MANKIND, in the first ages of society, before the establishment of law, order, and security, have little curiosity to find out those hidden chains of events which bind together the seemingly disjointed appearances of nature. A savage, whose subsistence is precarious, whose life is every day exposed to the rudest dangers, has no inclination to amuse himself with searching out what, when discovered, seems to serve no other purpose than to render the theatre of nature a more connected spectacle to his imagination. Many of these smaller incoherences, which in the course of things perplex philosophers, entirely escape his attention. Those more magnificent irregularities, whose grandeur he cannot overlook, call forth his amazement. Comets, eclipses, thunder, lightning, and other meteors, by their greatness, naturally overawe him, and he views them with a reverence that approaches to fear. His inexperience and uncertainty with regard to every thing about them, how they came, how they are to go, what went before, what is to come after them, exasperate his sentiment into terror and consternation. But our passions, as Father Malbranche observes, all justify themselves; that is, suggest to us opinions which justify them. As those appearances terrify him, therefore, he is disposed to believe every thing about them which can render them still more the objects of his terror. That they proceed from some intelligent, though invisible causes, of whose vengeance and displeasure they are either the signs or the effects, is the notion of all others most capable of enhancing this passion, and is that, therefore, which he is most apt to entertain. To this, too, that cowardice and pusillanimity, so natural to man in his uncivilized state, still more disposes him; unprotected by the laws of society, exposed, defenceless, he feels his weakness upon all occasions; his strength and security upon none.
But all the irregularities of nature are not of this awful or terrible kind. Some of them are perfectly beautiful and agreeable. These,339therefore, from the same impotence of mind, would be beheld with love and complacency, and even with transports of gratitude; for whatever is the cause of pleasure naturally excites our gratitude. A child caresses the fruit that is agreeable to it, as it beats the stone that hurts it. The notions of a savage are not very different. The ancient Athenians, who solemnly punished the axe which had accidentally been the cause of the death of a man, erected altars, and offered sacrifices to the rainbow. Sentiments not unlike these, may sometimes, upon such occasions, begin to be felt even in the breasts of the most civilized, but are presently checked by the reflection, that the things are not their proper objects. But a savage, whose notions are guided altogether by wild nature and passion, waits for no other proof that a thing is the proper object of any sentiment, than that it excites it. The reverence and gratitude, with which some of the appearances of nature inspire him, convince him that they are the proper objects of reverence and gratitude, and therefore proceed from some intelligent beings, who take pleasure in the expressions of those sentiments. With him, therefore, every object of nature, which by its beauty or greatness, its utility or hurtfulness, is considerable enough to attract his attention, and whose operations are not perfectly regular, is supposed to act by the direction of some invisible and designing power. The sea is spread out into a calm, or heaved into a storm, according to the good pleasure of Neptune. Does the earth pour forth an exuberant harvest? It is owing to the indulgence of Ceres. Does the vine yield a plentiful vintage? It flows from the bounty of Bacchus. Do either refuse their presents? It is ascribed to the displeasure of those offended deities. The tree which now flourishes and now decays, is inhabited by a Dryad, upon whose health or sickness its various appearances depend. The fountain, which sometimes flows in a copious, and sometimes in a scanty stream, which appears sometimes clear and limpid, and at other times muddy and disturbed, is affected in all its changes by the Naiad who dwells within it. Hence the origin of Polytheism, and of that vulgar superstition which ascribes all the irregular events of nature to the favour or displeasure of intelligent, though invisible beings, to gods, demons, witches, genii, fairies. For it may be observed, that in all polytheistic religions, among savages, as well as in the early ages of heathen antiquity, it is the irregular events of nature only that are ascribed to the agency and power of their gods. Fire burns, and water refreshes; heavy bodies descend, and lighter substances fly upwards, by the necessity of their own nature; nor was the invisible hand of Jupiter ever apprehended to be employed in those matters. But thunder and lightning, storms and sunshine, those more irregular events, were ascribed to his favour, or his anger. Man, the only designing power with which they were acquainted, never acts but either to stop or to alter the course which natural events would take, if left to themselves.340Those other intelligent beings, whom they imagined, but knew not, were naturally supposed to act in the same manner; not to employ themselves in supporting the ordinary course of things, which went on of its own accord, but to stop, to thwart, and to disturb it. And thus, in the first ages of the world, the lowest and most pusillanimous superstition supplied the place of philosophy.
But when law has established order and security, and subsistence ceases to be precarious, the curiosity of mankind is increased, and their fears are diminished. The leisure which they then enjoy renders them more attentive to the appearances of nature, more observant of her smallest irregularities, and more desirous to know what is the chain which links them together. That some such chain subsists betwixt all her seemingly disjointed phenomena, they are necessarily led to conceive; and that magnanimity and cheerfulness which all generous natures acquire who are bred in civilized societies, where they have so few occasions to feel their weakness, and so many to be conscious of their strength and security, renders them less disposed to employ, for this connecting chain, those invisible beings whom the fear and ignorance of their rude forefathers had engendered. Those of liberal fortunes, whose attention is not much occupied either with business or with pleasure, can fill up the void of their imagination, which is thus disengaged from the ordinary affairs of life, no other way than by attending to that train of events which passes around them. While the great objects of nature thus pass in review before them, many things occur in an order to which they have not been accustomed. Their imagination, which accompanies with ease and delight the regular progress of nature, is stopped and embarrassed by those seeming incoherences; they excite their wonder, and seem to require some chain of intermediate events, which, by connecting them with something that has gone before, may thus render the whole course of the universe consistent and of a piece. Wonder, therefore, and not any expectation of advantage from its discoveries, is the first principle which prompts mankind to the study of Philosophy, of that science which pretends to lay open the concealed connections that unite the various appearances of nature; and they pursue this study for its own sake, as an original pleasure or good in itself, without regarding its tendency to procure them the means of many other pleasures.
Greece, and the Greek colonies in Sicily, Italy, and the Lesser Asia, were the first countries which, in these western parts of the world, arrived at a state of civilized society. It was in them, therefore, that the first philosophers, of whose doctrine we have any distinct account, appeared. Law and order seem indeed to have been established in the great monarchies of Asia and Egypt, long before they had any footing in Greece: yet, after all that has been said concerning the learning of the Chaldeans and Egyptians, whether there ever was in those nations341any thing which deserved the name of science, or whether that despotism which is more destructive of security and leisure than anarchy itself, and which prevailed over all the East, prevented the growth of Philosophy, is a question which, for want of monuments, cannot be determined with any degree of precision.
The Greek colonies having been settled amid nations either altogether barbarous, or altogether unwarlike, over whom, therefore, they soon acquired a very great authority, seem, upon that account, to have arrived at a considerable degree of empire and opulence before any state in the parent country had surmounted that extreme poverty, which, by leaving no room for any evident distinction of ranks, is necessarily attended with the confusion and misrule which flows from a want of all regular subordination. The Greek islands being secure from the invasion of land armies, or from naval forces, which were in those days but little known, seem, upon that account too, to have got before the continent in all sorts of civility and improvement. The first philosophers, therefore, as well as the first poets, seem all to have been natives, either of their colonies, or of their islands. It was from thence that Homer, Archilochus, Stesichorus, Simonides, Sappho, Anacreon, derived their birth. Thales and Pythagoras, the founders of the two earliest sects of philosophy, arose, the one in an Asiatic colony, the other in an island; and neither of them established his school in the mother country.
