‹‹‹ Vitr. 8.3.2 | Table of Contents | Vitr. 8.3.4 ›››
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.
3Consequently those who think that they have excavated sources of springs at the height of such hills find themselves mistaken when they open up their excavations. Suppose a bronze vase filled not to the very lips, but containing two thirds of the quantity of water which forms its capacity, and with a cover placed upon it. When it is subjected to a very hot fire, the water must become thoroughly heated, and from the rarity of its nature it greatly expands by taking in the heat, so that it not only fills the vase but raises its cover by means of the currents of air in it, and swells and runs over. But if you take the cover off, the expanding forces are released into the open air, and the water settles down again to its proper level. So it is with the sources of springs. As long as they are confined in narrow channels, the currents of air in the water rush up in bubbles to the top, but as soon as they are given a wider outlet, they lose their air on account of the rarity peculiar to water, and so settle down and resume their proper level.