performed in a shorter or longer time in proportion to the length of the pendulum itself. A pendulum of 3 feet 8½ French lines in length, makes 3600 vibrations in an hour; i.e., each vibration is performed in a second of time, and for that reason it is called a second pendulum. But a pendulum of 9 inches 2¼ French lines makes 7200 vibrations in an hour, or two vibrations in a second of time, and is called a half-second pendulum. Hence, in constructing a wheel whose revolution must be performed in a given time, the time of the vibrations of the pendulum which regulates its motion must be considered. Supposing, then, that the pendulum AB makes 7200 vibrations in an hour, let us consider how the wheel E shall take up an hour in making one revolution. This entirely depends on the number of teeth in the wheels and pinions. If the balance-wheel consists of 30 teeth, it will turn once in the time that the pendulum makes 60 vibrations; for at every turn of the wheel, the same tooth acts once on the pallets I, and once on the pallet K, which occasions two separate vibrations in the pendulum; and the wheel having 30 teeth, it occasions twice 30, or 60 vibrations. Consequently, this wheel must perform 120 revolutions in an hour; because 60 vibrations, which it occasions at every revolution, are contained 120 times in 7200, the number of vibrations performed by the pendulum in an hour. Now in order to determine the number of teeth for the wheels E, F, and their pinions e f, it must be remarked, that one revolution of the wheel E must turn the pinion e as many times as the number of teeth in the pinion is contained in the number of teeth in the wheel. Thus, if the wheel E contains 72 teeth, and the pinion e 6, the pinion will make twelve revolutions in the time that the wheel makes one; for each tooth of the wheel drives forward a tooth of the pinion, and when the six teeth of the pinion are moved, a complete revolution is performed; but the wheel E has by that time only advanced six teeth, and has still 66 to advance before its revolution be completed, which will occasion 11 more revolutions of the pinion. For the same reason, the wheel F having 60 teeth, and the pinion f 6, the pinion will make 10 revolutions while the wheel performs one. Now, the wheel F being turned by the pinion e, makes 12 revolutions for one of the wheel E; and the pinion f makes ten revolutions for one of the wheel F; consequently, the pinion f performs 10 times 12 or 120 revolutions in the time the wheel E performs one. But the wheel G, which is turned by the pinion f, occasions 60 vibrations in the pendulum each time it turns round; consequently the wheel G occasions 60 times 120 or 7200 vibrations of the pendulum while the wheel E performs one revolution; but 7200 is the number of vibrations made by the pendulum in an hour, and consequently the wheel E performs but one revolution in an hour; and so of the rest.

From this reasoning, it is easy to discover how a clock may be made to go for any length of time without being wound up: 1. By increasing the number of teeth in the wheels. 2. By diminishing the number of teeth in the pinions. 3. By increasing the length of the cord that sustains the weight; and lastly, by adding to the number of wheels and pinions. But, in proportion as the time is augmented, if the weight continues the same, the force which it communicates to the last wheel GH will be diminished.

It only remains to take notice of the number of teeth in the wheels which turn the hour and minute hands.

The wheel E performs one revolution in an hour; the wheel NN, which is turned by the axis of the wheel E, must likewise make only one revolution in the same time; and the minute-hand is fixed to the barrel of this wheel. The wheel N has 30 teeth, and acts upon the wheel O, which has likewise 30 teeth, and the same diameter; consequently the wheel O takes one hour to a revolution; now the wheel O carries the pinion p, which has six teeth, and which acts upon the wheel qq of 72 teeth; consequently the pinion p makes 12 revolutions while the wheel qq makes one, and of course the wheel qq takes 12 hours to one revolution; and upon the barrel of this wheel the hour-hand is fixed. We shall conclude with remarking, that all that has been said here concerning the revolutions of the wheels, &c., is equally applicable to watches as to clocks.

Of the Mechanism of a Watch.

Watches, as well as clocks, are composed of wheels and pinions, and a regulator to direct the quickness or slowness of the wheels, and of a spring which communicates motion to the whole machine. But the regulator and spring of a watch are vastly inferior to the weight and pendulum of a clock, neither of which can be employed in watches. In place of a pendulum, therefore, we are obliged to use a balance (fig. 4.) to regulate the motion of a watch; and of a spring (fig. 6.) which serves in place of a weight, to give motion to the wheels and balance.

The wheels of a watch, like those of a clock, are placed in a frame formed of two plates and four pillars. Fig. 3. represents the inside of a watch, after the plate (fig. 5.) is taken off. A is the barrel which contains the spring (fig. 6.) the chain is rolled about the barrel, with one end of it fixed to the barrel A, and the other to the fusee B.

When a watch is wound up, the chain which was upon the barrel winds about the fusee, and by this means the spring is stretched; for the interior end of the spring is fixed by a hook to the immovable axis, about which the barrel revolves; the exterior end of the spring is fixed to the inside of the barrel, which turns upon an axis. It is therefore easy to perceive how the spring extends itself, and how its elasticity forces the barrel to turn round, and consequently obliges the chain which is upon the fusee to unfold and turn the fusee; the motion of the fusee is communicated to the wheel CC; then, by means of the teeth, to the pinion c, which carries the wheel D; then to the pinion d, which carries the wheel E; then to the pinion e, which carries the wheel F; then to the pinion f, upon which is the balance-wheel G, whose pivot runs in the pieces A called a pontance, and B called the follower, which are fixed on the plate, fig. 5. This plate, of which only a part is represented, is applied to that of (fig. 3.) in such a manner, that the pivots of the wheels enter into holes made in the plate (fig. 3.) Thus the impressed force of the spring is communicated to the wheels; and the pinion f, being then connected to the wheel F, obliges it to turn, (fig. 7.) This wheel acts upon the pallets of the verge 1, 2, (fig. 4.) the axis of which carries the balance... HH, (fig. 4.) The pivot I, in the end of the verge, enters into the hole c in the potance A (fig. 5.) In this figure the palettes are represented; but the balance is on the other side of the plate, as may be seen in fig. 11. The pivot 3 of the balance enters into a hole of the cock BC, (fig. 10) a perspective view of which is represented in (fig. 12.) Thus the balance turns between the cock and the potance c, (fig. 5.) as in a kind of cage. The action of the balance-wheel upon the palettes 1, 2, (fig. 4.) is the same with what we have described with regard to the same wheel in the clock; i.e., in a watch the balance wheel obliges the balance to vibrate backwards and forward like a pendulum. At each vibration of the balance a palette allows a tooth of the balance-wheel to escape, so that the quickness of the motion of the wheels is entirely determined by the quickness of the vibrations of the balance, and these vibrations of the balance and motion of the wheels are produced by the action of the spring.

But the quickness or slowness of the vibrations of the balance depend not solely upon the action of the great spring, but chiefly upon the action of the spring a, b, c, called the spiral spring, (fig. 14.) situated under the balance H, and represented in perspective (fig. 11.) The exterior end of the spiral is fixed to the pin a, (fig. 14.)