What was the particular system of either of those two philosophers, or whether their doctrine was so methodized as to deserve the name of a system, the imperfection, as well as the uncertainty of all the traditions that have come down to us concerning them, make it impossible to determine. The school of Pythagoras, however, seems to have advanced further in the study of the connecting principles of nature, than that of the Ionian philosopher. The accounts which are given of Anaximander, Anaximenes, Anaxagoras, Archelaus, the successors of Thales, represent the doctrines of those sages as full of the most inextricable confusion. Something, however, that approaches to a composed and orderly system, may be traced in what is delivered down to us concerning the doctrine of Empedocles, of Archytas, of Timæus, and of Ocellus the Lucanian, the most renowned philosophers of the Italian school. The opinions of the two last coincide pretty much; the one, with those of Plato; the other, with those of Aristotle; nor do those of the two first seem to have been very different, of whom the one was the author of the doctrine of the Four Elements, the other the inventor of the Categories; who, therefore, may be regarded as the founders, the one, of the ancient Physics; the other, of the ancient Dialectic; and, how closely these were connected will appear hereafter. It was in the school of Socrates, however, from Plato and Aristotle, that Philosophy first received that form, which introduced her, if one342may say so, to the general acquaintance of the world. It is from them, therefore, that we shall begin to give her history in any detail. Whatever was valuable in the former systems, which was at all consistent with their general principles, they seem to have consolidated into their own. From the Ionian philosophy, I have not been able to discover that they derived anything. From the Pythagorean school, both Plato and Aristotle seem to have derived the fundamental principles of almost all their doctrines. Plato, too, appears to have borrowed something from two other sects of philosophers, whose extreme obscurity seems to have prevented them from acquiring themselves any extensive reputation; the one was that of Cratylus and Heraclitus; the other was Xenophanes, Parmenides, Melissus, and Zeno. To pretend to rescue the system of any of those ante-Socratic sages, from that oblivion which at present covers them all, would be a vain and useless attempt. What seems, however, to have been borrowed from them, shall sometimes be marked as we go along.
There was still another school of philosophy, earlier than Plato, from which, however, he was so far from borrowing any thing, that he seems to have bent the whole force of his reason to discredit and expose its principles. This was the philosophy of Leucippus,Democritus, and Protagoras, which accordingly seems to have submitted to his eloquence, to have lain dormant, and to have been almost forgotten for some generations, till it was afterwards more successfully revived by Epicurus.
OFall the phenomena of nature, the celestial appearances are, by their greatness and beauty, the most universal objects of the curiosity of mankind. Those who surveyed the heavens with the most careless attention, necessarily distinguished in them three different sorts of objects; the Sun, the Moon, and the Stars. These last, appearing always in the same situation, and at the same distance with regard to one another, and seeming to revolve every day round the earth in parallel circles, which widened gradually from the poles to the equator, were naturally thought to have all the marks of being fixed, like so many gems, in the concave side of the firmament, and of being carried round by the diurnal revolutions of that solid body: for the azure sky, in which the stars seem to float, was readily apprehended, upon account of the uniformity of their apparent motions, to be a solid body, the roof or outer wall of the universe, to whose inside all those little sparkling objects were attached.
The Sun and Moon, often changing their distance and situation, in regard to the other heavenly bodies, could not be apprehended to be attached to the same sphere with them. They assigned, therefore, to343each of them, a sphere of its own; that is, supposed each of them to be attached to the concave side of a solid and transparent body, by whose revolutions they were carried round the earth. There was not, indeed, in this case, the same ground for the supposition of suchasphere as in that of the Fixed Stars; for neither the Sun nor the Moon appear to keep always at the same distance with regard to any one of the other heavenly bodies. But as the motion of the Stars had been accounted for by an hypothesis of this kind, it rendered the theory of the heavens more uniform, to account for that of the Sun and Moon in the same manner. The sphere of the sun they placed above that of the Moon; as the Moon was evidently seen in eclipses to pass betwixt the Sun and the Earth. Each of them was supposed to revolve by a motion of its own, and at the same time to be affected by the motion of the Fixed Stars. Thus, the Sun was carried round from east to west by the communicated movement of this outer sphere, which produced his diurnal revolutions, and the vicissitudes of day and night; but at the same time he had a motion of his own, contrary to this, from west to east, which occasioned his annual revolution, and the continual shifting of his place with regard to the Fixed Stars. This motion was more easy, they thought, when carried on edgeways, and not in direct opposition to the motion of the outer sphere, which occasioned the inclination of the axis of the sphere of the Sun, to that of the sphere of the Fixed Stars; this again produced the obliquity of the ecliptic, and the consequent changes of the seasons. The moon, being placed below the sphere of the Sun, had both a shorter course to finish, and was less obstructed by the contrary movement of the sphere of the Fixed Stars, from which she was farther removed. She finished her period, therefore, in a shorter time, and required but a month, instead of a year, to complete it.
The Stars, when more attentively surveyed, were some of them observed to be less constant and uniform in their motions than the rest, and to change their situations with regard to the other heavenly bodies; moving generally eastward, yet appearing sometimes to stand still, and sometimes even, to move westwards. These, to the number of five, were distinguished by the name of Planets, or Wandering Stars, and marked with the particular appellations of Saturn, Jupiter, Mars, Venus, and Mercury. As, like the Sun and Moon, they seem to accompany the motion of the Fixed Stars from east to west, but at the same time to have a motion of their own, which is generally from west to east; they were each of them, as well as those two great lamps of heaven, apprehended to be attached to the inside of a solid concave and transparent sphere, which had a revolution of its own, that was almost directly contrary to the revolution of the outer heaven, but which, at the same time, was hurried along by the superior violence and greater rapidity of this last.
This is the system of concentric Spheres, the first regular system of344Astronomy, which the world beheld, as it was taught in the Italian school before Aristotle, and his two contemporary philosophers, Eudoxus and Callippus, had given it all the perfection which it is capable of receiving. Though rude and inartificial, it is capable of connecting together, in the imagination, the grandest and the most seemingly disjointed appearances in the heavens. The motions of the most remarkable objects in the celestial regions, the Sun, the Moon, the Fixed Stars, are sufficiently connected with one another by this hypothesis. The eclipses of these two great luminaries are, though not so easily calculated, as easily explained, upon this ancient, as upon the modern system. When these early philosophers explained to their disciples the very simple causes of those dreadful phenomena, it was under the seal of the most sacred secrecy, that they might avoid the fury of the people, and not incur the imputation of impiety, when they thus took from the gods the direction of those events, which were apprehended to be the most terrible tokens of their impending vengeance. The obliquity of the ecliptic, the consequent changes of the seasons, the vicissitudes of day and night, and the different lengths of both days and nights in the different seasons, correspond too, pretty exactly, with this ancient doctrine. And if there had been no other bodies discoverable in the heavens, besides the Sun, the Moon, and the Fixed Stars, this hypothesis might have stood the examinations of all ages and gone down triumphant to the remotest posterity.
