stitutes a pendulum. During its entire swing it occupies three seconds. Now observe I can amplify the distance of the swing at pleasure, but the time in which the ball travels this distance is always the same. Suppose I make it swing through a distance of twelve feet, then (making allowance for the law that regulates the movements of a pendulum) four feet would roughly represent one second, and one inch would represent the one-forty-eighth of a second. Thus by amplifying the extent of the swing and subdividing we get a notion of small periods of time, such as the one-hundredth or the one -thousandth of a second. If I shorten the length of the string of the pendulum, the ball swings faster, but through a shorter distance in each swing. Carry this on until the ball, suppose it now to be very small, moves, say a hundred times backwards and forwards in a second, and you pass on in thought to the delicate instruments we shall now consider.
The first instrument, devised by Thomas Young, is the revolving cylinder. Suppose this cylinder revolved at a uniform rate by means of clockwork. Suppose the surface of the
