VIII
1
2
From what has been said it must be inferred that in nature there exists a principle of activity of enormous force. For there is nothing that does not become more active through tension; and it is no less true, nothing will be found capable of tension from another body unless it have in itself capacity of tension.41 In the same way we say that nothing could be moved by another body without possessing the quality of mobility in itself. But what element can be conceived more likely to possess tension in itself than air? Will any one deny that it can be subject to that force after seeing how it tosses about the earth with its mountains, houses, and walls and towers, and great cities with their inhabitants, seas, and whole coast-lines? The tension of air is proved, too, by its velocity and expansion. Illustrations of these properties are common: in an instant the eye extends its sight over many miles; a single voice resounds at the same moment through whole cities; light does not creep forth little by little, but is shed simultaneously over the whole world.