As to their motion, one may make that depend on the circular movement of the entire heaven. One may also suppose that the stars move, while the heaven itself is immoveable; for there is nothing to prevent the idea that originally, before the formation of the world, they may have received, by the appointment of fate, an impulse from east to west, and that now their movement continues in consequence of their heat, as the fire naturally proceeds onwards in order to seek the aliment which suits it.
The intertropical movements of the sun and moon may depend, either on the obliquity impressed by fate on the heaven at certain determined epochs, or on the resistance of the air, or on the fact that these ignited bodies stand in need of being nourished by a matter suitable to their nature, and that this matter fails them; or finally, they may depend on the fact of their having originally received an impulse which compels them to move as they do describing a sort of spiral figure. The sensible evidence does not in the least contradict these different suppositions, and all those of the same kind which one can form, having always a due regard to what is possible, and bringing back each phenomenon to its analogous appearances in sensible facts, without disquieting oneself about the miserable speculations of the astronomers.
The evacuations and subsequent replenishings of the moon may depend either on a conversion of this body, or on the different forms which the air when in a fiery state can adopt, or perhaps to the interposition of another body, or lastly, to some one of the causes by which one gives account of the analogous phenomena which pass under our eyes. Provided, however, that one does not obstinately adopt an exclusive mode of explanation; and that, for want of knowing what is possible for a man to explain, and what is inaccessible to his intelligence; one does not throw oneself into interminable speculations.
It may also be possibly the case that the moon has a light of her own, or that she reflects that of the sun. For we see around us many objects which are luminous of themselves, and many others which have only a borrowed light. In a word, one will not be arrested by any of the celestial phenomena, provided that one always recollects that there are many explanations possible; that one examines the principles and reasons which agree with this mode of explanation, and that one does not proceed in accounting for the facts which do not agree with this method, to suffer oneself to be foolishly carried away, and to propose a separate explanation for each phenomenon, sometimes in one way, and sometimes in another.
The appearance of a face in the orb of the moon may depend either on a displacement of its parts, or on the interposition of some obstacle, or on any other cause capable of accounting for such an appearance. For one must not neglect to apply this same method to all the heavenly phenomena; for, from the moment when one comes to any point of contradiction to the evidence of the senses, it will be impossible to possess perfect tranquillity and happiness.
The eclipses of the sun and moon may depend either on the fact that these stars extinguish themselves, a phenomenon which we often see produced under our eyes, or on the fact of other bodies, the earth, the heaven, or something else of the same kind interposing between them and us. Besides, we must compare the different modes of explanation appropriate to phenomena, and recollect that it is not impossible that many causes may at one and the same time concur in their production.
He says the same thing in the twelfth book of his treatise on Nature; and adds that the eclipses of the sun arise from the fact that it penetrates into the shade of the moon, to quit it again presently; and the eclipses of the moon from the fact of its entering into the shade of the earth. We also find the same doctrine asserted by Diogenes, the Epicurean, in the first book of his Select Opinions.
The regular and periodical march of these phenomena has nothing in it that ought to surprise us, if we only attend to the analogous facts which take place under our eyes. Above all things let us beware of making the Deity interpose here, for that being we ought to suppose exempt from all occupation and perfectly happy; otherwise we shall be only giving vain explanations of the heavenly phenomena, as has happened already to a crowd of authors. Not being able to recognize what is really possible, they have fallen into vain theories, in supposing that for all phenomena there was but one single mode of production, and in rejecting all other explanations which are founded on probability; they have adopted the most unreasonable opinions, for want of placing in the front the study of the heavenly phenomena, and of sensible facts, which ought to serve to explain the first.
The differences in the length of nights and days may arise from the fact that the passage of the sun above the earth is more or less rapid; and more or less slow, according to the length of the regions which it has to pass through. Or, again, to the fact that certain regions are passed through more rapidly than others, as is seen to be the case by our own eyes, in those things to which we can compare the heavenly phenomena. As to those who on this point admit only one explanation as possible, they put themselves in opposition to facts, and lose sight of the bounds set to human knowledge.
The prognostics which are derived from the stars may, like those which we borrow from animals, arise from a simple coincidence. They may also have other causes, for example, some change in the air; for these two suppositions both harmonize equally with facts, but it is impossible to distinguish in what case one is to attribute them to the one cause or to the other.
The clouds may be formed either by the air condensed under the pressure of the winds, or by the agency of atoms set apart for that end, or by emanations from the earth and waters, or by other causes. For there are a great number which are all equally able to produce this effect. When the clouds clash with one another, or undergo any transformation, they produce showers; and the long rains are caused by the motion of the clouds when moved from places suitable to them through the air, when a more violent inundation than usual takes place, from collections of some masses calculated to produce these effects.
Thunder possibly arises from the movement of the winds revolving in the cavities of the clouds; of which we may see an image in vessels in our own daily use. It may also arise from the noise of fire acted upon by the wind in them, and from the tearings and ruptures of the clouds when they have received a sort of crystaline consistency. In a word, experience drawn from our senses teaches us that all these phenomena, and that one in particular, may be produced in many different manners.
One may also assign different causes to the lightning; either the shock and collision of the clouds produce a fiery appearance, which is followed by lightning; or the lighting up of the clouds by the winds, produces this luminous appearance; or the mutual pressure of the clouds, or that of the wind against them, disengages the lightning. Or one might say that the interception of the light diffused from the stars, arrested for a time in the bosom of the clouds, is driven from them subsequently by their own movements, and by those of the winds, and so escapes from their sides; that the lightning is an extremely subtle light that evaporates from the clouds; that the clouds which carry the thunder are collected masses of fire; that the lightning arises from the motion of the fire, or from the conflagration of the wind, in consequence of the rapidity and continuousness of its motion. One may also attribute the luminous appearance of lightning to the rupture of the clouds under the action of the winds, or to the fall of inflammable atoms. Lastly, one may easily find a number of other explanations, if one applies to sensible facts, in order to search out the analogies which they present to the heavenly phenomena.
