Page:Popular Science Monthly Volume 29.djvu/194

182 in which the water drove a wheel that marked the hours by a hand. The old Romans used this water-clock; but, when their empire was destroyed, all Western Europe forgot the existence of such a thing. In the year 807, the Caliph of Bagdad, Haroun-al-Raschid, sent to Charlemagne a water-clock of this kind. Soon after we learn that, instead of the running water, a weight was used for turning the wheel. But whether the clock was run by water or by a weight it was always a hard matter to have the hours of the same length. The escapement, which we shall speak of presently, made one hour more nearly the length of every other hour. The machine for telling the hours was, for many years, called the horologe, or "hour-teller." The word "clock" was applied only to the bell that struck the hours. It sounds very much like the Saxon, French, and German words that mean "bell." About nine hundred Years ago horologes were brought into England by the Catholic clergy. Very large horologes were built into the towers at Canterbury Cathedral, in 1292; at Westminster, in 1290; at Exeter Cathedral, in 1317—the striking part of which is still in use; at the cathedrals of Wells and Peterborough; and at St. Albans Abbey in 1326. A smaller horologe was made for Charles V of France in 1370, by a German named Vick.

Horologes, or clocks, would have remained in this imperfect state until to-day if it had not been for the invention of the pendulum, which means "something that swings." You all remember the story of Galileo, who, when a boy, watched the chandelier as it swung to and fro in the cathedral at Florence. The young boy noticed that it moved with great regularity. If it had moved all the way around the point where it was held, or suspended, it would have made a circle; but as it moved only a small part of the way, it moved in what is called the "arc" of a circle. Galileo saw that it took just as long a time to go from one end of the arc to the other as it did to return. This is called isochronism, or "equal times." In 1620, several years after Galileo's discovery, Huygens first used the pendulum to regulate the movement of a clock. You may see how this is done by looking at Fig. 1. We have here the simplest form of clock-work, or "movement," as it is called. A wheel, with teeth on the edge, turns on a pin, i, by the force of the weight h, the string being wound about what is called a "barrel" at i. If there is no way of stopping the wheel, it will run down very fast and very unevenly. Here is just where the pendulum becomes useful. The pendulum is a long wire, a c, the part c being enlarged into what is called a "bob." The pendulum swings on the point a. It has an arm, d g, fastened to it and swinging with it. The points of this arm are called the "pallets." When the pendulum is in the position marked by the black line you will see that the wheel is stopped by the pallet d. But, when the pendulum swings to the place marked by the dotted line, the pallet d moves out to e. This lets the wheel move a little; but, before it