The fact that maps are good guides to action, and are made on the basis of its roundness, should be sufficient. It would make all the difference to world politics if the earth were flat. It would mean that there were new regions to be found, and that ambitious statesmen could annex them instead of invading their neighbours. Children learn about the firmament in Bible classes, and unlearn it in Science classes. Centrifugal force would produce a fearful strain in the firmament, but perhaps if there were a firmament it could stand it. The most dramatic of all the proofs of the earth's rotation was the experiment which Foucault made a hundred years ago with a very long pendulum suspended from the dome of the Pantheon in Paris. If Blackett is right in his recent guess that all spinning bodies are magnets, the earth's magnetism is a further bit of evidence that it is spinning.