XV
Some entertain the following opinion: The earth is porous at many points, possessing not merely those first shafts which it received as ventilators at its creation, but many subsequently opened up by various changes. In some places water has washed away the soil that was on the surface; part has been eaten away by torrents, while parts have been exposed by the disruptive action of great tides. Through the interstices thus produced air enters. If it so happen now that the sea has shut it in and driven it deeper, and the waves prevent its escape by the same road, egress and regress being alike closed, the air rolls about within the earth. Its natural tendency is to hurry straight forward, but as that path is closed, it presses upward and lashes the earth, whose weight lies heavy upon it.