العلم الطبيعي في زمن نيرون (المسائل الطبيعية)

XIII. There may be two mock suns simultaneously. Some think the one is a reflection of the other, the clouds acting as mirrors set opposite to one another. Mock suns, especially in the South, are a sign of rain 36 XIV. Other celestial fires. “_Cave meteors_,” “_Barrel meteors_,” “_Chasms_,” with a brief description of each. The rapidity of their flight, just as of lightning, deceives the sight. Their origin and cause. They indicate wind 37 XV. _Gleams_ (_flashes_, σέλα). Their production and motions, varieties of them. Some do damage. Some are analogous to comets. “_Bearded_,” “_torches_,” “_cypress_” are different kinds. “_Beams_” and “_barrels_” may be of the same class. A curious case where such an appearance raised an alarm of fire. They are real fires. On the contrary, rainbows and halos are mere reflections. Mirrors have this wonderful power of false presentation 39 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.…