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Some approve the following explanation: The earth contains, they assert, many hollow recesses and a great quantity of air. This air, under pressure of the gross darkness, of necessity freezes. Then remaining sluggish and unmoved it ceases to circulate and turns into water. Just as on earth a change in the density of the atmosphere produces rain, so beneath the earth the change of density starts a river or a stream. In the former case the air above our heads cannot long remain sluggish and heavy; for sometimes it is rarefied by the sun’s heat, sometimes expanded by the wind’s force. There are, therefore, long intervals between falls of rain. But underground the forces, whatever they are, that turn air into water, are constant—perpetual darkness, everlasting cold, inert density; they can, therefore, supply without a break the sources of fountain or flood. We Stoics are satisfied that the earth is interchangeable in its elements. So all this air that she has exhaled in her interior, since it is not taken up by the free atmosphere, condenses and is forthwith converted into moisture.