Perhaps the most primitive of all time-measuring devices was a stick or pole planted upright in a sunny place. The position of the shadow which it cast marked time. The sun-dial was a development of this simple device. It consisted essentially of two parts: a flat plate of metal marked off much like the dial of a modern clock or watch, and an upright piece, usually also of metal, fastened to the center of the dial. To make the direction of the shadow uniform for any given hour throughout the year, the upright piece was made parallel to the axis of the earth. As the earth rotated on its axis the shadow cast by the upright piece moved from point to point on the dial, measuring the flight of time. The sun-dial was in use among the earliest nations. Herodotus is authority for the statement that the Greeks borrowed it from the Babylonians. The sun-dial was obviously of no use on cloudy days or dark nights, and even in sunny weather it could not accurately or delicately indicate the passage of time. However, it continued in use so long that to the end of the seventeenth century the art of dialling was considered a necessary element of every course in mathematics. Another ancient invention for measuring time was the water-clock. Water was permitted to drop from a small orifice in a containing vessel. The period required for emptying the vessel marked a unit of time. Its principle was the same as the common hour-glass, according to which time is measured by the slow dropping of sand from one receptacle into another. The water-clock was used by the ancient Chaldeans and the Hindoos, and also by the Greeks and Romans. Demosthenes mentions its use in the courts of justice at Athens. In order to mark the hours of the day, the Saxon King Alfred the Great is said to have made wax candles twelve inches in length, each marked at equal distances. The burning of six of these candles in succession consumed, roughly, just twenty-four hours. To prevent the wind from extinguishing them they were inclosed in cases of thin, white, transparent horn. The candles thus inclosed were the ancestors of the modern lantern. Our word _clock_ comes from the Anglo-Saxon verb _clocean_ meaning "to strike," "to give out a sound." It is impossible to ascertain by whom clocks were invented, or when or where. It is fairly clear, however, that a Benedictine Monk named Gerbert, who afterward became Pope Sylvester II, made a clock for the German city of Magdeburg a little before the year 1000 A.D. Clocks may have been made before this, but if so it would be hard to establish the fact. In Gerbert's clock weights were the motive power for the mechanism. Weight clocks were used in the monasteries of Europe in the eleventh century, but it is probable that these early clocks struck a bell at certain intervals as a call to prayer, and did not have dials for showing the time of day. [Illustration: A "GRANDFATHER'S CLOCK," BELONGING TO WILLIAM PENN]