Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/34

24 place at which the pallet is stopped by the pin Iv 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 is 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-ivheel 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 &quot; strike and silent &quot; apparatus, which we shall therefore not describe, especially as it is seldom used now. If the clock is required to strike quarters, a third &quot; part&quot; or train of wheels 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-bells than two, the hammers are generally raised by a chime-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 order required for the tune of the chimes. The quarters are usually made to let off the hour, and this con nection 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 tunes into confusion, but the old locking-plate move ment, as in turret clocks ; and therefore, if we conceive the hour lifting-piece connected with the quarter locking- plate, as it is with the wheel N, iu fig. 15, it is evident that the pin will discharge the hour striking part as the fourth quarter finishes. 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 them selves have a rack and snail, &c., just like the hours, ex cept 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 placed that when it falls for the fourth quarter (its greatest drop), it falls against the hour lifting-piece some where between and N, so as to raise it and the click C. Then the pin Q will be caught by the click Q?, and so tho lifting-piece will remain up until all the teeth of the quar ter-rack are gathered up ; and as that is done, it may be made to disengage the click Q?, and so complete the let ting off the hour striking part. This click Q? 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 pulled as by a person in bed, just like repeating watches, which are rarely made now, on account of the difficulty of keep ing 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 (not yet described), as far as it is allowed to be wound up by the position of a 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 easy to see that the number of blows struck by the two quarter hammers may thus be made to depend upon the extent to which the spring that drives the train is wound up; and it may even be made to indicate half-quarters ; for instance, 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.

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Hitherto we have supposed all clocks to be kept going by a weight. But, as is well known, many of them are driven by a spring coiled up in a barrel. In this respect they differ nothing from watches, and therefore for consideration of the construction of parts belonging to the spring reference is made to the article. It may, 