Concerning the First Principle of Cold (11-12)

And therefore water drawn from a fountain soonest congeals; for the more of cold in the air overcomes the less of cold in the water. Thus if a man takes cold water out of a well and puts it into a vessel, and then lets the vessel down again into the well, so that it may not touch the water but hang for some time in the air, the water will be much colder. Whence it is apparent, that the coldness of the water is not the first cause of coldness, but the coldness of the air. For you do not find that any of your great rivers are ever thoroughly frozen, by reason of their depth. For the air doth not pierce through the whole; only so much as it can seize and embrace with its cold quality generally freezes, and no more. Therefore the barbarians never cross over frozen rivers till they have sent a fox before to try the depth of the ice. For if the ice be not very thick, but only superficial, the fox, perceiving it by the noise of the water floating underneath, returns. And some there are that melt the ice with hot water to make way for their lines, when they go to catch fish in winter. So that nothing suffers from cold in the depth of the water. Nevertheless, so great has been the alteration of the upper parts of the water by congelation, that several vessels riding in the stream have been bruised and broken by the forcible compressure and griping of the congelation; as we have heard from them who lately had their winter quarters with Caesar upon the Danube. And indeed, what happens to ourselves is sufficient to demonstrate the truth of this. For after hot bathings and sweatings, we are most sensible of cold, at what time, our bodies being open and the skin relaxed, we give a freer entrance to the cold together with the ambient air. And after the very same manner the water itself suffers. For