Physical Science in the Time of Nero

BOOK I. [meteors, Halo, Rainbow, Mock Sun, Etc.]

PREFACE

PAGE

Contrast between human (moral) philosophy and divine (natural, physical). The sublime character of the latter which lifts us above the contemplation of the littlenesses of the earth and earthly life to the knowledge of God and His nature. Compared with astronomical conceptions and dimensions the world of man is but as a threshing-floor, the haunt of ants. The mind of man attains its true height in contemplation and investigation of these sublime facts. Some of the problems thus raised 3

CHAP.

I. Meteoric fires--she-goat, kid, etc. Occasions of their appearance; connection of portent with event. Explanation of the phenomena. They may be due to pressure of the atmosphere. Aristotle attributes them to the effect of terrestrial evaporation: difference of density causes various outbursts of this kind. They are analogous to lightning, but less violent 8

II. Halos. Produced by the light of a heavenly body striking the surrounding air and forming a circle as a stone does when thrown into a pond. Formed far away from the heavenly body and comparatively near the earth in the region of the wind. Require a particular state of the atmosphere neither too dense nor too thin. More frequent at night than day for this reason: by day the sun rarefies the air too much by its heat. Method of dissipation gives indication of wind or rain. Calmness a condition of formation, as in the analogous case of water 12

III. Rainbows. Generally by day, produced by inequalities of surface and density in clouds. Another species seen in a burst pipe or a fuller at work. Various explanations. Light and shade will not explain the varied colours. Some explain the rainbow as a confused reflection of the sun from individual drops of rain: every bounded surface, large or small, thus reflects--fish-pond and dew-drop equally. Aristotle attributes the confusion of colours to weakness of human sight; parallels may be found in persons whose sight is abnormally weak. As the innumerable drops, apparently without intervals, fall, human vision fails to distinguish severally the reflections of the sun, which thus become blended and confused. Vision is similarly deceived in the case of an oar in water, apples in a glass globe, etc., even in the size and movements of the sun himself. At any rate the rainbow requires both sun and cloud, and these opposite to each other. These two in operation produce the varieties of colour 16

IV. That the rainbow is an image is shown by the relation of sun to cloud in position, by the rapidity of formation and dispersion. Artemidorus’ explanation of the shape of the cloud (concave), and the consequent position of the red in the rainbow 22

V. Arguments to show that the cloud is coloured by the sun, like a dove’s neck or a peacock’s tail, and that the rainbow is not a reflection of the sun. The position (opposite) would be equally necessary in this case. Answer to this contention by Posidonius. The colour effects. Author agrees with Posidonius’ position but not his arguments. The only proof is the geometrical one 23

VI. Arguments from the size--never more than a semicircle--and shape of the bow. As the colour, whether real or reflected, is derived from the sun, so must also the shape be. The size is accounted for by the magnifying power of water, glass, etc. The sun as he appears in the rainbow is seen through moisture 28

VII. The arguments from the dispersion of the sun’s rays through glass (prism). Contention that they confirm author’s view 30

VIII. The form once more; why it is never larger than a semicircle. A wrong explanation refuted. Explanation of Aristotle’s remark as to the seasons of rainbows, in summer only in the morning or evening, in autumn at any time 31

IX. Streaks or weather-galls. Merely abortive or imperfect rainbows 33

X. Relations and differences of halos, bows, and weather-galls 34

XI. Mock suns. Their appearance and position in relation to the sun. They are a reflection of the sun in a suitable medium 34

XII. The formation of a mock sun may be compared to the image of the sun in eclipse as seen reflected in a dish of oil or pitch: the medium must be adapted to give the impression. The mock sun requires a certain consistency of cloud, failing which, a different effect is produced--obscuration, dissipation, etc. 35

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. 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 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 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 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

XIV. There is nothing inconsistent with this in the explanation given (I. xv.) of meteors. There may be an analogy with what is observed in cases of fire when isolated groups of houses take fire through gradual accumulation of heat 65

XV. Some (Stoics) think that spontaneous combustion takes place in the air 66

XVI. Difference between a flash of lightning and a bolt 66

XVII. Some explain the noise of thunder as due to hot meeting cold, as in the case of hot iron plunged into water 67

XVIII. Anaximander attributes the effects to air and explains all the phenomena by reference to it 67

XIX. Anaxagoras says it is the ether that acts on the lower atmosphere to produce them: it sends out fire 68

XX. Diogenes of Apollonia thinks that fire and air interact, producing one another, as may be observed in the various phenomena 68

XXI. Authorities discarded: independent explanation. A flash and a bolt are fire in some form: they differ only in degree 69

XXII. Analogy of fire on earth: it must apply above. Lightning due either to impact or friction. Hurricanes are a sufficient cause of the former 70

XXIII. Clouds and air may through friction also be a cause. The fires so produced are insubstantial and evanescent 71

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