700 CLOCKS AND WATCHES scape wheel is 30, and if the pendulum is 39'1 inches in length, it will revolve once in a minute, because one tooth will escape at every double vibration (sometimes called a complete vibration), or every two seconds. If the pinion has 7 leaves, and the third wheel, which drives it, has 56 teeth, the latter will revolve once in 8 minutes ; and if its pinion has 8 leaves, each leaf will pass a certain point every minute ; and therefore, if the centre wheel has 60 teeth, it will revolve once in an hour. If the pinion of the centre wheel has 8 leaves, and there are 96 teeth in the great wheel, the latter will turn round once in 12 hours. This arrangement formerly existed in clocks before the use of the minute hand, but since then wheels separate from the train have been used to move the hands at the proper rate. In the engraving a back view of the wheels is given, not placed in relation to each other just as they are when in actual use, but every wheel, following in order from below upward, placed behind its prede- cessor, for the purpose of showing the pinions. The wheels may be arranged in this way, but they are generally placed alternately in front of and behind each other, for economy of space. The second wheel, as has been stated, moves the minute hand. The pinion by which the great wheel drives it is called the centre pinion. This is on the back side of the wheel, but it carries another pinion in front, called the can- non pinion, which is placed on the arbor so that it may be turned by using a certain amount of force, an operation which is required in setting. It is upon a tubular barrel of this cannon pinion that the minute hand is placed. The cannon pinion has a certain number of leaves, which play into a wheel having, we will say, four times as many teeth, which latter has a pinion with a certain number of leaves which again play into another wheel having three times as many teeth. This wheel, called the hour wheel, will then turn round once in 12 hours, and upon its barrel, which is placed over the cannon pinion, the hour hand is fixed. The time during which a clock can be made to run from one winding to another, measured by the number of times the scape wheel can be made to revolve, depends upon the number of teeth in the train of wheels, the distance through which the weight falls, and the length of the pendulum. The number of teeth may be regulated by the number of wheels in the train, or by the number of teeth in each wheel and pinion. If the weight falls through a small space, the number of teeth must be in- creased, and this is usually done by increasing the number of wheels, which again requires the gravity of the weight to be increased. The number of teeth in the train remaining the same, the duration of running may be increas- ed by increasing the distance through which the weight falls. About the year 1840 Prof. Wheatstone exhibited to the royal society of London a clock dial, the hands of which were moved by a wheel acted upon by a small elec- tro-magnet at intervals, the current being formed and broken by means of the oscillations of the pendulum of a common clock. Through this device the same time may be indicated in several distant places simultaneously. In 1848 successful experiments were made upon this principle by the United States coast survey between Cincinnati and Pittsburgh, a distance of 400 miles. A clock placed in the electric circuit recorded its beats at all the offices along the line by means of Morse's apparatus. The first clock, however, which had any of its own parts moved by electricity, was constructed by Alexander Bain of Edinburgh. In this elec- tricity was used as a motive power in place of the usual weight or spring, and the pendulum was not only employed as a regulator, but as a motor. The bob of the pendulum was formed of a coil of wire which became a magnet at intervals of the oscillations, and, passing over the poles of permanent magnets placed near the ends of the arc of oscillation, was alter- nately attracted by each. In some of the clocks the two magnets were temporary, and the re- versal of their poles by one of the devices used in electrical apparatus caused an alternate at- traction and repulsion of the pendulum. Mr. Shepherd exhibited at the international exhibi- tion in London of 1851 a clock in which there was an electrical gravity escapement, the pal- lets being raised by temporary magnets. A description of it may be found in Wood's " Curiosities of Clocks and Watches," and also in Mr. Denison's treatise. WATCHES. The first watches must have been very imperfect time- keepers, as they were not supplied with that necessary piece of apparatus which answers the place of the pendulum in clocks, viz., the balance and balance spring, and the escape- ment consisted only of a verge and a crown wheel, of a similar form to that used in the verge escapement clocks. The train of wheels was moved, like that in the modern watch, by a main spring, which is a coil of ribbon-shaped, finely tempered steel, placed around the arbor of the going barrel, having one end attached to the arbor and the other to the inside of the barrel. The train of wheels in a watch is much the same as in a clock, and indeed a watch may be considered as a small clock, in which the weight and pendulum are replaced by the main spring and the balance, which latter part is composed of the balance wheel and balance or hair spring. The devices for regulating the motion of the scape wheel by means of a lev- er armed with two pallets, against which the teeth of the wheel are caused to exert their force, are much alike in the modern detached lever watch and a good regulator clock ; but as the arc in which the pendulum swings varies but very little, while that in which the balance wheel vibrates varies considerably from differ- ent causes (the principal one being the motion given to the watch in carrying it in the pocket), it is apparent that there must be considerable variation in the mode of applying the devices.
