There are two kinds of striking works use in clocks. The older of them, which is still used in most foreign clocks, and in turret clocks in England also, will now aloe the striking of any hour to either omitted or repeated, without making the next hour strike wrong; whereas, in that which is used in English house clocks, the number of blows to be struck depends merely on the position of a wheel attached to the going part; and therefore the striking of any hour may omitted or repeated without deranging the following ones. It turret clocks there is no occasion for the repeating movements; and for the purpose of describing the other, which is called the locking-plate movement, we may we refer to fig. 22, which is the front view of a large clock, striking both hours and quarters on this plan. In the hour part (on the left), you observe a bent lever BAH, called the "lifting-piece," of which the end H has just been left off by the snail on the hour-wheel 40 of the going part; and at the other end there are two stops on the back side of the lever, one behind and rather below the other; and against the upper one a pin in the end of a short lever 9 B, which is fixed to the arbor of the fly, is now resting, and thereby the train is stopped from running, and the clock from striking any more. The stops are shown on the quarter lifting-piece in the figure (27) of the Westminster clock. We omit the description of the action of the wheels, because it is evident enough. At D may be seen a piece projecting from the lever AB, and dropping into a notch in the wheel 78. That wheel is the locking-wheel or locking-plate; and it has in reality notches such as D all round it, at distances 2, 3, up to 12, from any given point in the circumference, which may be considered as marked off into 78 spaces, that being the number of blows struck in 12 hours. These notches are shown in the locking-plate of the quarter part in fig. 22, but not in the hour part, for want of size to show them distinctly.
When the arm AB is the lifting-piece is raised by the snail depressing the other end H, a few minutes before the hour, the fly-pin slips past the first of the stops at B, but is stopped by the second and lower one, until the lever is dropped again exactly at the hour. Thus the pin can pass, and would go once round, allowing the train to go on a little; but before it has got once round, AB has been lifted again high enough to carry both stops out of the way of the fly-pin, by means of the cylinder with two slices taken off it, which is set on the arbor of the wheel 90, and on which the end of the lifting-piece rests, with a small roller to diminish the friction. If the clocks has only to strike one, the lifting-piece will then drop again, and the fly-pin will be caught by the first step, having made (according to the numbers of the teeth given in fig. 22) 5 turns. But if it has to strike more, the locking-wheel comes into action. That wheel turns with the train, being either driven by pinion 20 on the arbor of the great wheel, or by a gathering pallet on the arbor of the second wheel, like G in fig. 15; and when once the lifting-piece is lifted out of a notch in the locking-plate, it cannot fall again until another notch has come under the but D; and as the distance of the notches is proportioned to the hours, the locking-plate thus determines the number of blows struck. It may occur to the reader, that the cylinder 10 and roller are not really wanted, and that the locking-plate would do as well without; and sometimes clocks are so made, but it is not safe, for the motion of the locking-plate is so slow that unless everything is very carefully adjusted and no shake left, the corner of the notch may not have gor fairly under the bit D before the fly has got once round, and then the lifting-piece will drop before the clock can strike at all; or it may hold on too long and strike 13l, and St Pauls clock did once at midnight, when it was heard at Windsor by a sentinel.
Small French clock, which generally have the striking part made in this way, very commonly strike the half hours also, by having a wide slit, like that for one oclock, in the locking-plate at every hour. But such clocks are unfit for any place except a room, as they strike one three times between 12 and 2, and accordingly turret clocks, or even large house clocks, are never made so. Sir E Beckett has lately introduced the plan of making turret clocks strike one at all the half hours except 12 _ and 1 _, so that any striking of one that is heard between 11 _ and 2 _ must needs be one oclock. This is done by having a 12-hour wheel driven by the going part, either continuously or by a gathering pallet moving that wheel only once an hour, and it has two high steps which come under another piece like D in the lifting detent a little before 12 _ and 1 _ so as to prevent it falling when let off by the snail. In the English or rack striking movement, to be presently described, the same thing may be done by a kind of star wheel with flat ends to the rays, attached to the 12-hour snail, which will let the rack fall enough to strike one at every half hour, but with two longer rays to prevent it falling at all at 12 _ and 1 _; or it would be better to let those rays be means of an intervening lever, prevent the lifting piece from falling, as that would involve less friction of the tail of the rack.
