I shall be ready to allow that air is the cause of this form of 1 destructive earthquake. But I shall have some criticism to offer as to the method by which it enters the ground. Does it enter by fine openings that the eye cannot detect, or by larger and more evident ones? Does it come from the depths of the earth, or does it pass through the surface too? The last-mentioned view seems inconceivable. In our bodies the skin keeps out air, which finds no entrance except that through which it is inhaled. And even when taken in by us, it cannot settle except in the looser portion of the body. It does not remain among the sinews or muscle, but in the bowels and the open 2 vessels of our internal organs. The same arrangement may be suspected in regard to the earth’s interior from the very fact that the movement in an earthquake is not on the surface of the earth or about the surface, but beneath in the lowest parts. A proof of this is that seas of immense depth are tossed up, no doubt from the movement of the ground over which they spread. It is therefore probable that the earth 3 is moved in its depths, and that the air is formed there in the immense caverns. Nay, says some critic, but just as when we shiver from cold a trembling follows, so, too, the earth is shaken by air affecting it from without. This I deny can by any possibility occur. Why, the earth must get a chill in order to have the same happen to it as to us, whom an external affection drives into a shuddering fit. I should quite allow that the earth shows symptoms of much the same kind as 4 we do, but the cause is wholly different. An injury of a deeper kind, more toward its centre, must affect it, the very strongest proof of which may be found in the fact that when through violent earthquake the soil is laid open in wide destruction, the chasm sometimes takes in and buries whole cities. Thucydides tells us that, about the time of the 5 Peloponnesian War, the island of Atalanta, either wholly, or, at any rate, for the most part, was swallowed up. You may take Posidonius for witness that the same thing happened to Sidon. But we do not require evidence of this. Within our own memory the earth has been torn by internal movement, adjoining places have been rent asunder, whole plains have disappeared. I will now explain how I suppose this sort of thing to occur. XXV