The Essays of Adam Smith

by Adam Smith

Nothing now embarrassed the system of Copernicus, but the difficulty which the imagination felt in conceiving bodies so immensely ponderous as the Earth and the other Planets revolving round the Sun with such incredible rapidity. It was in vain that Copernicus pretended, that, notwithstanding the prejudices of sense, this circular motion might be as natural to the Planets, as it is to a stone to fall to the ground. The imagination had been accustomed to conceive such 373 objects as tending rather to rest than motion. This habitual idea of their natural inertness was incompatible with that of their natural motion. It was in vain that Kepler, in order to assist the fancy in connecting together this natural inertness with their astonishing velocities, talked of some vital and immaterial virtue, which was shed by the Sun into the surrounding spaces, which was whirled about with his revolution round his own axis, and which, taking hold of the Planets, forced them, in spite of their ponderousness and strong propensity to rest, thus to whirl about the centre of the system. The imagination had no hold of this immaterial virtue, and could form no determinate idea of what it consisted in. The imagination, indeed, felt a gap, or interval, betwixt the constant motion and the supposed inertness of the Planets, and had in this, as in all other cases, some general idea or apprehension that there must be a connecting chain of intermediate objects to link together these discordant qualities. Wherein this connecting chain consisted, it was, indeed, at a loss to conceive; nor did the doctrine of Kepler lend it any assistance in this respect. That doctrine, like almost all those of the philosophy in fashion during his time, bestowed a name upon this invisible chain, called it an immaterial virtue, but afforded no determinate idea of what was its nature.

Des Cartes was the first who attempted to ascertain, precisely, wherein this invisible chain consisted, and to afford the imagination a train of intermediate events, which, succeeding each other in an order that was of all others the most familiar to it, should unite those incoherent qualities, the rapid motion, and the natural inertness of the Planets. Des Cartes was the first who explained wherein consisted the real inertness of matter; that it was not in an aversion to motion, or in a propensity to rest, but in a power of continuing indifferently either at rest of in motion, and of resisting, with a certain force, whatever endeavoured to change its state from the one to the other. According to that ingenious and fanciful philosopher, the whole of infinite space was full of matter, for with him matter and extension were the same, and consequently there could be no void. This immensity of matter, he supposed to be divided into an infinite number of very small cubes; all of which, being whirled about upon their own centres, necessarily gave occasion to the production of two different elements. The first consisted of those angular parts, which, having been necessarily rubbed off, and grinded yet smaller by their mutual friction, constituted the most subtle and movable part of matter. The second consisted of those little globules that were formed by the rubbing off of the first. The interstices betwixt these globules of the second element was filled up by the particles of the first. But in the infinite collisions, which must occur in an infinite space filled with matter, and all in motion, it must necessarily happen that many of the globules of the second element should be broken and grinded down into the first. The quantity 374 of the first element having been thus increased beyond what was sufficient to fill up the interstices of the second, it must, in many places, have been heaped up together, without any mixture of the second along with it. Such, according to Des Cartes, was the original division of matter. Upon this infinitude of matter thus divided, a certain quantity of motion was originally impressed by the Creator of all things, and the laws of motion were so adjusted as always to preserve the same quantity in it, without increase, and without diminution. Whatever motion was lost by one part of matter, was communicated to some other; and whatever was acquired by one part of matter, was derived from some other: and thus, through an eternal revolution, from rest to motion, and from motion to rest, in every part of the universe, the quantity of motion in the whole was always the same.

But, as there was no void, no one part of matter could be moved without thrusting some other out of its place, nor that without thrusting some other, and so on. To avoid, therefore, an infinite progress, he supposed that the matter which any body pushed before it, rolled immediately backwards, to supply the place of that matter which flowed in behind it; and as we may observe in the swimming of a fish, that the water which it pushes before it, immediately rolls backward, to supply the place of what flows in behind it, and thus forms a small circle or vortex round the body of the fish. It was, in the same manner, that the motion originally impressed by the Creator upon the infinitude of matter, necessarily produced in it an infinity of greater and smaller vortices, or circular streams: and the law of motion being so adjusted as always to preserve the same quantity of motion in the universe, those vortices either continued for ever, or by their dissolution gave birth to others of the same kind. There was, thus, at all times, an infinite number of greater and smaller vortices, or circular streams, revolving in the universe.

But, whatever moves in a circle, is constantly endeavouring to fly off from the centre of its revolution. For the natural motion of all bodies is in a straight line. All the particles of matter, therefore, in each of those greater vortices, were continually pressing from the centre to the circumference, with more or less force, according to the different degrees of their bulk and solidity. The larger and more solid globules of the second element forced themselves upwards to the circumference, while the smaller, more yielding, and more active particles of the first, which could flow, even through the interstices of the second, were forced downwards to the centre. They were forced downwards to the centre, notwithstanding their natural tendency was upwards to the circumference; for the same reason that a piece of wood, when plunged in water, is forced upwards to the surface, notwithstanding its natural tendency is downwards to the bottom; because its tendency downwards is less strong than that of the particles of water, which, therefore, 375 if one may say so, press in before it, and thus force it upwards. But there being a greater quantity of the first element than what was necessary to fill up the interstices of the second, it was necessarily accumulated in the centre of each of these great circular streams, and formed there the fiery and active substance of the Sun. For, according to that philosopher, the Solar Systems were infinite in number, each Fixed Star being the centre of one: and he is among the first of the moderns, who thus took away the boundaries of the Universe; even Copernicus and Kepler, themselves, having confined it within, what they supposed, to be the vault of the Firmament.