If it gained the belief of mankind by its plausibility, it attracted their wonder and admiration; sentiments that still more confirmed their belief, by the novelty and beauty of that view of nature which it presented to the imagination. Before this system was taught in the world, the earth was regarded as, what it appears to the eye, a vast, rough, and irregular plain, the basis and foundation of the universe, surrounded on all sides by the ocean, and whose roots extended themselves through the whole of that infinite depth which is below it. The sky was considered as a solid hemisphere, which covered the earth, and united with the ocean at the extremity of the horizon. The Sun, the Moon, and all the heavenly bodies rose out of the eastern, climbed up the convex side of the heavens, and descended again into the western ocean, and from thence, by some subterraneous passages, returned to their first chambers in the east. Nor was this notion confined to the people, or to the poets who painted the opinions of the people; it was held by Xenophanes, founder of the Eleatic philosophy, after that of the Ionian and Italian schools, the earliest that appeared in Greece. Thales of Miletus too, who, according to Aristotle, represented the Earth as floating upon an immense ocean of water, may have been nearly of the same opinion; notwithstanding what we are told by Plutarch and Apuleius concerning his astronomical discoveries, all of which must plainly have been of a much later date. To those345who had no other idea of nature, besides what they derived from so confused an account of things, how agreeable must that system have appeared, which represented the Earth as distinguished into land and water, self-balanced and suspended in the centre of the universe, surrounded by the elements of Air and Ether, and covered by eight polished and crystalline Spheres, each of which was distinguished by one or more beautiful and luminous bodies, and all of which revolved round their common centre, by varied, but by equable and proportionable motions. It seems to have been the beauty of this system that gave Plato the notion of something like an harmonic proportion, to be discovered in the motions and distances of the heavenly bodies; and which suggested to the earlier Pythagoreans, the celebrated fancy of the Music of the Spheres; a wild and romantic idea, yet such as does not ill correspond with that admiration, which so beautiful a system, recommended too by the graces of novelty, is apt to inspire.
Whatever are the defects which this account of things labours under, they are such, as to the first observers of the heavens could not readily occur. If all the motions of the Five Planets cannot, the greater part of them may, be easily connected by it; they and all their motions are the least remarkable objects in the heavens; the greater part of mankind take no notice of them at all; and a system, whose only defect lies in the account which it gives of them, cannot thereby be much disgraced in their opinion. If some of the appearances too of the Sun and Moon, the sometimes accelerated and again retarded motions of those luminaries but ill correspond with it; these, too, are such as cannot be discovered but by the most attentive observation, and such as we cannot wonder that the imaginations of the first enquirers should slur over, if one may say so, and take little notice of.
It was, however, to remedy those defects, that Eudoxus, the friend and auditor of Plato, found it necessary to increase the number of the Celestial Spheres. Each Planet is sometimes observed to advance forward in that eastern course which is peculiar to itself, sometimes to retire backwards, and sometimes again to stand still. To suppose that the sphere of the planet should by its own motion, if one may say so, sometimes roll forwards, sometimes roll backwards, and sometimes do neither the one nor the other, is contrary to all the natural propensities of the imagination, which accompanies with ease and delight any regular and orderly motion, but feels itself perpetually stopped and interrupted, when it endeavours to attend to one so desultory and uncertain. It would pursue, naturally and of its own accord, the direct or progressive movement of the Sphere, but is every now and then shocked, if one may say so, and turned violently out of its natural career by the retrograde and stationary appearances of the Planet, betwixt which and its more usual motion, the fancy feels a want of connection, a gap or interval, which it cannot fill up, but by supposing346some chain of intermediate events to join them. The hypothesis of a number of other spheres revolving in the heavens, besides those in which the luminous bodies themselves were infixed, was the chain with which Eudoxus endeavoured to supply it. He bestowed four of these Spheres upon each of the five Planets; one in which the luminous body itself revolved, and three others above it. Each of these had a regular and constant, but a peculiar movement of its own, which it communicated to what was properly the Sphere of the Planet, and thus occasioned that diversity of motions observable in those bodies. One of these Spheres, for example, had an oscillatory motion, like the circular pendulum of a watch. As when you turn round a watch, like a Sphere upon its axis, the pendulum will, while turned round along with it, still continue to oscillate, and communicate to whatever body is comprehended within it, both its own oscillations and the circular motion of the watch; so this oscillating Sphere, being itself turned round by the motion of the Sphere above it, communicated to the Sphere below it, that circular, as well as its own oscillatory motions; produced by the one, the daily revolutions: by the other, the direct, stationary, and retrograde appearances of the Planet, which derived from a third Sphere that revolution by which it performed its annual period. The motions of all these Spheres were in themselves constant and equable, such as the imagination could easily attend to and pursue, and which connected together that otherwise incoherent diversity of movements observable in the Sphere of the Planet. The motions of the Sun and Moon being more regular than those of the Five Planets, by assigning three Spheres to each of them, Eudoxus imagined he could connect together all the diversity of movements discoverable in either. The motion of the Fixed Stars being perfectly regular, one Sphere he judged sufficient for them all. So that, according to this account, the whole number of Celestial Spheres amounted to twenty-seven. Callippus, though somewhat younger, the contemporary of Eudoxus, found that even this number was not enough to connect together the vast variety of movements which he discovered in those bodies, and therefore increased it to thirty-four. Aristotle, upon a yet more attentive observation, found that even all these Spheres would not be sufficient, and therefore added twenty-two more, which increased their number to fifty-six. Later observers discovered still new motions, and new inequalities, in the heavens. New Spheres were therefore still to be added to the system, and some of them to be placed even above that of the Fixed Stars. So that in the sixteenth century, when Fracostorio, smit with the eloquence of Plato and Aristotle, and with the regularity and harmony of their system, in itself perfectly beautiful, though it corresponds but inaccurately with the phenomena, endeavoured to revive this ancient Astronomy, which had long given place to that of Ptolemy and Hipparchus, he found it necessary to multiply347the number of Celestial Spheres to seventy-two; neither were all these found to be enough.
This system had now become as intricate and complex as those appearances themselves, which it had been invented to render uniform and coherent. The imagination, therefore, found itself but little relieved from that embarrassment, into which those appearances had thrown it, by so perplexed an account of things. Another system, for this reason, not long after the days of Aristotle, was invented by Apollonius, which was afterwards perfected by Hipparchus, and has since been delivered down to us by Ptolemy, the more artificial system of Eccentric Spheres and Epicycles.
In this system, they first distinguished between the real and apparent motion of the heavenly bodies. These, they observed, upon account of their immense distance, must necessarily appear to revolve in circles concentric with the globe of the Earth, and with one another: but that we cannot, therefore, be certain that they really revolve in such circles, since, though they did not, they would still have the same appearance. By supposing, therefore, that the Sun and the other Planets revolved in circles, whose centres were very distant from the centre of the Earth; that consequently, in the progress of their revolution, they must sometimes approach nearer, and sometimes recede further from it, and must to its inhabitants appear to move faster in the one case, and slower in the other, those philosophers imagined they could account for the apparently unequal velocities of all those bodies.
By supposing, that in the solidity of the Sphere of each of the Five Planets there was formed another little Sphere, called an Epicycle, which revolved round its own centre, at the same time that it was carried round the centre of the Earth by the revolution of the great Sphere, betwixt whose concave and convex sides it was inclosed; in the same manner as we might suppose a little wheel inclosed within the outer circle of a great wheel, and which whirled about several times upon its own axis, while its centre was carried round the axis of the great wheel, they imagined they could account for the retrograde and stationary appearances of those most irregular objects in the heavens. The Planet, they supposed, was attached to the circumference, and whirled round the centre of this little Sphere, at the same time that it was carried round the earth by the movement of the great Sphere. The revolution of this little Sphere, or Epicycle, was such, that the Planet, when in the upper part of it; that is, when furthest off and least sensible to the eye; was carried round in the same direction with the centre of the Epicycle, or with the Sphere in which the Epicycle was inclosed: but when in the lower part, that is, when nearest and most sensible to the eye; it was carried round a direction contrary to that of the centre of the Epicycle: in the same manner as every point in the upper part of the outer circle of a coach-wheel revolves forward in the348same direction with the axis, while every point, in the lower part, revolves backwards in a contrary direction to the axis. The motions of the Planet, therefore, surveyed from the Earth, appeared direct, when in the upper part of the Epicycle, and retrograde, when in the lower. When again it either descended from the upper part to the lower, or ascended from the lower to the upper, it appeared stationary.
