stretches out their branches, or sets them on high, but the tension and unity of air? [38] Or, except in a body of uniform texture. [39] Nisard translates, What imparts movement, in man, to the vital principle? VII Some writers believe that the air is rent and separated into small 1 parts with void spaces, as they suppose, between. They consider the easy flight of birds through it a proof that it has not a compact body, but has large empty spaces: fowls, great and small, pass through it without difficulty. But this is a mistake. For water also affords the same easy motion, and there is no doubt of its unity. When it 2 receives bodies, it always retreats in the direction opposite to them. This the Stoics call displacement, in Greek it is peristasis,[40] which takes place in air just as it does in water. For it literally stands round every body by which it is pressed. There is no need to assume an admixture of vacuum with the element. But more of this another time. [40] περίστασις = a standing around. The Latin equivalent in the text is _circumstantia_, rendered “displacement.” VIII From what has been said it must be inferred that in nature there 1 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 2 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. [41] The reading at several points is so uncertain that one cannot be at all sure of the meaning. Probably the whole passage is very corrupt. So far as the main theme is concerned, the argument seems to be, As mobility is a presupposition of motion, so tensibility is a necessary condition of actual tension produced in a body by another body. One is tempted to employ “elasticity,” but the term contains implications with which the author was apparently unfamiliar. IX