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—it is not composed 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 is proved by the effects of air, which tosses about mountains, houses, walls, etc. The propagation of sight and sound proves the same. . . . .	58
IX. Its tension is seen in raising water, as in the jet in the amphitheatre. Proofs from a ship upborne by 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 atmosphere 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