HISTORICAL DEVELOPMENT. Among the prede cessors of the clock, as time-measurers, are the sun-dial. the clepsydra or water-clock, and the hour-glass. (See DIAL: CLEPSYDRA; HOUR GLASS.) The clepsydra vas a graduated trans parent vase, in which water trickled through a bole in the bottom at such a rate that the reced ing water marked the passage of time. In the hour-glass sand was substituted for water. Among Eastern nations a great many curious mechanical devices were introduced into the con struction of the clepsydra: the water was made to llow in tears from the eyes of automata; a floating statue. falling with the liquid, pointed to the passing hours, as indicated on the side of the glass; finally, a mechanism was introduced by which the water, as it fell, drop by drop, turned a little wheel. which moved the bands on the fare of a dial, and so marked the hour. The next step was the construction of a time-indi cator, whose hands were moved by the action of falling weights instead of that of falling water. When this step was taken. and the first true clock constructed, is uncertain. Its invention is claimed by many peoples, from the Chinese, B.C. 2000, to the Germans of the eleventh century. Certain it is that clocks were in general use in churches and monasteries throughout the latter part of the Middle Ages, and that these ancient tower-clocks were the progenitors of all our mod ern timekeepers.
The oldest clock of which we have a complete description was set up in the tower of the palace of Charles V. of France, in 1379. by a German named Henry De Vick. This primitive clock was constructed on the mechanical principle which is the basis of all modern timekeepers. This principle, as formulated by E. A. Marsh, is "that the power stored up in a raised weight or coiled spring shall be communicated to a train of wheels which are set revolv ing, and that the force or motion shall be cut up into a succession of minute but equal impulses by convert ing a rotary into a vibratory motion. The last and quickest wheel of the train shall have its teeth so formed that they are caught and escape alternately. and hence the wheel is called the `seape - wheel,' a n d .
from its resemblance to a crown. the 'crown wheel.' The bar and staff, which. with its projections, catch and release the teeth, is termed the 'escapement?. and it is through this device that the rotary is converted into the backward and forward motion." The accompanying sketch of De Viek's clock is use ful not only from its historical interest. but also because, from its comparative simplicity. it will form a groundwork for further explanation of the mechanism of clocks in their more compli cated form. It will be readily understood, from a
glance at the annexed figure (Fig. 1 ) , that as the weight A tends to uncoil the cord and set in motion the cylinder B round its axis, the mo tion will he successively conununieated to the various toothed wheels in the figure, and finally to the crown-wheel or wheel, I; the teeth of which so act on the two small ers or pallets, b projecting f r o m and forming part of the suspended upright spindle or vertical axis, KM, on which is fixed the regulator or balance. Lb, that an alternating or vibrator- instead of a circular tion of the balance itself is the result. The hands of the clock are attached the wheel N, also set in motion IT the cylinder B. Now, unless there were some cheek upon the motion.
it is manifest that the heavy weight A would go rapidly to the ground, causing the wheels to rotate, the beance to vibrate, and the hands to go round with increasing velocity. In order to prevent this rapid unwind'ng of the clockwork, and adjust it to the more deliberate measurement of time, the balance is, in De Vick's clock, loaded with two weights, en, vn : and the farther these are removed from the axis or spindle, E.M.
the more heavily they will resist and coun teract the escapement of the levers, and the rapidity of the rota tion of the escape ment-wheel, till the clock he brought to go neither too quick nor too slow.
Upon this simple plan it is probable that all clocks were eonstructed until the seventeenth century, when the principle of the pendulum was plied to the science of horology. The erty of a pendulums known as its isochronism (q.v.) constitutes its value to elock-mechanism—that. when a suspended body is swinging. any increase or decrease in its speed will not change the num ber of vibrations it makes in a given them. but only the length of the are it describes. This law of the pendulum was discovered by Galileo, and was first applied (probably) to clockwork by Huygens, about 16.17. The two accompanying cuts show how the horizontal swing of the balance, as maintained in De Vick's clock, was converted the vertical swing of the pendulum. By taking oil one of the weights and hanging the balance in an upright position, it becomes a pendulum. Ten years later Dr. invented an escapement which enabled a weaker support to carry a heavier pendulum. Subsequent improvements in the (-sea llement and pendulum ( see ESCAPEMENT : PENDULUM), and in the use of the spring (see WArcu) in place of the pendulum, have brought the mechanism of timekeepers down to the present degree of perfec tion.