HISTORY

OF THE

ANCIENT PHYSICS.

FRO

ROM arranging and methodizing the Syftem of the Heavens, Philofophy defcended to the confideration of the inferior parts of Nature, of the Earth, and of the bodies which immediately furround it. If the objects, which were here presented to its view, were inferior in greatness or beauty, and therefore less apt to attract the attention of the mind, they were more apt, when they came to be attended to, to embarrass and perplex it, by the variety of their species, and by the intricacy and feeming irregularity of the laws or orders of their fucceffion. The species of objects in the Heavens are few in number; the Sun, the Moon, the Planets, and the Fixed Stars, are all which those philofophers could distinguish. All the changes too, which are ever obferved in these bodies, evidently arise from fome difference in the velocity and direction of their several motions; but the variety of meteors in the air, of clouds, rainbows, thunder, lightning, winds, rain, hail, fnow, is vastly greater; and the order of their fucceffion feems to be still more irregular and unconstant. The fpecies of foffils, minerals, plants, animals, which are found in the Waters, and near the furface of the Earth, are still more intricately diverfified; and if we

regard

regard the different manners of their production, their mutual influence in altering, deftroying, fupporting one another, the orders of their fucceffion feem to admit of an almost infinite variety. If the imagination, therefore, when it confidered the appearances in the Heavens, was often perplexed, and driven out of its natural career, it would be much more exposed to the fame embarraffment, when it directed its attention to the objects which the Earth prefented to it, and when it endeavoured to trace their progress and fucceffive revolutions.

To introduce order and coherence into the mind's conception of this feeming chaos of diffimilar and disjointed appearances, it was neceffary to deduce all their qualities, operations, and laws of fucceffion, from thofe of fome particular things, with which it was perfectly acquainted and familiar, and along which its imagination could glide fmoothly and easily, and without interruption. But as we would in vain attempt to deduce the heat of a ftove from that of an open chimney, unless we could fhow that the fame fire which was expofed in the one, lay concealed in the other; fo it was impoffible to deduce the qualities and laws of fucceffion, obferved in the more uncommon appearances of Nature, from thofe of fuch as were more familiar, if those customary objects were not fuppofed, however difguifed in their appearance, to enter into the compofition of those rarer and more fingular phænomena. To render, therefore, this lower part of the great theatre of nature a coherent spectacle to the imagination, it became neceffary to fuppofe, firft, That all the ftrange objects of which it confifted were made up out of a few, with which the mind was extremely familiar: and fecondly, That all their qualities, operations, and rules of fucceffion, were no more than different diverfifications of thofe to which it had long been accustomed, in thefe primary and elementary objects.

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Of

Of all the bodies of which thefe inferior parts of the universe seem to be compofed, thofe with which we are moft familiar, are the Earth, which we tread upon; the Water, which we every day use; the Air, which we conftantly breath; and the Fire, whofe benign influence is not only required for preparing the common neceffaries of life, but for the continual fupport of that vital principle which actuates both plants and animals. Thefe, therefore, were by Empedocles, and the other philofophers, of the Italian school, fuppofed to be the elements, out of which, at least, all the inferior parts of nature were compofed. The familiarity of thofe bodies to the mind, naturally difpofed it to look for fome refemblance to them in whatever elfe was prefented to its confideration. The difcovery of fome fuch refemblance united the new object to an affortment of things, with which the imagination was perfectly acquainted. And if any analogy could be observed betwixt the operations and laws of fucceffion of the compound, and thofe of the fimple objects, the movement of the fancy, in tracing their progress, became quite smooth, and natural, and eafy. This natural anticipation, too, was still more confirmed by fuch a flight and inaccurate analysis of things, as could be expected in the infancy of fcience, when the curiofity of mankind, grasping at an account of all things before it had got full fatisfaction with regard to any one, hurried on to build, in imagination, the immense fabric of the universe. The heat, obferved in.. both plants and animals, feemed to demonftrate, that Fire made a part of their compofition. Air was not lefs neceffary for the fubfiftance of both, and feemed, too, to enter into the fabric of animals by refpiration, and into that of plants by fome other means. The juices which circulated through them fhowed how much of their texture was owing to Water. And their refolution into Earth by putrefaction, difcovered that this element had not been left out in their original formation. A fimilar analysis seemed to shew the same principles in most other compound bodies.

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The vaft extent of those bodies seemed to render them, upon another account, proper to be the great ftores out of which nature compounded all the other fpecies of things. Earth and Water divide almost the whole of the terrestrial globe between them. The thin transparent covering of the Air furrounds it to an immense height upon all fides. Fire, with its attendant, light, seems to defcend from the celeftial regions, and might, therefore, either be fuppofed to be diffused through the whole of those ætherial spaces, as well as to be condensed and conglobated in those luminous bodies, which sparkle across them, as by the Stoics; or, to be placed immediately under the sphere of the Moon, in the region next below them, as by the Peripatetics, who could not reconcile the devouring nature of Fire with the supposed unchangeable effence of their folid and crystalline spheres.

The qualities, too, by which we are chiefly accustomed to characterize and distinguish natural bodies, are all of them found, in the highest degree in those Four Elements. The great divifions of the objects, near the surface of the Earth, are thofe into hot and cold, moist and dry, light and heavy. These are the most remarkable properties of bodies; and it is upon them that many of their other moft fenfible qualities and powers feem to depend. Of thefe, heat and cold were naturally enough regarded by those first enquirers into nature, as the active, moisture and drynefs, as the paffive qualities of matter. It was the temperature of heat and cold which feemed to occafion the growth and dissolution of plants and animals; as appeared evident from the effects of the change of the seasons upon both. A proper degree of moisture and drynefs was not lefs neceffary for these purposes; as was evident from the different effects and productions of wet and dry feafons and foils. It was the heat and cold, however, which actuated and determined thofe two otherwife inert qualities of things, to a ftate either of rest or motion.

Gravity