diffused atmosphere occupies the hidden spaces underground. If that is true, of necessity the earth must often be moved, since it is full of a most movable substance. No one, I suppose, can doubt that there is nothing so restless, so capricious, so fond of disturbance as air. XVII It follows, therefore, that air should obey the law of its being; 1 what is wont to be moved will sometimes move other things. And when? Whenever its free course is checked. As long as it is not hindered it flows quietly along. When it is opposed and held back it becomes furious, bursting all obstacles just like that Araxes that ever spurned a bridge. As long as the river has a free easy channel it rolls down its waters 2 in due and regular succession. But if through chance or by human agency rocks are placed in its way to check its course, then it gathers fresh strength from the barrier, and the more numerous the obstacles opposed to it, the greater the force that it musters to overcome them. For all the water that accumulates behind, constantly increases, and being at last unable to bear its own weight manifests its violence through the havoc it works in its descent, and escapes headlong down its channel, bearing the very obstacles that blocked its path. The same thing occurs with air, only that, in proportion to its greater strength 3 and mobility, it is the more rapidly carried onward, and bursts the more violently all that encloses it. From this, of course, there is a disturbance in the part of the ground under which the struggle has occurred. The truth of this assertion may be proved from the consideration that often when an earthquake has taken place, involving a breach of only some part of the earth, wind has issued from it for several days. This is recorded to have taken place in the earthquake 4 in which Chalcis suffered, as you will find in Asclepiodotus, Posidonius’ pupil, in his discussion of my own topic of Physical Inquiries. In other authors, too, you will find it stated that after a chasm had opened up at one spot, in no long time wind issued from it, having no doubt made for itself the way along which it travelled. XVIII The chief cause of earthquake, therefore, is air, an element 1 naturally swift and shifting from place to place. As long as it is not stirred, but lurks in a vacant space, it reposes innocently, giving no trouble to objects round it. But when any cause coming upon it from without rouses it, or compresses it, and drives it into a narrow space, in the first instance, to be sure, it merely retires and roams about its enclosure. But when opportunity of escape is cut off, and resistance meets it on all hands, then ... With deep murmur of the mountain It roars around the barriers;...