But, though, by the eccentricity of the great Sphere, they were thus able, in some measure, to connect together the unequal velocities of the heavenly bodies, and by the revolutions of the little Sphere, the direct, stationary, and retrograde appearances of the Planets, there was another difficulty that still remained. Neither the Moon, nor the three superior Planets, appear always in the same part of the heavens, when at their periods of most retarded motion, or when they are supposed to be at the greatest distance from the Earth. The apogeum therefore, or the point of greatest distance from the Earth, in the Spheres of each of those bodies, must have a movement of its own, which may carry it successively through all the different points of the Ecliptic. They supposed, therefore, that while the great eccentric Sphere revolved eastwards round its centre, that its centre too revolved westwards in a circle of its own, round the centre of the Earth, and thus carried its apogeum through all the different points of the Ecliptic.
But with all those combined and perplexed circles; though the patrons of this system were able to give some degree of uniformity to the real directions of the Planets, they found it impossible so to adjust the velocities of those supposed Spheres to the phenomena, as that the revolution of any one of them, when surveyed from its own centre, should appear perfectly equable and uniform. From that point, the only point in which the velocity of what moves in a circle can be truly judged of, they would still appear irregular and inconstant, and such as tended to embarrass and confound the imagination. They invented, therefore, for each of them, a new Circle, called the Equalizing Circle, from whose centre they should all appear perfectly equable: that is, they so adjusted the velocities of these Spheres, as that, though the revolution of each of them would appear irregular when surveyed from its own centre, there should, however, be a point comprehended within its circumference, from whence its motions should appear to cut off, in equal times, equal portions of the Circle, of which that point was supposed to be the centre.
Nothing can more evidently show how much the repose and tranquillity of the imagination is the ultimate end of philosophy, than the invention of this Equalizing Circle. The motions of the heavenly bodies had appeared inconstant and irregular, both in their velocities and in their directions. They were such, therefore, as tended to embarrass and confound the imagination, whenever it attempted to trace them. The invention of Eccentric Spheres, of Epicycles, and of the349revolution of the centres of the Eccentric Spheres, tended to allay this confusion, to connect together those disjointed appearances, and to introduce harmony and order into the mind’s conception of the movements of those bodies. It did this, however, but imperfectly; it introduced uniformity and coherence into their real directions. But their velocities, when surveyed from the only point in which the velocity of what moves in a Circle can be truly judged of, the centre of that Circle, still remained, in some measure, inconstant as before; and still, therefore, embarrassed the imagination. The mind found itself somewhat relieved from this embarrassment, when it conceived, that how irregular soever the motions of each of those Circles might appear, when surveyed from its own centre, there was, however, in each of them, a point, from whence its revolution would appear perfectly equable and uniform, and such as the imagination could easily follow. Those philosophers transported themselves, in fancy, to the centres of these imaginary Circles, and took pleasure in surveying from thence, all those fantastical motions, arranged, according to that harmony and order, which it had been the end of all their researches to bestow upon them. Here, at last, they enjoyed that tranquillity and repose which they had pursued through all the mazes of this intricate hypothesis; and here they beheld this, the most beautiful and magnificent part of the great theatre of nature, so disposed and constructed, that they could attend, with delight, to all the revolutions and changes that occurred in it.
These, the System of Concentric, and that of Eccentric Spheres, seem to have been the two Systems of Astronomy, that had most credit and reputation with that part of the ancient world, who applied themselves particularly to the study of the heavens. Cleanthes, however, and the other philosophers of the Stoical sect who came after him, appear to have had a system of their own, quite different from either. But though justly renowned for their skill in dialectic, and for the security and sublimity of their moral doctrines, those sages seem never to have had any high reputation for their knowledge of the heavens; neither is the name of any one of them ever counted in the catalogue of the great astronomers, and studious observers of the Stars among the ancients. They rejected the doctrine of the Solid Spheres; and maintained, that the celestial regions were filled with a fluid ether, of too yielding a nature to carry along with it, by any motion of its own, bodies so immensely great as the Sun, Moon, and Five Planets. These, therefore, as well as the Fixed Stars, did not derive their motion from the circumambient body, but had each of them, in itself, and peculiar to itself, a vital principle of motion, which directed it to move with its own peculiar velocity, and its own peculiar direction. It was by this internal principle that the Fixed Stars revolved directly from east to west in circles parallel to the Equator, greater or less, according to their distance or nearness to the Poles, and with velocities so proportioned,350that each of them finished its diurnal period in the same time, in something less than twenty-three hours and fifty-six minutes. It was, by a principle of the same kind, that the Sun moved westward, for they allowed of no eastward motion in the heavens, but with less velocity than the Fixed Stars, so as to finish his diurnal period in twenty-four hours, and, consequently, to fall every day behind them, by a space of the heavens nearly equal to that which he passes over in four minutes; that is, nearly equal to a degree. This revolution of the Sun, too, was neither directly westwards, nor exactly circular; but after the Summer Solstice, his motion began gradually to decline a little southwards, appearing in his meridian to-day, further south than yesterday; and to-morrow still further south than to-day; and thus continuing every day to describe a spiral line round the Earth, which carried him gradually further and further southwards, till he arrived at the Winter Solstice. Here this spiral line began to change its direction, and to bring him gradually, every day, further and further northwards, till it again restored him to the Summer Solstice. In the same manner they accounted for the motion of the Moon, and that of the Five Planets, by supposing that each of them revolved westwards, but with directions and velocities, that were both different from one another, and continually varying; generally, however, in spherical lines, and somewhat inclined to the Equator.
This system seems never to have had the vogue. The system of Concentric as well as that of Eccentric Spheres gives some sort of reason, both for the constancy and equability of the motion of the Fixed Stars, and for the variety and uncertainty of that of the Planets. Each of them bestows some sort of coherence upon those apparently disjointed phenomena. But this other system seems to leave them pretty much as it found them. Ask a Stoic, why all the Fixed Stars perform their daily revolutions in circles parallel to each other, though of very different diameters, and with velocities so proportioned that they all finish their period at the same time, and through the whole course of it preserve the same distance and situation with regard to one another? He can give no other answer, but that the peculiar nature, or if one may say so, the caprice of each Star directs it to move in that peculiar manner. His system affords him no principle of connection, by which he can join together, in his imagination, so great a number of harmonious revolutions. But either of the other two systems, by the supposition of the solid firmament, affords this easily. He is equally at a loss to connect together the peculiarities that are observed in the motions of the other heavenly bodies; the spiral motion of them all; their alternate progression from north to south, and from south to north; the sometimes accelerated, and again retarded motions of the Sun and Moon; the direct retrograde and stationary appearances of the Planets. All these have, in his system,351no bond of union, but remain as loose and incoherent in the fancy, as they at first appeared to the senses, before philosophy had attempted, by giving them a new arrangement, by placing them at different distances, by assigning to each some peculiar but regular principle of motion, to methodize and dispose them into an order that should enable the imagination to pass as smoothly, and with as little embarrassment, along them, as along the most regular, most familiar, and most coherent appearances of nature.