Lightning precedes thunder, either because it is produced at the same moment that the wind falls on the cloud, while the noise is only heard at the instant when the wind has penetrated into the bosom of the cloud; or, perhaps, the two phenomena being simultaneous, the lightning arrives among us more rapidly than the noise of the thunderbolt, as is in fact remarked in other cases when we see at a distance the clash of two objects.
The thunderbolt may be produced either by a violent condensation of the winds, or by their rapid motion and conflagration. It may arise from the fact of the winds meeting in places which are too dense, in consequence of the accumulation of clouds, and then a portion of the current detaches itself and proceeds towards the lower situations; or else it may be caused by the fire which is contained in the bosom of the clouds precipitating itself downwards. As one may suppose that an immense quantity of fire being accumulated in the clouds dilates, violently bursting the substance which envelops it, because the resistance of the center hinders it from proceeding further. This effect is especially produced in the neighborhood of high mountains; and, accordingly, they are very frequently struck with the thunderbolts. In short, one may give a number of explanations of the thunderbolt; but we ought above all things to be on our guard against fables, and this one will easily be if one follows faithfully the sensible phenomena in the explanation of these things which are not perceived, except indirectly.
Hurricanes (πρηστῆρες) may be caused either by the presence of a cloud which a violent wind sets in motion and precipitates with a spiral movement towards the lower regions, or by a violent gust which bears a cloud into the neighborhood of some other current, or else by the mere agitation of the wind by itself, when air is brought together from the higher regions and compressed without being able to escape on either side, in consequence of the resistance of the air which surrounds it; when the hurricane descends towards the earth, then there result whirlwinds in proportion to the rapidity of the wind that has produced them; and this phenomenon extends over the sea also.
Earthquakes may arise from the wind penetrating into the interior of the earth, or from the earth itself receiving incessantly the addition of exterior particles, and being in incessant motion as to its constituent atoms, being in consequence disposed to a general vibration. That which permits the wind to penetrate is the fact that falls take place in the interior, or that the air being impressed by the winds insinuates itself into the subterraneous caverns. The movement which numberless falls and the reaction of the earth communicate to the earth, when this motion meets bodies of greater resistance and solidity, is sufficient to explain the earthquakes. One might, however, give an account of them in several other ways.
Winds are caused either by the successive and regular addition of some foreign matter, or else by the reunion of a great quantity of water; and the differences of the winds may arise from the fact that some portions of this same matter fall into the numerous cavities of the earth, and are divided there.
Hail is produced by an energetic condensation acting on the ethereal particles which the cold embraces in every direction; or in consequence of a less violent condensation acting however on aqueous particles, and accompanied by division, in such a manner as to produce, at the same time, the reunion of certain elements and of the collective masses; or by the rupture of some dense and compact mass which would explain at the same time, the numerousness of the particles and their individual hardness. As to the spherical form of the hail, one may easily account for that by admitting that the shocks which it receives in every direction make all the angles disappear, or else that at the moment when the different fragments are formed, each of them is equally embraced on all sides by aqueous or ethereal particles.
Snow may be produced by a light vapor full of moisture which the clouds allow to escape by passages intended for that end, when they are pressed in a corresponding manner by other clouds and set in motion by the wind. Subsequently, these vapors become condensed in their progress under the action of the cold which surrounds the clouds in the lower regions. It may also be the case that this phenomenon is produced by clouds of a slight density as they become condensed. In this case the snow which escapes from the clouds would be the result of the contact or approximation of the aqueous particles, which in a still more condensed state produce hail. This effect is most especially produced in the air. Snow, again, may result from the collection of clouds previously condensed and solidified; or from a whole army of other causes.
Dew proceeds from a reunion of particles contained in the air calculated to produce this moist substance. These particles may be also brought from places which are moist or covered with water (for in those places, above all others, it is that dew is abundant). These then reunite, again resume their aqueous form, and fall down. The same phenomenon takes place in other cases before our own eyes under many analogies.
Hoarfrost is dew congealed by the influence of the cold air that surrounds it.
Ice is formed either by the detrition of round atoms contained in the water, and the reunion at scalene and acute angles of the atoms which exist in the water, or by an addition from without of these latter particles, which penetrating into the water, solidify it by driving away an equal amount of round atoms.
The rainbow may be produced by the reflection of the solar rays on the moist air; or it may arise from a particular property of light and air, in virtue of which these particular appearances of color are formed, either because the shades which we perceive result directly from this property, or because on the contrary it only produces one single shade which, reflecting itself on the nearest portions of the air, communicates to them the tints which we observe. As to the circular form of the rainbow, that depends either on the fact of the sight perceiving an equal distance in every direction, or the fact of the atoms taking this form when reuniting in the air; or it may be caused by its detaching from the air which moves towards the moon, certain atoms which, being reunited in the clouds, give rise to this circular appearance.
The lunar halo arises from the fact of the air, which moves towards the moon from all quarters, uniformly intercepting the rays emitted by this star, in such a way as to form around it a sort of circular cloud which partially veils it. It may also arise from the fact of the moon uniformly rejecting from all quarters, the air which surrounds it, in such a manner as to produce this circular and opaque covering. And perhaps this opaqueness may be caused by some particles which some current brings from without; perhaps also, the heat communicates to the moon the property of emitting, by the pores in its surface, the particles by which this effect is produced.