Going Barrels. Equation Clocks.
A clock which is capable of going accurately must have some contrivance to keep it going while you are winding it up. In the old-fashioned house clocks, which were would up by merely pulling one of the strings, and in which one such winding served for both the going and striking parts, this was done by what is called the endless chain of Huyghens, which consists of a string or chain with then ends joined together, and passing over two pulleys on the arbors of the great wheels, with deep grooves and spikes in them, to prevent the chain from slipping. In one of the two loops or festoons which hang from the upper pulleys is a loose pulley without spikes, carrying the clock-weights, and in the other small weight only heavy enough to keep the chain close to the upper pulleys. Now, suppose one of those pulleys to be on the arbor of the great wheel of the striking part, with a ratchet ad click, and the other pulley fixed to the arbor of the great wheel of the going part; then (whenever the clock is not striking) you may pull of the weight by pulling down that part of the string which hangs from the other side of the striking part; and yet the weight will be acting on the going part all the time. And it would be just the same if you would up the striking part and its pulley with a key, instead of pulling the string and also the same, if there were no striking part at all, but the second pulley were put on a black arbor, except that in the case the weight would take twice as long to run down, supposing that the striking part generally requires the same weight x fall as the going part.
This kind of gong barrel, however, is evidently not suited to the delicacy of an astronomical of an astronomical clock; and Harrisons going ratchet is now universally adopted in such clocks, and also in chronometers and watches for keeping the action of the train on the escapement during the winding. Fog. 14 (in which the same letters are used as in the corresponding parts of fig 1). Shows its construction. The click of the barrel-ratchet R is set upon another larger ratchet-wheel, with its teeth pointing the opposite way, and its click rT is set in the clock-frame. That ratchet is connected with the great wheel by a spring ss pressing against the two pins s in the ratchet and s in the wheel. When you wind up the weight (which is equivalent to taking if off), the click Tr prevents that ratchet from turning back or to the right; and as the spring ss is kept by the weight in a state of tension equivalent to the weight itself it will drive the wheel to the left for a short distance, when its end s is held fast, with the same force as if that end was pulled forward by the weight; and as the great wheel has to move very little during the short time the clock is winding, the spring will keep the clock going long enough.
In the commoner kind of turret clocks a more simple apparatus is used, which goes by the name of the bolt and shutter, because it consists of a weighted lever with a broad end, which shuts up the winding-hole until you lift it, and then a spring-bolt attached to the lever, or its arbor, runs into the teeth of one of the wheels, and the weight of the lever keeps the train going until the bolt has run itself out of gear. In the common construction of this apparatus there is nothing to ensure its being raised high enough to keep in gear the whole time of winding, if the man loiters over it. For this purpose Sir E. Beckett has the arbor of the bolt and shutter made to pump in and out of gear; and, instead of the shutter covering the winding-hole, it ends in a circular are advanced just far enough to prevent to the key or winder from being put on, by obstructing a ring set on the end of the pipe. In order to get the winder on, you must raise the lever high enough for the arc to clear the ring. During the two or three minutes which the clock may take to wind, the arc will be descending again without also pulling the maintaining power out of gear; so that even if it is constructed to keep in action then minutes, if required, still it will never remain in action longer than the actual time of winding. The circular arc must be think enough, or have a projecting flange added to it deep enough, to prevent the winder being put on by merely pushing back the maintaining power lever without lifting it.
In large clocks with a train remontoire, or even with a gravity escapement, it is hardly safe to use a spring going barrel, because it is very likely to be exhausted too much wind up the remontoire, or raise the gravity pallets, before the winding is finished, if it takes more than two or three minutes; whereas, with the common escapements, the wheel has only to escape, as the pendulum will keep itself going for some time without any impulse.
It would occupy too much space to describe the various contrivances for making clocks shows the variations of solar compared with mean time (called equation clocks), the days of the month, periods of the moon, and other phenomena. The old day of the month clocks, required setting at the end of every month which has not 31 days, and have long been obsolete. Clocks are now made even to provide for leap year. But we doubt whether practically anybody ever takes his day of the month from a clock face, especially as the figures are too small to be seen except quite near. Several persons have taken parents for methods of exhibiting the time by figures appearing through a hole in the dial, on the principle of the "numbering machine." But they do not reflect that no such figures, on any practicable scale, are as conspicuous as a pair of hands; and that nobody really reads the figures on a dial, but judges of the time in a moment from the position of the hands; for which reason the minute hand should be straight and plain, while the hour hand has a "heart" near the end; 12 large marks and 48 small one make a more distinguishable dial than one with figures; and the smaller the figures are the better, as they only tend to obscure the hands.
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