Page:The American Cyclopædia (1879) Volume IV.djvu/711

 CLOCKS AND WATCHES 699 FIG. 5. Pin-wheel Escape- ment of Lepaute. dulum the action of the pins is downward on both pallets, which have the impulse faces cut to nearly the same angle, the one on the shorter and outside arm being rather the more oblique when they are to receive the pins on a level with the axis of the scape wheel. It will be observed that the pallets may be made to receive the pins at any part of the revo- lution of the wheel by changing their posi- tion with the pendu- lum'; and it is also evident that the form of the pins and pallets may be such as to either produce a re- coil of the scape wheel or a dead beat. An- other kind are called gravity escapements, be- cause the impulse is not given to the pallets directly by the action of the train weight, but by another weight or spring which is caused to act for a sufficient space of time at every beat of the pendulum. There are several forms of gravity escapements, asMudge's, Cumming's, Hardy's, Kater's, Bloxam's, and others. In order that a timepiece may be kept running, there must be a contrivance for winding up the weight or spring. As it is evident that this must be done without reversing the motion of the train, there has to be provided an arrange- ment for turning the barrel backward without turning the great wheel. This is effected by means of a ratchet wheel, which is prevented by a click from moving on the great wheel in the direction of the going of the train, but is free to move in the contrary direction. With- out some further contrivance, however, than what is necessary to prevent the great wheel from being turned back, the clock would soon stop, because the propelling power is taken away during the winding. Such a contrivance is called a main- taining power or going barrel, the principal form of which ha present use is Harrison's main- taining spring and ratchet, or going barrel. The oldest of all is Huyghens's endless chain shown in fig. 6, which is so contrived as not to take the weight off the barrel during the winding. By pulling down the small weight the large weight is raised with- out taking the tension of the cord off from the going wheel at all. The pulley may be placed upon the arbor of the great wheel of the striking part, which must then be attached to it by a ratchet and click. Harrison's going barrel is represented Fio. 6. Huy- gens's End- less Chain. in the train of wheels in a clock shown in fig. 7. The weight moves the great wheel c through the spring d ; but during winding the smaller ratchet is turned to the right, taking off the weight, while the spring, acting against the larger ratchet, impels the great wheel a sufficient time to keep it going, and thereby to maintain the motion of the scape wheel and the impulse of its teeth against the pallets. Com- mon clocks, many of them keeping good time, are now made to be driven by a main spring instead of a weight, in the manner of a watch. Being portable, and occupying less space, they are more convenient and cheaper; but the best clocks, used for regulators, are driven by weights. It will now be proper to describe the train of wheels and principal parts of a clock. The train comprises those wheels through which the motive power, the weight or spring, exerts its force upon the pallets connected with the FIG. 7. Train of Wheels in a Clock. pendulum. These wheels are made to act upon each other by means of pinions, which are a kind of small cog wheels. The cogs on the wheels proper are called teeth, while those of the pinions are called leaves. The axis upon which a wheel or pinion turns is called the arbor. The train of wheels in a good modern eight-day clock generally consists of four. In fig. 7, a is the first or great wheel, upon whose arbor is the barrel over which the cord passes to which is suspended the weight. The second or centre wheel, &, whose pinion c is driven by the great wheel, always turns round once in an hour, and is therefore made to turn the minute hand. It drives the pinion e of the third wheel d, which again drives the pinion of the scape wheel f. This last is the fastest- going wheel in the tram, and is the one that acts upon the pallets connected with the pen- dulum. The usual number of teeth in the