Such were the systems of Astronomy that, in the ancient world, appear to have been adopted by any considerable party. Of all of them, the system of Eccentric Spheres was that which corresponded most exactly with the appearances of the heavens. It was not invented till after those appearances had been observed, with some accuracy, for more than a century together; and it was not completely digested by Ptolemy till the reign of Antoninus, after a much longer course of observations. We cannot wonder, therefore, that it was adapted to a much greater number of the phenomena, than either of the other two systems, which had been formed before those phenomena were observed with any degree of attention, which, therefore, could connect them together only while they were thus regarded in the gross, but which, it could not be expected, should apply to them when they came to be considered in the detail. From the time of Hipparchus, therefore, this system seems to have been pretty generally received by all those who attended particularly to the study of the heavens. That astronomer first made a catalogue of the Fixed Stars; calculated, for six hundred years, the revolutions of the Sun, Moon, and Five Planets; marked the places in the heavens, in which, during all that period, each of those bodies should appear; ascertained the times of the eclipses of the Sun and Moon, and the particular places of the Earth in which they should be visible. His calculations were founded upon this system, and as the events corresponded to his predictions, with a degree of accuracy which, though inferior to what Astronomy has since arrived at, was greatly superior to any thing which the world had then known, they ascertained, to all astronomers and mathematicians, the preference of his system, above all those which had been current before.
It was, however, to astronomers and mathematicians, only, that they ascertained this; for, notwithstanding the evident superiority of this system, to all those with which the world was then acquainted, it was never adopted by one sect of philosophers.
Philosophers, long before the days of Hipparchus, seem to have abandoned the study of nature, to employ themselves chiefly in ethical, rhetorical, and dialectical questions. Each party of them too, had by this time completed their peculiar system or theory of the universe, and no human consideration could then have induced them to give up any part of it. That supercilious and ignorant contempt too, with352which at this time they regarded all mathematicians, among whom they counted astronomers, seems even to have hindered them from enquiring so far into their doctrines as to know what opinions they held. Neither Cicero nor Seneca, who have so often occasion to mention the ancient systems of Astronomy, takes any notice of that of Hipparchus. His name is not to be found in the writings of Seneca. It is mentioned but once in those of Cicero, in a letter to Atticus, but without any note of approbation, as a geographer, and not as an astronomer. Plutarch, when he counts up, in his second book, concerning the opinions of philosophers, all the ancient systems of Astronomy, never mentions this, the only tolerable one which was known in his time. Those three authors, it seems, conversed only with the writings of philosophers. The elder Pliny, indeed, a man whose curiosity extended itself equally to every part of learning, describes the system of Hipparchus, and never mentions its author, which he has occasion to do often, without some note of that high admiration which he had so justly conceived for his merit. Such profound ignorance in those professed instructors of mankind, with regard to so important a part of the learning of their own times, is so very remarkable, that I thought it deserved to be taken notice of, even in this short account of the revolutions of the philosophy of the ancients.
Systems in many respects resemble machines. A machine is a little system, created to perform, as well as to connect together, in reality, those different movements and effects which the artist has occasion for. A system is an imaginary machine invented to connect together in the fancy those different movements and effects which are already in reality performed. The machines that are first invented to perform any particular movement are always the most complex, and succeeding artists generally discover that, with fewer wheels, with fewer principles of motion, than had originally been employed, the same effects may be more easily produced. The first systems, in the same manner, are always the most complex, and a particular connecting chain, or principle, is generally thought necessary to unite every two seemingly disjointed appearances: but it often happens, that one great connecting principle is afterwards found to be sufficient to bind together all the discordant phenomena that occur in a whole species of things. How many wheels are necessary to carry on the movements of this imaginary machine, the system of Eccentric Spheres! The westward diurnal revolution of the Firmament, whose rapidity carries all the other heavenly bodies along with it, requires one. The periodical eastward revolutions of the Sun, Moon, and Five Planets, require, for each of those bodies, another. Their differently accelerated and retarded motions require, that those wheels, or circles, should neither be concentric with the Firmament, nor with one another; which, more than any thing, seems to disturb the harmony of the universe. The353retrograde and stationary appearance of the Five Planets, as well as the extreme inconstancy of the Moon’s motion, require, for each of them, an Epicycle, another little wheel attached to the circumference of the great wheel, which still more interrupts the uniformity of the system. The motion of the apogeum of each of those bodies requires, in each of them, still another wheel, to carry the centres of their Eccentric Spheres round the centre of the Earth. And thus, this imaginary machine, though, perhaps, more simple, and certainly better adapted to the phenomena than the Fifty-six Planetary Spheres of Aristotle, was still too intricate and complex for the imagination to rest in it with complete tranquillity and satisfaction.
It maintained its authority, however, without any diminution of reputation, as long as science was at all regarded in the ancient world. After the reign of Antoninus, and, indeed, after the age of Hipparchus, who lived almost three hundred years before Antoninus, the great reputation which the earlier philosophers had acquired, so imposed upon the imaginations of mankind, that they seem to have despaired of ever equalling their renown. All human wisdom, they supposed, was comprehended in the writings of those elder sages. To abridge, to explain, and to comment upon them, and thus show themselves, at least, capable of understanding some of their sublime mysteries, became now the only road to reputation. Proclus and Theon wrote commentaries upon the system of Ptolemy; but, to have attempted to invent a new one, would then have been regarded, not only as presumption, but as impiety to the memory of their so much revered predecessors.
The ruin of the empire of the Romans, and, along with it, the subversion of all law and order, which happened a few centuries afterwards, produced the entire neglect of that study of the connecting principles of nature, to which leisure and security can alone give occasion. After the fall of those great conquerors and civilizers of mankind, the empire of the Caliphs seems to have been the first state under which the world enjoyed that degree of tranquillity which the cultivation of the sciences requires. It was under the protection of those generous and magnificent princes, that the ancient philosophy and astronomy of the Greeks were restored and established in the East; that tranquillity, which their mild, just, and religious government diffused over their vast empire, revived the curiosity of mankind, to inquire into the connecting principles of nature. Thefameof the Greek and Roman learning, which was then recent in the memories of men, made them desire to know, concerning these abstruse subjects, what were the doctrines of the so much renowned sages of those two nations.
They translated, therefore, into the Arabian language, and studied, with great eagerness, the works of many Greek philosophers, particularly of Aristotle, Ptolemy, Hippocrates, and Galen. The superiority which they easily discovered in them, above the rude essays which354their own nation had yet had time to produce, and which were such, we may suppose, as arise every where in the first infancy of science, necessarily determined them to embrace their systems, particularly that of Astronomy: neither were they ever afterwards able to throw off their authority. For, though the munificence of the Abassides, the second race of the Caliphs, is said to have supplied the Arabian astronomers with larger and better instruments than any that were known to Ptolemy and Hipparchus, the study of the sciences seems, in that mighty empire, to have been either of too short, or too interrupted a continuance, to allow them to make any considerable correction in the doctrines of those old mathematicians. The imaginations of mankind had not yet got time to grow so familiar with the ancient systems, as to regard them without some degree of that astonishment which their grandeur and novelty excited; a novelty of a peculiar kind, which had at once the grace of what was new, and the authority of what was ancient. They were still, therefore, too much enslaved to those systems, to dare to depart from them, when those confusions which shook, and at last overturned the peaceful throne of the Caliphs, banished the study of the sciences from that empire. They had, however, before this, made some considerable improvements: they had measured the obliquity of the Ecliptic, with more accuracy than had been done before. The tables of Ptolemy had, by the length of time, and by the inaccuracy of the observations upon which they were founded, become altogether wide of what was the real situation of the heavenly bodies, as he himself indeed had foretold they would do. It became necessary, therefore, to form new ones, which was accordingly executed by the orders of the Caliph Almamon, under whom, too, was made the first mensuration of the Earth that we know off, after the commencement of the Christian era, by two Arabian astronomers, who, in the plain of Sennaar, measured two degrees of its circumference.
