At this point, if you have no objection, one may raise the question why a whirlwind occurs. In rivers, when their course has been without any obstacle for a long distance, the channel is a straight, uniform one. But when they meet some boulder that juts from the bank, the 2 stream is driven back and whirls the waters in a circle without a way of escape, so that in their revolution they are constantly sucked in toward the centre to form a whirlpool. In like manner the wind pours out in full force as long as no obstacle stands in the way. But when it is reflected from some jutting projection, or is massed in a quarter which combines to form a thin downward channel, then it revolves upon its own axis, and produces an eddy similar to that in which, as we have just said, the water revolves. This revolving wind, which 3 always traverses the same spot and is roused to fury by the mere giddy whirling, is a whirlwind. If it is a very fierce one, and revolves longer than ordinary, it ignites and causes what the Greeks call a fire-wind (πρηστήρ), which is just a fiery whirlwind. The bursting of such winds from the clouds produces almost all the disasters by which herds are carried off and ships lifted, bodily, right out of the water. Further, some winds produce different ones by dispersing the air and driving it before them in other directions than that toward which they themselves have bent their course. It occurs to me at the moment to mention a parallel to wind that 4 may be drawn from drops of moisture. The single drops may begin to incline downwards and be on the verge of giving way, but yet do not manage to fall. When, however, several have united and the mass has imparted strength, then they are said to flow and to move. So, as long as there are slight movements of the atmosphere disturbed at several points, they do not produce wind. The latter begins only when all those movements are united and concentrated in a single effort. Air differs from wind in degree alone. A more violent air is a wind; air in turn is gently flowing atmosphere. XIV Let me now recall a remark that I had made early in this book, 1 namely, that wind issues from cave or inner recess of earth. The whole earth is not of solid compact constitution down to its lowest foundations, but at many points is hollow, ... hung over dark retreats.