WEIGHTS AND MEASURES 33 



that the lamps suspended by long chains from the ceil- 

 ing swung to and fro. The question entered his head 

 as to whether or not the period, i.e. time of one com- 

 plete swing, depended upon the amplitude of the 

 swing, that is, upon how far the swinging lamp departed 

 from the vertical. 



The problem then was to find out if the period 

 changed as the motion damped down. The lengths 

 of these pendula were comparatively long, so that 

 their periods were matters of several seconds. Galileo 

 timed the swings by counting his pulse. In the interval 

 between pulse beats he had a convenient unit of time 

 for his observations. Although the pulse rate varies 

 from person to person and from time to time in the 

 same person, for a few minutes, to determine which 

 of two moving bodies is traveling its path in the shorter 

 time, it might give a fair indication. 



Up to this time pendulum clocks were unknown, and 

 Galileo's discovery that the period was independent 

 of the amplitude was the basis of Huyghens' later use 

 of a pendulum to control the escapement and hence 

 the rate of turning of the wheels of a clock. In a clock 

 the springs or weights turn the wheels and the function 

 of the pendulum is to allow only a definite part of a 

 rotation for each swing, the amount depending upon 

 the relation of the number of cogs on the wheels. The 

 tendency of the motion of the pendulum to die out is 

 cared for by arranging the mechanism so that as it 

 passes through the middle point in its swing it receives 

 a little kick which keeps it going and overcomes the 

 friction of the bearing and of the air. 



The earliest clocks were somewhat similar in prin- 



