i8z TEE POPULAR SCIEXCE MONTHLY. 



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 a. p.. 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 bv water or bv 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 

 nearlv the length of everv other hour. The machine for telling the 

 hours was, for manv Tears, 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 bv the Catholic clergv. Very large horologes were 

 built into the towers at Canterbury Cathedral, in 1292 ; at "Westmin- 

 ster, 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 Abbev 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 swing-.'* 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 voung bov noticed that it 

 moved with great regularitv. If it had moved all the wav 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 u 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 regu- 

 late the movement of a clock. You may see how this is done by look- 

 ing at Fig. 1. We have here the simplest form of clock-work, or 

 14 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 n 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 pen- 

 dulum 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 



