38.

In the mountainous country is a river, the Silas, on the surface of which nothing will float. Democritus, who had travelled over a large part of Asia, disbelieves this, and Aristotle does not credit it, although atmospheres exist so rare that no bird can sustain its flight in them. Vapours also, which ascend (from some substances), attract and absorb, as it were, whatever is flying over them; as amber attracts straw, and the magnet iron, and perhaps there may be in water a similar power.

As these matters belong to physics and to the question of floating bodies, these must be referred to them. At present we must proceed to what follows, and to the subjects more nearly relating to geography.