2Water collected from showers possesses wholesome qualities, because it consists of the lightest and most subtle particles of all springs, which, cleansed by the action of the air, and loosened by the tempests, descend upon the earth: and the reason why showers do not fall so often upon plains as they do on mountains or their vicinity is, because the vapours ascending from the earth at sunrise, to whatever part of the heavens they incline, drive the air before them, and, being in motion, receive an impetus from the air which rushes after them.
2The air rushing on, and driving in every direction the vapour before it, creates gales, and blasts, and eddies of wind. Hence the winds, wherever they travel, extract from springs, rivers, marshes, and from the sea, when heated by the sun, condensed vapours, which rise and form clouds. These, borne up by the winds when they come against the sides of mountains, from the shock they sustain, as well as from storms, swell, and, becoming heavy, break and disperse themselves on the earth.
3The vapours, clouds, and exhalations which rise from the earth, seem to depend on its retention of intense heat, great winds, cold moisture, and its large proportion of water. Thus when, from the coolness of the night, assisted by the darkness, winds arise, and clouds are formed from damp places, the sun, at its rising, striking on the earth with great power, and thereby heating the air, raises the vapours and the dew at the same time.
4A corroboration of this may be seen in a hot bath; for it is absurd to suppose that there can be a spring above its ceiling; and yet that, when warmed by the heated air from the furnace, attracts the moisture from the pavement, whence it is carried up to the vaulting of the ceiling, where it hangs. For hot vapours always ascend, and at first, from their lightness, do not fall down, but as soon as condensed, their gravity prevents buoyancy, and they drop on the heads of the bathers. In the same manner the atmospheric air, when warmed by the sun, raises the moisture from all places, and gathers it to the clouds: for the earth acted upon by heat, drives out its moisture, as heat drives out perspiration from the human body.
5This is manifest from the winds, among which, those that blow from the coldest quarters, as the north, and the north-east, bring dry and pure air, but the south and other winds, which blow from the direction of the sun’s course, are very damp, and always bring showers with them, because they reach us heated by the torrid regions, and imbibing vapours from the countries they pass over, transport them to the northern quarters.
6That this is the case, is evident from an inspection of the sources of rivers, as marked in geographical charts; as also from the descriptions of them, wherein we find that the largest, and greatest number are from the north. First, in India, the Ganges and Indus spring from Mount Caucasus: in Syria, the Tigris and Euphrates: in Asia, and especially in Pontus, the Borysthenes, Hypanis and Tanaïs: in Colchis, the Phasis: in France, the Rhône: in Belgium, the Rhine: southward of the Alps, the Timavus and Po: in Italy, the Tiber: in Maurusia, which we call Mauritania, the river Dyris, from Mount Atlas, which, rising in a northern region, proceeds westward to the lake Heptabolus, where, changing its name, it is called the Niger, and thence from the lake Heptabolus, flowing under barren mountains, it passes in a southern direction, and falls into the marsh Coloe, which encircles Meroe, a kingdom of the southern Ethiopians. From this marsh turning round near the rivers Astasoba, Astabora, and many others, it passes through mountains to the Cataract, and falling down towards the north it passes between Elephantis and Syene and the Thebaic Fields in Egypt, where it receives the appellation of the Nile.
7That the source of the Nile is in Mauritania, is certain, because on the other side of the Mount Atlas are other springs whose course is towards the western ocean, in which are found the ichneumon, the crocodile, and other animals and fishes of a similar nature, the hippopotamus excepted.
8Since, therefore, all the large known rivers in the world seem to flow from the north, and towards the land of Africa, because those are in the southern regions under the sun’s course, where there is little moisture, and but few springs and rivers, it follows that those sources which are in the north and north-east, are much better than others, unless they run over a sulphureous, aluminous, or bituminous soil, for their quality is thereby changed, and whether hot or cold, their water is then of bad smell and taste. It is not that water, by its nature, is hot, but when cold, it is heated by running over a hot soil, and issues warm from the earth through the different pores: it does not, however, long remain in that state, but soon becomes cold; whereas, if it were naturally hot, it would not so soon grow cool; for it does not lose its taste, smell, and colour, which, from the purity of its nature, remain unchanged and discoloured.