In all cases, the locking-plate must be considered as divided into as many parts as the number of blows to be struck in 12 hours, i.e. 78, 90, or 88, according as half hours are or are not struck; and it must have the same number of teeth, driven by a pinion on the striking wheel arbor of as many teeth as the striking cams, or in the same ratio.
Fig. 15 is a front view of a common English house clock with the face taken off, showing the repeating or rack striking movement. Here, as in fig 1, 1, M is the hour-wheel, on the pipe of which the minute-hand is set, N the reversed hour-wheel, and n its pinion, driving the 12-hours wheel H, on whose socket is fixed what is called the snail Y, which belongs to the striking work exclusively. The hammer is raised by the eight pins in the rim of the second wheel in the striking train, in the manner which is obvious.
The hammer does not quite touch the bell, as it would jar in striking if it did, and prevent the full sound; and if you observe the form of the hammer-shank at the arbor where the spring S acts upon it, you will see that the spring both drives the hammer against the bell when the tail T is raised, and also checks nit just before it reaches the bell, and so the blow on the ell is given by the hammer having acquired momentum enough to go a little farther than its place of rest. Sometimes two springs are used, one for impelling the hammer, and the other for checking it. A piece of vulcanized India-rubber, tied round the pillar just where the hammer-shank nearly touches it, forms a good a check spring as anything. But nothing will check the chattering of a heavy hammer, except making it lean forward so as to act, partially at least, by its weight. The pinion of the striking-wheel generally has eight leaves, the same number as the pins; and as a clock strikes 78 blows in 12 hours, the great wheel will turn in that time if it has 78 teeth instead of 96, which the great wheel of the going part has for a centre pinion of eight. The striking-wheel drives the wheel above it once round for each blow, and that wheel drives a fourth (in which you observe a single pin P), six, or any other integral number of turns, for one turn on its own, and that drives a fan-fly to moderate the velocity of the train by the resistance of the air, an expedient at least as old as De Vicks clock in 1370.
The wheel N is so adjusted that, within a few minutes of the hour, the pin in it raises the lifting-piece LONF so far that that piece lifts the click C our of the teeth of the rack BKRV, which immediately falls back (helped by a spring near the bottom) as far as its tail V can go by reason of the snail Y, against it falls; and it is so arranged that the number of teeth which pass the click is proportionate to the depth of the snail; and as there is one step in the snail for each hour, and it goes round with the hour-hand, the rack always drops just as many teeth as the number of the hour to be struck. This drop makes the noise of "giving warning." But the clock is not yet ready to strike till the lifting piece has fallen again; for, as soon as the rack was let off the tail of the thing called the gathering pallet G, on the prolonged arbor of the third wheel, was enabled to pass the pin K of the rack on which it was pressing before, and the striking train began to move; but before the fourth wheel had got half round, its pin P was caught by the end of the lifting-piece, which is bent back and goes through a hole in the plate, and when raised stands in the way of the pin P, so that the train cannot go on till the lifting-piece drops, which it does exactly at the hour, by the pin N then slipping past it. Then the train is free; the striking wheel begins to lift the hammer, and the gathering pallet gathers up the rack, a tooth for each blow, until it has returned to the place at which the pallet is stopped by the pin K coming under it. In this figure the lifting-piece is prolonged to F, where there is a string hung to it, as this is the proper place for such a string when it is wanted for the purpose of learning the hour in the dark, and not (as it is generally put) on the click C; for if it put there and you hold the string a little too long, the clock will strike too many; and if the string accidentally sticks in the case, it will go on striking till it is run down; neither of which things can happen when the string is put on the lifting-piece.
The snail is sometimes set on a separate stud with the apparatus called a star-wheel and jumper; but as this only increases the cost without any advantage that we can see, we omit any further reference to it. On the left side of the frame we have placed a lever x, with the letters st below it, and si above. If it is pushed up to si, the other end will come against a pin in the rack, and prevent it from falling, and will thus make the clock silent; and this is much more simple than the old-fashioned "strike and silent" apparatus, which we shall therefore not describe, especially as it is seldom used now.