The victorious arms of the Saracens carried into Spain the learning, as well as the gallantry, of the East; and along with it, the tables of Almamon, and the Arabian translations of Ptolemy and Aristotle; and thus Europe received a second time, from Babylon, the rudiments of the science of the heavens. The writings of Ptolemy were translated from Arabic into Latin; and the Peripatetic philosophy was studied in Averroes and Avicenna with as much eagerness and as much submission to its doctrines in the West, as it had been in the East.
The doctrine of the Solid Spheres had, originally, been invented, in order to give a physical account of the revolutions of the heavenly bodies, according to the system of Concentric Circles, to which that doctrine was very easily accommodated. Those mathematicians who invented the doctrine of Eccentric Circles and Epicycles, contented themselves with showing, how, by supposing the heavenly bodies to revolve in such orbits, the phenomena might be connected together,355and some sort of uniformity and coherence be bestowed upon their real motions. The physical causes of those motions they left to the consideration of the philosophers; though, as appears from some passages of Ptolemy, they had some general apprehension, that they were to be explained by a like hypothesis. But, though the system of Hipparchus was adopted by all astronomers and mathematicians, it never was received, as we have already observed, by any one sect of philosophers among the ancients. No attempt, therefore, seems to have been made amongst them, to accommodate to it any such hypothesis.
The schoolmen, who received, at once, from the Arabians, the philosophy of Aristotle, and the astronomy of Hipparchus, were necessarily obliged to reconcile them to one another, and to connect together the revolutions of the Eccentric Circles and Epicycles of the one, by the solid Spheres of the other. Many different attempts of this kind were made by many different philosophers: but, of them all, that of Purbach, in the fifteenth century, was the happiest and the most esteemed. Though his hypothesis is the simplest of any of them, it would be in vain to describe it without a scheme; neither is it easily intelligible with one; for, if the system of Eccentric Circles and Epicycles was before too perplexed and intricate for the imagination to rest in it with complete tranquillity and satisfaction, it became much more so, when this addition had been made to it. The world, justly indeed, applauded the ingenuity of that philosopher, who could unite, so happily, two such seemingly inconsistent systems. His labours, however, seem rather to have increased than to have diminished the causes of that dissatisfaction, which the learned soon began to feel with the system of Ptolemy. He, as well as all those who had worked upon the same plan before, by rendering this account of things more complex, rendered it more embarrassing than it had been before.
Neither was the complexness of this system the sole cause of the dissatisfaction, which the world in general began, soon after the days of Purbach, to express for it. The tables of Ptolemy having, upon account of the inaccuracy of the observations on which they were founded, become altogether wide of the real situation of the heavenly bodies, those of Almamon, in the ninth century, were, upon the same hypothesis, composed to correct their deviations. These again, a few ages afterwards, became, for the same reason, equally useless. In the thirteenth century, Alphonsus, the philosophical King of Castile, found it necessary to give orders for the composition of those tables, which bear his name. It is he, who is so well known for the whimsical impiety of using to say, that, had he been consulted at the creation of the universe, he could have given good advice; an apophthegm which is supposed to have proceeded from his dislike to the intricate system of Ptolemy. In the fifteenth century, the deviation of the Alphonsine tables began to be as sensible, as those of Ptolemy and356Almamon had been before. It appeared evident, therefore, that, though the system of Ptolemy might, in the main, be true, certain corrections were necessary to be made in it before it could be brought to correspond with exact precision to the phenomena. For the revolution of his Eccentric Circles and Epicycles, supposing them to exist, could not, it was evident, be precisely such as he represented them; since the revolutions of the heavenly bodies deviated, in a short time, so widely from what the most exact calculations, that were founded upon his hypothesis, represented them. It had plainly, therefore, become necessary to correct, by more accurate observations, both the velocities and directions of all the wheels and circles of which his hypothesis is composed. This, accordingly, was begun by Purbach, and carried on by Regiomontanus, the disciple, the continuator, and theperfecterof the system of Purbach; and one, whose untimely death, amidst innumerable projects for the recovery of old, and the invention and advancement of new sciences, is, even at this day, to be regretted.
When you have convinced the world, that an established system ought to be corrected, it is not very difficult to persuade them that it should be destroyed. Not long, therefore, after the death of Regiomontanus, Copernicus began to meditate a new system, which should connect together the new appearances, in a more simple as well as a more accurate manner, than that of Ptolemy.
The confusion, in which the old hypothesis represented the motions of the heavenly bodies, was, he tells us, what first suggested to him the design of forming a new system, that these, the noblest works of nature, might no longer appear devoid of that harmony and proportion which discover themselves in her meanest productions. What most of all dissatisfied him, was the notion of the Equalizing Circle, which, by representing the revolutions of the Celestial Spheres, as equable only, when surveyed from a point that was different from their centres, introduced a real inequality into their motions; contrary to that most natural, and indeed fundamental idea, with which all the authors of astronomical systems, Plato, Eudoxus, Aristotle, even Hipparchus and Ptolemy themselves, had hitherto set out, that the real motions of such beautiful and divine objects must necessarily be perfectly regular, and go on, in a manner, as agreeable to the imagination, as the objects themselves are to the senses. He began to consider, therefore, whether, by supposing the heavenly bodies to be arranged in a different order from that in which Aristotle and Hipparchus has placed them, this so much sought for uniformity might not be bestowed upon their motions. To discover this arrangement, he examined all the obscure traditions delivered down to us, concerning every other hypothesis which the ancients had invented, for the same purpose. He found, in Plutarch, that some old Pythagoreans had represented the Earth as revolving in the centre of the universe, like a wheel round its own axis; and that357others, of the same sect, had removed it from the centre, and represented it as revolving in the Ecliptic like a star round the central fire. By this central fire, he supposed they meant the Sun; and though in this he was very widely mistaken, it was, it seems, upon this interpretation, that he began to consider how such an hypothesis might be made to correspond to the appearances. The supposed authority of these old philosophers, if it did not originally suggest to him his system, seems, at least, to have confirmed him in an opinion, which, it is not improbable, that he had beforehand other reasons for embracing, notwithstanding what he himself would affirm to the contrary.
It then occurred to him, that, if the Earth was supposed to revolve every day round its axis, from west to east, all the heavenly bodies would appear to revolve, in a contrary direction, from east to west. The diurnal revolution of the heavens, upon this hypothesis, might be only apparent; the firmament, which has no other sensible motion, might be perfectly at rest; while the Sun, the Moon, and the Five Planets, might have no other movement beside that eastward revolution, which is peculiar to themselves. That, by supposing the Earth to revolve with the Planets, round the Sun, in an orbit, which comprehended within it the orbits of Venus and Mercury, but was comprehended within those of Mars, Jupiter, and Saturn, he could, without the embarrassment of Epicycles, connect together the apparent annual revolutions of the Sun, and the direct, retrograde, and stationary appearances of the Planets: that while the Earth really revolved round the Sun on one side of the heavens, the Sun would appear to revolve round the Earth on the other; that while she really advanced in her annual course, he would appear to advance eastward in that movement which is peculiar to himself. That, by supposing the axis of the Earth to be always parallel to itself, not to be quite perpendicular, but somewhat inclined to the plane of her orbit, and consequently to present to the Sun, the one pole when on the one side of him, and the other when on the other, he would account for the obliquity of the Ecliptic; the Sun’s seemingly alternate progression from north to south, and from south to north, the consequent change of the seasons, and different lengths of the days and nights in the different seasons.
