this. If you swing a pendulum truly backwards and forwards, without any wobble, it will go on swinging in the same way even though the Earth rotates underneath it. Here is a little pendulum mounted on a model earth: I set it swinging and turn the model earth round—the pendulum still goes on swinging in the same way: it takes no notice of the rotating Earth underneath, though I have turned the model a quarter turn (so that if the pendulum went with it, the swing would be at right angles to its former direction). If, however, there were people living on this model earth, they would not be conscious that I had turned it round: to them the pendulum would seem to change: to them it would seem to be swinging now at right angles to its former direction. And so our long pendulum will not really change, but will seem to us to be changing. But to turn our actual Earth a quarter round takes six hours: we should have to go home to tea long before that: we must be satisfied to watch the real pendulum for a few minutes only. Nevertheless we can see its apparent change if we magnify it in the ingenious way devised by Sir James Dewar.
Here is the pendulum tied back. Presently I am going to swing it by burning this string which ties it up. When the string breaks in burning, the pendulum will swing backwards and forwards. At one end of the swing it comes very close to this lamp which is throwing the shadow of the pendulum-wire on the screen, and you will then see the wire and the motion much magnified; so that we can easily tell whether the swing changes even a little bit. But it is not the pendulum that is changing, it is the Earth
