Thus the essential parts of a clock--an escapement, which is a device to control the power in a clock or watch so that it shall act intermittently on the time index, a motive power, which was then water or a weight, a dial to display the hours, and an index to point them out--were invented at this early age. But the art advanced practically no further for many centuries. The hour-glass is too familiar to need description. The incense sticks of the Chinese, the combustion of which proceeded so slowly and regularly as to render them available for time measures, were the precursors of the graduated candles. With the ungraduated sun-dial the Greeks fixed their times for bathing and eating. When the shadow was six feet long it was time to bathe, when twice that length it was time to sup. The clepsydra became in Greece a useful instrument to enforce the law in restricting loquacious orators and lawyers to reasonable limits in their addresses. And in Rome the sun-dials, the clepsydras and the hour-glass were used for the same purpose, and more generally than in Greece, to regulate the hours of business and pleasure. The graduated candles are chiefly notable as to their use, if not invention, by Alfred the Great in about 883. They were 12 inches long, divided into 12 parts, of which three would burn in one hour. In use they were shielded from the wind by thin pieces of horn, and thus the “horn lantern” originated. With them he divided the day into three equal parts, one for religion, one for public affairs, and one for rest and recreation. Useful clocks of wondrous make were described in the annals of the middle ages, especially in Germany, made by monks and others for Kings, monasteries and churches. The old Saxon and Teutonic words _cligga_, and _glocke_, signifying the striking of a bell, and from which the name clock is derived, indicates the early combination of striking and time-keeping mechanism. The records are scant as to the particulars of inventions in horology during the middle ages and down to the sixteenth century, but we know that weights, and trains of wheels and springs, and some say pendulums, were used in clockwork, and that the tones of hourly bells floated forth from the dim religious light of old cathedrals. They all appear to have involved in different forms the principle of the old clepsydra, using either weights or water as the motive power to drive a set of wheels and to move a pointer over the face of a dial.