If this new hypothesis thus connected together all these appearances as happily as that of Ptolemy, there were others which it connected together much better. The three superior Planets, when nearly in conjunction with the Sun, appear always at the greatest distance from the Earth, are smallest, and least sensible to the eye, and seem to revolve forward in their direct motion with the greatest rapidity. On the contrary, when in opposition to the Sun, that is, when in their meridian about midnight, they appear nearest the Earth, are largest, and most sensible to the eye, and seem to revolve backwards in their retrograde motion. To explain these appearances, the system of Ptolemy supposed358each of these Planets to be at the upper part of their several Epicycles, in the one case; and at the lower, in the other. But it afforded no satisfactory principle of connection, which could lead the mind easily to conceive how the Epicycles of those Planets, whose spheres were so distant from the sphere of the Sun, should thus, if one may say so, keep time to his motion. The system of Copernicus afforded this easily, and like a more simple machine, without the assistance of Epicycles, connected together, by fewer movements, the complex appearances of the heavens. When the superior Planets appear nearly in conjunction with the Sun, they are then in the side of their orbits, which is almost opposite to, and most distant from the Earth, and therefore appear smallest, and least sensible to the eye. But, as they then revolve in a direction which is almost contrary to that of the Earth, they appear to advance forward with double velocity; as a ship, that sails in a contrary direction to another, appears from that other, to sail both with its own velocity, and the velocity of that from which it is seen. On the contrary, when those Planets are in opposition to the Sun, they are on the same side of the Sun with the Earth, are nearest it, most sensible to the eye, and revolve in the same direction with it; but, as their revolutions round the Sun are slower than that of the Earth, they are necessarily left behind by it, and therefore seem to revolve backwards; as a ship which sails slower than another, though it sails in the same direction, appears from that other to sail backwards. After the same manner, by the same annual revolution of the Earth, he connected together the direct and retrograde motions of the two inferior Planets, as well as the stationary appearances of all the Five.
There are some other particular phenomena of the two inferior Planets, which correspond still better to this system, and still worse to that of Ptolemy. Venus and Mercury seem to attend constantly upon the motion of the Sun, appearing, sometimes on the one side, and sometimes on the other, of that great luminary; Mercury being almost always buried in his rays, and Venus never receding above forty-eight degrees from him, contrary to what is observed in the other three Planets, which are often seen in the opposite side of the heavens, at the greatest possible distance from the Sun. The system of Ptolemy accounted for this, by supposing that the centres of the Epicycles of these two Planets were always in the same line with those of the Sun and the Earth; that they appeared therefore in conjunction with the Sun, when either in the upper or lower part of their Epicycles, and at the greatest distance from him, when in the sides of them. It assigned, however, no reason why the Epicycles of these two Planets should observe so different a rule from that which takes place in those of the other three, nor for the enormous Epicycle of Venus, whose sides must have been forty-eight degrees distant from the Sun, while its centre was in conjunction with him, and whose diameter must have covered359more than a quadrant of the Great Circle. But how easily all these appearances coincide with the hypothesis, which represents those two inferior Planets revolving round the Sun in orbits comprehended within the orbit of the Earth, is too obvious to require an explanation.
Thus far did this new account of things render the appearances of the heavens more completely coherent than had been done by any of the former systems. It did this, too, by a more simple and intelligible, as well as more beautiful machinery. It represented the Sun, the great enlightener of the universe, whose body was alone larger than all the Planets taken together, as established immovable in the centre, shedding light and heat on all the worlds that circulated around him in one uniform direction, but in longer or shorter periods, according to their different distances. It took away the diurnal revolution of the firmament, whose rapidity, upon the old hypothesis, was beyond what even thought could conceive. It not only delivered the imagination from the embarrassment of Epicycles, but from the difficulty of conceiving these two opposite motions going on at the same time, which the system of Ptolemy and Aristotle bestowed upon all the Planets; I mean, their diurnal westward, and periodical eastward revolutions. The Earth’s revolution round its own axis took away the necessity for supposing the first, and the second was easily conceived when by itself. The Five Planets, which seem, upon all other systems, to be objects of a species by themselves, unlike to every thing to which the imagination has been accustomed, when supposed to revolve along with the Earth round the Sun, were naturally apprehended to be objects of the same kind with the Earth, habitable, opaque, and enlightened only by the rays of the Sun. And thus this hypothesis, by classing them in the same species of things, with an object that is of all others the most familiar to us, took off that wonder and that uncertainty which the strangeness and singularity of their appearance had excited; and thus far, too, better answered the great end of Philosophy.
Neither did the beauty and simplicity of this system alone recommend it to the imagination; the novelty and unexpectedness of that view of nature, which it opened to the fancy, excited more wonder and surprise than the strangest of those appearances, which it had been invented to render natural and familiar, and these sentiments still more endeared it. For, though it is the end of Philosophy, to allay that wonder, which either the unusual or seemingly disjointed appearances of nature excite, yet she never triumphs so much, as when, in order to connect together a few, in themselves, perhaps, inconsiderable objects, she has, if I may say so, created another constitution of things, more natural, indeed, and such as the imagination can more easily attend to, but more new, more contrary to common opinion and expectation, than any of those appearances themselves. As, in the instance before us, in order to connect together some seeming irregularities in the motions of360the Planets, the most inconsiderable objects in the heavens, and of which the greater part of mankind have no occasion to take any notice during the whole course of their lives, she has, to talk in the hyperbolical language of Tycho Brahe, moved the Earth from its foundations, stopped the revolution of the Firmament, made the Sun stand still, and subverted the whole order of the Universe.
Such were the advantages of this new hypothesis, as they appeared to its author, when he first invented it. But, though that love of paradox, so natural to the learned, and that pleasure, which they are so apt to take in exciting, by the novelties of their supposed discoveries, the amazement of mankind, may, notwithstanding what one of his disciples tells us to the contrary, have had its weight in prompting Copernicus to adopt this system; yet, when he had completed his Treatise of Revolutions, and began coolly to consider what a strange doctrine he was about to offer to the world, he so much dreaded the prejudice of mankind against it, that, by a species of continence, of all others the most difficult to a philosopher, he detained it in his closet for thirty years together. At last, in the extremity of old age, he allowed it to be extorted from him, but he died as soon as it was printed, and before it was published to the world.
When it appeared in the world, it was almost universally disapproved of, by the learned as well as by the ignorant. The natural prejudices of sense, confirmed by education, prevailed too much with both, to allow them to give it a fair examination. A few disciples only, whom he himself had instructed in his doctrine, received it with esteem and admiration. One of them, Reinholdus, formed, upon this hypothesis, larger and more accurate astronomical tables, than what accompanied the Treatise of Revolutions, in which Copernicus had been guilty of some errors in calculation. It soon appeared, that these Prutenic Tables, as they were called, corresponded more exactly with the heavens, than the Tables of Alphonsus. This ought naturally to have formed a prejudice in favour of the diligence and accuracy of Copernicus in observing the heavens. But it ought to have formed none in favour of his hypothesis; since the same observations, and the result of the same calculations, might have been accommodated to the system of Ptolemy, without making any greater alteration in that system than what Ptolemy had foreseen, and had even foretold should be made. It formed, however, a prejudice in favour of both, and the learned began to examine, with some attention, an hypothesis which afforded the easiest methods of calculation, and upon which the most exact predictions had been made. The superior degree of coherence, which it bestowed upon the celestial appearances, the simplicity and uniformity which it introduced into the real directions and velocities of the Planets, soon disposed many astronomers, first to favour, and at last to embrace a system, which thus connected together so happily,361the most disjointed of those objects that chiefly occupied their thoughts. Nor can any thing more evidently demonstrate, how easily the learned give up the evidence of their senses to preserve the coherence of the ideas of their imagination, than the readiness with which this, the most violent paradox in all philosophy, was adopted by many ingenious astronomers, notwithstanding its inconsistency with every system of physics then known in the world, and notwithstanding the great number of other more real objections, to which, as Copernicus left it, this account of things was most justly exposed.
