184 



THE POPULAR SCIENCE MONTHLY. 



equator it should be about thirty-nine inches long, to " vibrate," or go 

 from c to b in one second. At the latitude of Washington, where the 

 force of gravity is greater, the length is thirty-nine and one tenth inches. 

 At London, which is still farther north, the length is thirty-nine and 

 one seventh inches. A pendulum of the right length in London would 

 lose two and one quarter minutes a day at the equator. The pendulum 

 that vibrates from e to b in two seconds must be four times the length 

 of a one-second pendulum. The pendulum of the great clock at West- 

 minster moves once in two seconds. It is nearly fifteen feet long, and 

 it weighs seven hundred pounds the heaviest in the world. The 

 heavier and longer the pendulum, the more regularly will the clock 

 move. But pendulums may be too long and too heavy. Almost all of 

 the clocks that were made before the year 1800 had pendulums about 



thirty-nine inches long, and 

 they stood with their cases 

 over five feet high usually 

 in the corner of the room. 

 They were so clumsy that 

 only the machinery was ped- 

 dled about from place to 

 place the nearest cabinet- 

 maker being called upon to 

 make the case. By-and-by 

 it was found that, if, in Fig. 

 1, the pendulum would go 

 from c to b in one second, it 

 would go from c to b, back 

 again to c or twice as fast 

 if it were one quarter as 

 long. After that, clocks were 

 made short enough to stand 

 on a shelf. 



It had also been found 

 that the bob of the pendu- 

 lum, when moving in the arc 

 of a circle, was not reliable ; 

 but that all the trouble was 

 avoided if it moved in the 

 arc of a cycloid (or " like a 

 / circle"). This arrangement 

 is shown in Fig. 2. The pen- 

 dulum hano-s from a fixed 

 point, a, where it is fastened 

 securely. The upper end of 

 the wire is beaten into a very thin spring. When the bob b moves back 

 and forth, it does not move in the arc of the circle c d, but on the dotted 



Fig. 2. 



