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3Hence, those who think they have found springs at such a height as the tops of hills, are mistaken when they dig their wells. For as a brazen vase, not filled to the brim, but about two-thirds full of water, with a cover thereon, when subjected to the great heat of a fire communicates that heat to the water, this, from its natural porosity, receiving the heat and swelling out, not only fills the vase, but, raising the cover by the force of the steam, increases and boils over. If the cover be taken away, the steam passes off to the open air, and the water subsides. In the same manner, when springs are forced through narrow channels, the pressure of the air drives the bubbles of the water to the top; but as soon as they come into wide open channels, the pores of the liquid having vent, it subsides and returns to its natural level.
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