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There are some who think that, while air and no other cause produces earthquake, it operates in a different way from that which Aristotle supposed. Listen to what they say: Our body is irrigated with blood, and with air which courses everywhere along its own routes. We have some comparatively narrow vessels through which they cannot do more than pass; some wider, in which they accumulate, and from which they are distributed to the members. So this whole body of the earth at large has passages alike for water, which performs the function of blood, and for wind, which might be called simply the breath of its life. These two encounter each other at some points, at some points they are stationary. While in our bodies good health is enjoyed, the movement of the veins preserves its rate undisturbed; but when there is malady the pulse beats more rapidly, the deep breathing and panting betoken laboured, wearied effort. In like manner the earth remains unshaken while it maintains its natural position. But if any flaw occur in it, there is a shaking, just as of a body suffering from disease; for the air which flowed through it with regularity is violently smitten, and causes its veins to quiver; but not, let me add, in the way, described a little above,83 imagined by those who will have it that the earth is a living creature. In that case the earth, just as an animal does, would feel the agitation equally all over. When a fever seizes any of us, it does not delay for a time its attack upon some parts, but with uniform regularity spreads over them all.
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Perhaps you had better assume, therefore, that air from the surrounding atmosphere enters the earth. As long as it has free egress, it glides through it without doing harm; but if it meet some obstacle to block its way, then it is, to begin with, weighted with the atmosphere that pours in on the rear; by and by it escapes with difficulty through some chink, and makes its way with the greater violence the narrower the opening is. That cannot take place without a struggle, and a struggle involves shaking of the earth. But if the confined air cannot find even a chink by which to issue, it is massed and becomes furious, and is driven round in this direction and in that, overthrowing or bursting one thing after another. It is excessively subtle, and at the same time exceedingly powerful; it can worm its way into obstructions however great, splitting and scattering whatever it enters. When this occurs, then there is a regular tossing of the earth. For the earth either opens to give room to the wind, or, after giving room, is deprived of its foundation and subsides into the very cavern from which it allowed the wind to issue.