xii PHYSICAL SCIENCE 



CHAP. PAGE 



LVI. Heraclitus and Caecina think sheet lightning an intermittent 



incipient fire. Change in the pronunciation of the Latin word . 100 



LVII. Lightning is probably due to the air turning into fire through 

 rarefaction of the clouds. It is naturally most frequent in 

 summer. Sheet and forked lightning differ in degree, not in kind loo 



LVIII. Reasons for rapidity of lightning and its obliquity . 101 



LIX. Every story should have a moral. Death cannot be prevented ; 

 why fear it ? It is cowardly and silly. Death by lightning is 

 rather an honour than otherwise. Besides, fear is futile . . 102 



BOOK III 



ON FORMS OF WATER 

 PREFACE 



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. &quot; The principal 

 thing &quot; 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 ? . 1 1 6 



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 

 &quot; living &quot; 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 



