A VOYAGE IN SPACE
were lit. There was a lamp hung like a long pendulum from the roof: and the verger pulled it toward him to light it and then let it swing back. Galileo thought that as the swinging died down it would become either slower or quicker, and to test this he timed the swings with his pulse: to his amazement they were all made in the same time. He naturally tried other pendulums for himself, and found that they all had this property of keeping time though the swing may alter in size it does not alter in time. One way of realizing this property of the pendulum
is to take something else which swings to and fro, rather like a pendulum, and yet does not keep time, such as this pair of inclined planes AC and CD (Fig. 7), in which a groove is cut so that a ball B can run down AC and up the other side CD, the join at C being rounded so as to avoid having a bump which would stop the roll. The ball swings back and forward rather like a pendulum, as you see, but it does not keep time. The short swings at the end are made much more quickly than the long ones at the beginning. But when we swing a pendulum and let its swings die down there is no such quickening, and hence the usefulness of a pendulum in regulating clocks, and the usefulness of a clock is to tell people like lecturers to stop talking after a reasonable time, before you are thoroughly sick of them and their gravity and levity both.