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Rh You see what an important crisis it was when Newton suggested his experiment for proving that the Earth rotates: and in honour of him we will now make such an experiment, though not the one he suggested. I might perhaps have shown you his very experiment, or one founded upon it: for during the last few years the rotation of the Earth has been proved and measured in this way, using, however, the apparatus known as Atwood's machine, to drop the weight. Here is a fine Atwood's machine, which has often been used in this Institution: you see two weights, one heavier than the other, connected by a string over a pulley. The heavier weight falls, pulling the other up; but the fall is comparatively slow, and can be made as slow as we please by making the weights nearly equal. Now this slow fall gives the sideways "flick" due to the Earth's rotation plenty of time to develop. When the experiment is made just as Newton described it, the fall is so rapid that the "flick" has scarcely time to work: with Atwood's machine, it gets a much better opportunity. And where do you think this ingenious development of Newton's idea has been put into practice and the rotation of the Earth demonstrated and measured? Why! at the observatory of the Vatican, where 300 years ago things were made so unpleasant for Galileo merely because he asserted this rotation. Truly Time brings changes!

But this beautiful method of Father Hagen is perhaps not the easiest way for us to see the rotation of the Earth within a few minutes. We had better use the method of an ingenious Frenchman called Foucault, who used for the purpose a long pendulum like