If the clock is required to strike quarters, a third "part" on train of wheel is added on the right hand of the going part; and its general construction is the same as the hour-striking part; only there are two more bells, and two hammers so placed that one is raised a little after the other. If there are more quarter-barrel, which is merely a cylinder set on the arbor of the striking-wheel (in that case generally the third in the train), with short pins stuck into it in the proper places to raise the hammers in the other required for the tune of the chimes. The quarters are usually made to let off the hour, and this connection may be made in two ways. If the chimes are different in tune for each quarter, and not merely the same tune repeated two, three, and four times, the repetition movement must not be used for them as it would throw the times into confusion, but the old locking-plate movement, as in turret clocks; and therefore, if we conceive ten hour lifting-piece connected with the quarter locking-plate, as it is with the wheel N, in fig. 15, it is evident that pin will discharge the hour striking part as the fourth quarter finished.
But where the repetition movement is required for the quarters, the matter is not quite so simple. The principle of it may shortly be described thus. The quarters themselves have a rack and snail, &c., just like the hours, except that the snail is fixed on one of the hour-wheels M or N, instead of on the twelve-hour wheel, and has only four steps in it. Now suppose the quarter-rack to be so place that when it falls for the fourth quarter (its greatest drop), it falls against the hour lifting-piece some where between O and N, so as to raise it and the click C. Then the pin Q will be caught by the click Qq, and so the lifting-piece will remain up until all the teeth of the quarter-rack are gathered up; and as that is done, it may be made to disengage the click Qq, and so complete the letting off the hour striking part, this click Qq has no existence except where there are quarters.
These quarter clocks are sometimes made so as only to strike the quarters at the time when a string is pulledas by a person in bed, just like repeating watches, which are rarely made now, on account of the difficulty of keeping in order such a complicated machine in such a small space. In this case, the act of pulling the string to make the clock strike winds up the quarter-barrel, which is that of a spring clock (nor yet described), as far as it is allowed to be wound up by the position of s snail on the hour-wheel against which a lever is pulled, just as the tail of the common striking-rack falls against the snail on the twelve-hour wheel; and it is east to see that the number of blows struck by the two quarters hammers may thus be made to depend upon the extent to which the spring that drives the train wound up; and it may even be made to indicate half-quarters; for instances, if the snail has eight steps in it, the seventh of them may be just deep enough to let the two hammers strike three times, and the first of them once more, which would indicate 7 _ minutes to the hour. It is generally so arranged that the hour is struck first, and the quarters afterwards.
In connection with these bedroom clocks we ought to mention alarums. Perhaps the bets illustration of the mode of striking an alarum is to refer to either of the recoil escapements (fig. 3 and 4). If you suppose a short hammer instead of a long pendulum attached to the axis of the pallets, and the wheel to be driven with sufficient force, it will evidently swing the hammer rapidly backwards and forwards; and the position and length of the hammer-head may be so adjusted as to be strike a bell inside, first on one side and then on the other. Then as to the mode of letting off the alarum at the time required; if it was always to be let off at the same time, you would only have to set a pin in the twelve-hour wheel at the proper place to raise the lifting-piece which lets off the alarum at that time. But as you want it to be capable of alternation, this discharging pin must be set in another wheel (without teeth), which rides with a friction-spring on the socket of the twelve-hour wheel, with a small movable dial attached to it, having figures so arranged with reference to the pin that whatever figure is made to come to small pointer set as a trail to the hour hand, the alarum shall be let off at the hour. The letting of does not require the same apparatus as a common striking part, because an alarum has not to strike a definite number of blows, but to go on till it is run down; and therefore the lifting-piece is nothing but a lever with a top or hook upon it, which, when it is dropped, takes hold on one of the alarum wheels, and lets them go while it is raised high enough to disengage it. You just of course not wind up an alarum till within twelve hours of the time when it is wanted to go off.
The watchmans or tell-tale clock may be seen in one of the lobbies of the House of Commons, and in prisons, and some other places where they want to make sure of watchman being on the spot and awake all the night; it is a clock with a set of spikes, generally 48 or 96, sticking out all round the dial, and a handle somewhere in the case, by pulling which you can press in that one of the spikes which is opposite to it, or to some lever connected with it, for a few minutes; and it will be observed, that this wheel of spikes is carried round with the hour-hand, which is these clocks is generally a twelve-four hour one. It is evident that very spike which is seen still sticking out in the morning indicates that at the particular time to which that spike belongs the watchman was not there to push it inor at any rate, that he did not; and hence its name. At some other part of their circuits, the inner ends of the pins are carried over a roller or an inclined plane with pushed them out again ready for business the next night.
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