CLOCK, an instrument for measuring and indicating the time of day. From the earliest periods of human history man has sought to measure time. To pastoral or agricultural nations where the duties of each day were monotonous and bounded by the four great divisions of sunrise, midday, sunset, and midnight, extreme accuracy was not important. The first measure of time was the sun-dial, but this being of no service at night or on cloudy days, the hour-glass was invented, next the clepsydra, subsequently im proved by the addition of a toothed wheel and index or sort of dial driven by thc water which flowed from the bottom of the jar. These have been in use 2,000 years. The next improvement was the substitution of a weight for the water to turn the wheel. This has been attributed to Archimedes. Some contrivance was necessary to regulate the weight so as to make the index pass over equal spaces in equal times. This must be accom plished by a pendulum or escapement of some kind, and a rude escapement is at tributed to Gerbert, about A. D. 1000. A better one was that of De Vick in 1379.
Accuracy in marking time was not at tained, however, by this, though it was a great improvement. For 270 years there was no advance, but between 1641 and 1658 the idea of attaching the pallets of the escapement to the pendulum-rod and making the escapement horizontal oc curred both to Harris, an English clock maker, and Huyghens, a Dutch philoso pher. The anchor escapement of Dr. Hooke, invented in 1666-1680, and the dead-beat escapement of Graham in 1700, gave a new impulse to clockmaking. There has been no material change in the principles on which clocks are made, except in the substitution of steel springs for weights and in the finer movements, and in the addition of the hairspring to regulate still further the action of the escapement or pendulum, since 1700.
There have been a great variety of es capements invented and much more at tention paid to accuracy in the details and perfection of finish, but the principles are the same.
Considered as scientific instruments for the precise measurement of time, they may be divided into two classes accord ing to the character of the compensation of their pendulums, whether of the grid iron type or the mercurial pendulum. The first keeps a constant length of the pen dulum-rod by the difference of expansion of different metals with change of tem perature, and the other makes up for the lengthening of the rod with rise of tem perature by the greater expansion of a jar of mercury carried on the bed-plate of the pendulum, the rise in the center of gravity of this counterbalancing the lengthening of the sustaining rod. Clocks differ in another important particular, that of the escapement, whose function it is to be unlocked at each oscillation of the pendulum and thus allow the train of wheels to move forward a step, and also to transmit an impulse to the pen dulum just sufficient to counterbalance the friction caused by the unlocking of the escapement. In fine astronomical clocks either the Graham dead-beat or some form of gravity escapement is the one most generally used. In any of them the object to be attained is to make the work of unlocking and the impulse given to the pendulum to make up for it as nearly absolutely constant as possible. If this is not done the arc of vibration of the pendulum will vary, and with it the steady rate of the clock.
The manufacture of clocks in America began about 1800 in Connecticut, which is still the center of the industry in the United States.