Having begun a mighty task in my old age I must make up for lost time by hurrying on. The magnitude of it is actually an incentive to effort. Such studies are far superior to the historian’s task of recording the deeds of the robbers and butchers of mankind. The former raise us above the vicissitudes of fortune. “The principal thing” is to have a pure heart and clean hands, to escape slavery to self. The study of the universe exalts us to this 109 I. The cause of rivers and their varieties. Waters vary in amount at different seasons, in temperature, in medicinal qualities 114 II. Varieties of taste, weight, colour, utility to health, consistency 115 III. Gravitation or the force of air determines the flow of water. Surface and spring water: they may be combined as in Lake Fucinus 115 IV. Why is the sea not filled nor the earth drained dry by rivers? 116 V. Some hold that what flows into the sea returns by secret passages cleansed of its salinity 116 VI. Some think rain supplies the rivers, and in proof cite the interior of Africa as contrasted with Gaul and Germany 117 VII. Objections to this argument. Rain does not penetrate more than 10 feet. If the earth is dry, it absorbs the rain; if it is saturated, the rain runs off. Again, rivers rise in rocks and mountains, where what rain ever fell must have run off. Rich wells of “living” water are found in the driest ground at depth. Fountains well out at mountain tops 117 VIII. The interior of the earth is, according to some, a huge receptacle of fresh water 118 IX. Others think the air which is contained within the earth being prevented from circulating turns into water 119 X. But indeed the four elements are all interchangeable 120 XI. Though the supply of water is perennial, rivers and springs are intermittent 121 XII. The abundance of water is no difficulty, since it is a fourth part of the universe 123 XIII. The relation of water to the other elements. Thales’ silly notion that the earth sails in water like a ship at sea 124 XIV. The Egyptians divide each of the elements into male and female 125 XV. The veins of the earth resemble those of the human body. There are likewise the analogies of marrow, mucus, etc., of injuries and of bleeding, of parturition, perspiration, etc. 125