III
The lie of the ground makes water either stand or run; on a slope it flows down, a plain keeps it in, causing it to stagnate. Sometimes under pressure of air it is forced uphill; it is then driven, it does not flow. Surface water comes from rain; spring water from a natural fountain. There is, however, nothing to prevent surface and spring water in the same spot. This we see in Lake Fucinus, into which the streams drain all the rainfall of the surrounding mountains, while there are also large springs concealed under the surface of the lake itself. So, even when the torrents discharge into it in winter, it preserves its appearance unaltered.