Briefly summarizing the steps by which the mechanism of the modern watch has been de veloped from De Vick's model, we find: (1) The use of the spring as motive force: (2) substitu tion of balance-wheel, with hair-spring. for weights: (3) successive improvements in es capement; (4) introduction of enamel dials; (5) introduction of jewels as pivot bearings; (6) invention of compensating balance; (7) dis carding of fusee; (8) introduetion of automatic machinery for the construction of the parts.
The mechanism or 'movement' of a modern American watch consists of a series of wheels called the train, the first of which, named the bar rel, contains the main-spring. in which the power secured when the watch is wound up is stored, The train transmits to the desired points the motion stored up in the spring. At the end of the train is the escapement—a delicate and corn plea mechanism which so regulates the motive force stored up in the main-spring that it is expanded with uniformity and at the desired rate. The train of wheels is placed in a circular position as shown in Fig. 2 so as to accommodate the shape of the watch, but the order is best understood by arranging them in a straight line, as in Fig. 3. It has been seen that the first wheel of the train, the barrel, eontains the main-spring and is therefore connected with the stem-winding attachment. A little click or pawl, working in the teeth of a ratchet fastened to the main-spring arbor, keeps the latter from running backward. The second wheel of the train, called the centre wheel. is always placed in the centre of the watch, because connected with it on the same stair are the wheels whh-h drive the minute and hour hands. The centre wheel meshes into the third pinion, 111)011 \dile!' the third wheel is firmly fixed. This ttu•ns the fourth pinion, which carries a wheel gearing into the escape pinion, upon which the escape wheel is carried. The second or centre pinion is attached to a centre staff, which projects through the main plate of the watch. Upon this projecting portion is another pinion, whiell from its shape is ealled the cannon. pinion. The minute hand is fitted upon the hub of this pinion and its teeth drive a little wheel Nvhielt is commonly known as the minute wheel. Attached to this wheel is still another pinion geared to drive the hour wheel, which is mounted on the long hub of the cannon pinion. Upon the hub of the hour wheel the hour hand is placed. The number of teeth on each of these wheels and pinions (which are generally called the dial wheels) is so arranged that while the cannon pinion, carrying the minute hand, makes a com plete revolution in one hour, the hour wheel, carrying the hour hand, revolves once in twelve hours. 'l'he stem attachment for setting the
watch is connected with one or other of these dial wheels.
Returning now to the wheel train, we find that the fourth pinion has a pivot which projects through the frame plate and upon this pivot the second !land is mounted. Hence it necessary that the fourth pinion and wheel should make a revolution once a minute, or 60 times as fast as the centre staff.
This is accomplished by means of the proportioning of the centre wheel. third pinion and wheel, and fourth pillion to each other. Con sidering, now, the last wheel of the train and its va rious attachments, which are grouped together under the general name of escapement, we find that the group con sists of seven members: The escape wheel. the pallet. the fork, the roller and roller pin, the balance-whcel, and the hair-spring. The use of each part is explained far ther on. The combined object of the whole group is to stop and then set in motion again, at regular intervals. every wheel in the train— an action which causes the familiar tick of the watch. The teeth of the escape- wheel, instead of being cut on the epicycloidal curve like the other teeth of the train, so as to convey mo tion with as little l of power as possible. are constructed with an entirely different object and in an entirely different form. They form a series of fifteen hooks. An anchor-shaped piece. called the pallet, is so hung that first one and then the other of its horns is locked into the tooth of the escape wheel as it revolves. These locking horns are usually made of some precious stone. The unlocking is accomplished by a lever called the fork, which is attached to the pallet. The action of the fork is controlled by the bal ance-w•heel by means of a little disk under the balance-wheel called the roller. On the outer edge of this disk is the roller pin, which alter nately engages and releases the fork. It has al ready been shown that the action of the hair spring on the balance makes its vibrations iso chronous; by means of the fork disengaging at regular intervals the teeth of the escape wheel from the pallet and so freeing the whole train, this isochronous motion is conveyed to the en tire watch train. Fig. 4 of the accompanying cuts shows the action of the fork, roller, roller pin, pallet, and escape wheel, while Fig. 5 shows the hair-spring connected with the balance, with the roller and roller pin underneath.