CONTENTS 



CHAP. PAGE 



XVI. The "mirrored den" of Hostius Quadra . . . . 41 



XVII. The philosophy of the looking-glass. The evolution of mirrors. 

 Mirrors of full length are now used. They cost a fortune greater 

 than the Senate gave Scipio's daughters. A harmless necessary 

 device has become an instrument of luxury, the adornment of 

 women, the burden of men, nay, part of the kit of the soldier . 44 



BOOK II 



[THE NATURE OF AIR. THUNDER AND LIGHTNING] 



[I. -XI. PREFATORY to treatment of thunder and lightning, descriptive 

 of the nature of the air, in which these phenomena occur. ] 



I. Divisions of physical science astronomy, meteorology, geography. 



Cross divisions, e.g. earthquakes, belong to meteorology, being 

 produced by air ; so the earth, as a planet, belongs to Astronomy 

 but its properties belong to Geography . . . . . 51 



II. Unity and composition in bodies. The analogy of the seen applies 



to the unseen. The atmosphere is possessed of unity (unitas) . 52 



III. Parts and material of bodies distinguished. In the human body 

 blood is both ......... 54 



IV. The atmosphere is an integral part of the universe : has unity . 54 



V. The earth is both part and material of the universe. From it 



nourishment is supplied to the latter . . . . . 55 



VI. The atmosphere has unity is not compact of atoms, otherwise it 

 could not exert tension, which is one of its main features, with 

 endless manifestations . . . . . . . . 55 



VII. There is no vacuum in the air, as the analogy of.water shows . 57 



VIII. The exertion of tension presupposes tensibility, just as motion 

 does mobility. Its existence in air proved by the effects of air, 

 which tosses about mountains, houses, walls, etc. The propaga- 

 tion of sight and sound proves the same . . . . . 58 



IX. Its tension is seen in raising water, as in the jet in the amphi- 

 theatre. Proofs from a ship upborne of water, a quoit flung from 



a height, sound heard through a wall . . . . . 59 



X. Varieties of density and temperature in the atmosphere : the 



central layer is coldest ........ 60 



XI. The lower parts are most subject to change. The cause of this 

 is in part the earth, in part the sun, moon, and other stars. 

 So much by way of preface to explain the nature of that atmo- 

 sphere in which thunder and lightning occur . . . . 61 



XII. Lightning, thunder-bolt, thunder. All agree that they occur in 

 the clouds, but different explanations are given of their cause and 

 relations. Anaxagoras connects them with the ether ; Aristotle 

 says they are due to exhalations of various kinds, from the earth, 

 coming in contact with the clouds . . . . . . 62 



XIII. The fire cannot be inherent in the clouds and fall from them. 

 When it so comes it is forced ....... 64 



