Now that we have given the names of each of the different parts which compose the escapement, let us see how they perform their important work of governing the speed of the little machine for measuring time. In the escape wheel, the left arm of the pallet rests on the inclined top of one of the wheel teeth. This is the position of rest. If we wind up the mainspring of the watch it will immediately cause the main wheel to turn, and, of course, that will turn the next wheel, and so on to the escape wheel. When that wheel turns to the right, as it must, it will force back the arm of the pallet which swings on its arbor. In swinging out in this way it must also swing in the other pallet arm, and that movement will bring it directly in front of another wheel tooth, so that the wheel can turn no further. It is locked and will remain so until something withdraws it. When the pallet was swung so as to cause this locking, the fork was also moved, and as it enclosed the roller pin, that too was moved and carried with it the roller and the balance wheel, and in so doing it deflected the hair spring from its condition of rest. And as the spring tried to get back to its place of rest it carried back the balance also. In going back, the balance acquired a little momentum, and so could not stop when it reached its former position, but went a little further, and, of course, the roller and its pin also went along in company, the pin carrying the fork and the pallet swinging in the other direction, which unlocked the escape wheel tooth. Its inclined top gave the pallet a little “push” so that the first pallet was locked, forcing the fork and roller, and the balance and hair spring, to move in the opposite direction. And so the alternate actions proceed, and the balance wheel travels further each time, until it reaches the greatest amount which the force of the mainspring can give. But before this extreme is reached, the momentum of the revolving balance carries the roller pin entirely out of the fork. As the fork is allowed to move only just far enough to allow the pin to pass out, it simply waits until the fork returns and enters its place, only to escape again on the other side. And so the motions continue to the number of 18,000 times per hour. If that number can be exactly maintained, the watch will measure time perfectly. But if it should fall short of that exact number only once each hour, it would result in a loss of 4.8 seconds each day, or 2.4 minutes in one month. A watch as bad as that would not be allowed on a railroad. [Illustration: ACCURATE MEASUREMENTS ARE ESSENTIAL TO CORRECT TIME KEEPING]