THE PENDULUM. 39 



velocity, virithout changing its direction ; and the force of 

 gravity, which finally brings it to the ground. The curve line 

 which the ball describes is called in geometry aparab'ola. 



A pendulum consists of a line, or rod, to one end of which 

 a weight is attached, and it is suspended by the other to a 

 fixed point, about which it is made to vibrate. Without 

 being put in motion, a pendulum, like a plumb line, hangs 

 perpendicularly to the general surface of the earth, by which 

 it is attracted ; but if you raise a pendulum, gravity will 

 bring it back to its perpendicular position. It will, how- 

 ever, not remain stationary there, for the velocity it has re- 

 ceived during its descent will impel it onwards, and it will 

 rise on the opposite side to an equal height ; from thence it 

 is brought back by its gravity, and again driven by the im- 

 pulse of its velocity. Were it possible to remove the ob- 

 stacles occasioned by the resistance of the air, and by the 

 friction of the part by which it is suspended, the motion of 

 a pendulum would be perpetual, and its vibrations perfectly 

 regular ; being of equal distances, and performed in equal 

 times. The metallic rods of pendulums are expanded by 

 heat and contracted by cold ; clocks therefore will go faster 

 in winter, and slower in summer, for the longer a pendulum 

 is, the slower are its vibrations. The common remedy for 

 this inconvenience is raising or lowering the weight of the 

 pendulum, by means of a screw, as occasion may require. 

 Pendulums vibrate faster towards the poles, and slowest at 

 the equator. This is accounted for by the earth's diameter 

 being greater through the equator than through the poles. 

 All bodies on the earth's surface are drawn to its centre by 

 the force of gravity ; and more powerfully as the square of 

 their distance is less. Hence, if one portion of the earth's 

 surface be farther from its centre than another, the force of 

 gravity on a pendulum in one place must be less than in 

 another ; and consequently the pendulum will vibrate slower 

 or faster according to its situation. And this is found to be 

 actually the case. 



It was from observing the difference in the vibrations of 

 pendulums of the same length, that the difference of gravity 

 was discovered, and the true figure of the earth ascer- 

 tained. Pendulums vibrating seconds, at London, are 

 thirty-nine inches and two-tenths in length ; but at the equa- 

 tor about thirty-nine inches and one-tenth. Pendulums 