It was adopted, however, nor can this be wondered at, by astronomers only. The learned in all other sciences, continued to regard it with the same contempt as the vulgar. Even astronomers were divided about its merit; and many of them rejected a doctrine, which not only contradicted the established system of Natural Philosophy, but which, considered astronomically only, seemed, to them, to labour under several difficulties.
Some of the objections against the motion of the Earth, that were drawn from the prejudices of sense, the patrons of this system, indeed, easily enough got over. They represented, that the Earth might really be in motion, though, to its inhabitants, it seemed to be at rest; and that the Sun and Fixed Stars might really be at rest, though from the Earth they seemed to be in motion; in the same manner as a ship, which sails through a smooth sea, seems to those who are in it, to be at rest, though really in motion; while the objects which she passes along, seem to be in motion, though really at rest.
But there were some other objections, which, though grounded upon the same natural prejudices, they found it more difficult to get over. The earth had always presented itself to the senses, not only as at rest, but as inert, ponderous, and even averse to motion. The imagination had always been accustomed to conceive it as such, and suffered the greatest violence, when obliged to pursue, and attend it, in that rapid motion which the system of Copernicus bestowed upon it. To enforce their objection, the adversaries of this hypothesis were at pains to calculate the extreme rapidity of this motion. They represented, that the circumference of the Earth had been computed to be above twenty-thousand miles: if the Earth, therefore, was supposed to revolve every day round its axis, every point of it near the equator would pass over above twenty-three thousand miles in a day; and consequently, near a thousand miles in an hour, and about sixteen miles in a minute; a motion more rapid than that of a cannon ball, or even than the swifter progress of sound. The rapidity of its periodical revolution was yet more violent than that of its diurnal rotation. How, therefore, could the imagination ever conceive so ponderous a body to be naturally endowed with so dreadful a movement? The Peripatetic Philosophy, the only philosophy then known in the world, still further confirmed362this prejudice. That philosophy, by a very natural, though, perhaps, groundless distinction, divided all motion into Natural and Violent. Natural motion was that which flowed from an innate tendency in the body, as when a stone fell downwards: Violent motion, that which arose from external force, and which was, in some measure, contrary to the natural tendency of the body, as when a stone was thrown upwards, or horizontally. No violent motion could be lasting; for, being constantly weakened by the natural tendency of the body, it would soon be destroyed. The natural motion of the Earth, as was evident in all its parts, was downwards, in a straight line to the centre; as that of fire and air was upwards, in a straight line from the centre. It was the heavens only that revolved naturally in a circle. Neither, therefore, the supposed revolution of the Earth round its own centre, nor that round the Sun, could be natural motions; they must therefore be violent, and consequently could be of no long continuance. It was in vain that Copernicus replied, that gravity was, probably, nothing else besides a tendency in the different parts of the same Planet, to unite themselves to one another; that this tendency took place, probably, in the parts of the other Planets, as well as in those of the Earth; that it could very well be united with a circular motion; that it might be equally natural to the whole body of the Planet, and to every part of it; that his adversaries themselves allowed, that a circular motion was natural to the heavens, whose diurnal revolution was infinitely more rapid than even that motion which he had bestowed upon the Earth; that though a like motion was natural to the Earth, it would still appear to be at rest to its inhabitants, and all the parts of it to tend in a straight line to the centre, in the same manner as at present. But this answer, how satisfactory soever it may appear to be now, neither did nor could appear to be satisfactory then. By admitting the distinction betwixt natural and violent motions, it was founded upon the same ignorance of mechanical principles with the objection. The systems of Aristotle and Hipparchus supposed, indeed, the diurnal motion of the heavenly bodies to be infinitely more rapid than even that dreadful movement which Copernicus bestowed upon the Earth. But they supposed, at the same time, that those bodies were objects of a quite different species, from any we are acquainted with, near the surface of the Earth, and to which, therefore, it was less difficult to conceive that any sort of motion might be natural. Those objects, besides, had never presented themselves to the senses, as moving otherwise, or with less rapidity, than these systems represented them. The imagination, therefore, could feel no difficulty in following a representation which the senses had rendered quite familiar to it. But when the Planets came to be regarded as so many Earths, the case was quite altered. The imagination had been accustomed to conceive such objects as tending rather to rest than motion; and this idea of their natural363inertness, encumbered, if one may say so, and clogged its flight whenever it endeavoured to pursue them in their periodical courses, and to conceive them as continually rushing through the celestial spaces, with such violent and unremitting rapidity.
Nor were the first followers of Copernicus more fortunate in their answers to some other objections, which were founded indeed in the same ignorance of the laws of motion, but which, at the same time, were necessarily connected with that way of conceiving things, which then prevailed universally in the learned world.
If the earth, it was said, revolved so rapidly from west to east, a perpetual wind would set in from east to west, more violent than what blows in the greatest hurricanes; a stone, thrown westwards would fly to a much greater distance than one thrown with the same force eastwards; as what moved in a direction, contrary to the motion of the Earth, would necessarily pass over a greater portion of its surface, than what, with the same velocity, moved along with it. A ball, it was said, dropped from the mast of a ship under sail, does not fall precisely at the foot of the mast, but behind it; and in the same manner, a stone dropped from a high tower would not, upon the supposition of the Earth’s motion, fall precisely at the bottom of the tower, but west of it, the Earth being, in the mean time, carried away eastward from below it. It is amusing to observe, by what, subtile and metaphysical evasions the followers of Copernicus endeavoured to elude this objection, which before the doctrine of the Composition of Motion had been explained by Galileo, was altogether unanswerable. They allowed, that a ball dropped from the mast of a ship under sail would not fall at the foot of the mast, but behind it; because the ball, they said, was no part of the ship, and because the motion of the ship was natural neither to itself nor to the ball. But the stone was a part of the earth, and the diurnal and annual revolutions of the Earth were natural to the whole, and to every part of it, and therefore to the stone. The stone, therefore, having naturally the same motion with the Earth, fell precisely at the bottom of the tower. But this answer could not satisfy the imagination, which still found it difficult to conceive how these motions could be natural to the earth; or how a body, which had always presented itself to the senses as inert, ponderous, and averse to motion, should naturally be continually wheeling about both its own axis and the Sun, with such violent rapidity. It was, besides, argued by Tycho Brahe, upon the principles of the same philosophy which had afforded both the objection and the answer, that even upon the supposition, that any such motion was natural to the whole body of the Earth, yet the stone, which was separated from it, could no longer be actuated by that motion. The limb, which is cut off from an animal, loses those animal motions which were natural to the whole. The branch, which is cut off from the trunk, loses that vegetative364motion which is natural to the whole tree. Even the metals, minerals, and stones, which were dug out from the bosom of the Earth, lose those motions which occasioned their production and increase, and which were natural to them in their original state. Though the diurnal and annual motion of the Earth, therefore, had been natural to them while they were contained in its bosom, it could no longer be so when they were separated from it.