The centre of each vortex being thus occupied by the most active and movable parts of matter, there was necessarily among them, a more violent agitation than in any other part of the vortex, and this violent agitation of the centre cherished and supported the movement of the whole. But, among the particles of the first element, which fill up the interstices of the second, there are many, which, from the pressure of the globules on all sides of them, necessarily receive an angular form, and thus constitute a third element of particles less fit for motion than those of the other two. As the particles, however, of this third element were formed in the interstices of the second, they are necessarily smaller than those of the second, and are, therefore, along with those of the first, urged down towards the centre, where, when a number of them happen to take hold of one another, they form such spots upon the surface of the accumulated particles of the first element, as are often discovered by telescopes upon the face of that Sun which enlightens and animates our particular system. Those spots are often broken and dispelled, by the violent agitation of the particles of the first element, as has hitherto happily been the case with those which have successively been formed upon the face of our Sun. Sometimes, however, they encrust the whole surface of that fire which is accumulated in the centre; and the communication betwixt the most active and the most inert parts of the vortex being thus interrupted, the rapidity of its motion immediately begins to languish, and can no longer defend it from being swallowed up and carried away by the superior violence of some other like circular stream; and in this manner, what was once a Sun, becomes a Planet. Thus, the time was, according to this system, when the Moon was a body of the same kind with the Sun, the fiery centre of a circular stream of ether, which flowed continually round her; but her face having been crusted over by a congeries of angular particles, the motion of this circular stream began to languish, and could no longer defend itself from being absorbed by the more violent vortex of the Earth, which was then, too, a Sun, and which chanced to be placed in its neighbourhood. The Moon, therefore, became a Planet, and revolved round the Earth. In process of time, the same fortune, which had thus befallen the Moon, befell also 376 the Earth; its face was encrusted by a gross and inactive substance; the motion of its vortex began to languish, and it was absorbed by the greater vortex of the Sun: but though the vortex of the Earth had thus become languid, it still had force enough to occasion both the diurnal revolution of the Earth, and the monthly motion of the Moon. For a small circular stream may easily be conceived as flowing round the body of the Earth, at the same time that it is carried along by that great ocean of ether which is continually revolving round the Sun; in the same manner, as in a great whirlpool of water, one may often see several small whirlpools, which revolve round centres of their own, and at the same time are carried round the centre of the great one. Such was the cause of the original formation and consequent motions of the Planetary System. When a solid body is turned round its centre, those parts of it, which are nearest, and those which are remotest from the centre, complete their revolutions in one and the same time. But it is otherwise with the revolutions of a fluid; the parts of it which are nearest the centre complete their revolutions in a shorter time, than those which are remoter. The Planets, therefore, all floating, in that immense tide of ether which is continually setting in from west to east round the body of the Sun, complete their revolutions in a longer or a shorter time, according to their nearness or distance from him. There was, however, according to Des Cartes, no very exact proportion observed betwixt the times of their revolutions and their distances from the centre. For that nice analogy, which Kepler had discovered betwixt them, having not yet been confirmed by the observations of Cassini, was, as I before took notice, entirely disregarded by Des Cartes. According to him, too, their orbits might not be perfectly circular, but be longer the one way than the other, and thus approach to an Ellipse. Nor yet was it necessary to suppose, that they described this figure with geometrical accuracy, or even that they described always precisely the same figure. It rarely happens, that nature can be mathematically exact with regard to the figure of the objects she produces, upon account of the infinite combinations of impulses, which must conspire to the production of each of her effects. No two Planets, no two animals of the same kind, have exactly the same figure, nor is that of any one of them perfectly regular. It was in vain, therefore, that astronomers laboured to find that perfect constancy and regularity in the motions of the heavenly bodies, which is to be found in no other parts of nature. These motions, like all others, must either languish or be accelerated, according as the cause which produces them, the revolution of the vortex of the Sun, either languishes, or is accelerated; and there are innumerable events which may occasion either the one or the other of those changes.

It was thus, that Des Cartes endeavoured to render familiar to the imagination, the greatest difficulty in the Copernican system, the rapid 377 motion of the enormous bodies of the Planets. When the fancy had thus been taught to conceive them as floating in an immense ocean of ether, it was quite agreeable to its usual habits to conceive, that they should follow the stream of this ocean, how rapid soever. This was an order of succession to which it had been long accustomed, and with which it was, therefore, quite familiar. This account, too, of the motions of the Heavens, was connected with a vast, an immense system, which joined together a greater number of the most discordant phenomena of nature, than had been united by any other hypothesis; a system in which the principles of connection, though perhaps equally imaginary, were, however, more distinct and determinate, than any that had been known before; and which attempted to trace to the imagination, not only the order of succession by which the heavenly bodies were moved, but that by which they, and almost all other natural objects, had originally been produced.—The Cartesian philosophy begins now to be almost universally rejected, whilst the Copernican system continues to be universally received. Yet it is not easy to imagine, how much probability and coherence this admired system was long supposed to derive from that exploded hypothesis. Till Des Cartes had published his principles, the disjointed and incoherent system of Tycho Brahe, though it was embraced heartily and completely by scarce any body, was yet constantly talked of by all the learned, as, in point of probability, upon a level with Copernicus. They took notice, indeed, of its inferiority with regard to coherence and connection, expressing hopes, however, that these defects might be remedied by some future improvements. But when the world beheld that complete, and almost perfect coherence, which the philosophy of Des Cartes bestowed upon the system of Copernicus, the imaginations of mankind could no longer refuse themselves the pleasure of going along with so harmonious an account of things. The system of Tycho Brahe was every day less and less talked of, till at last it was forgotten altogether.

The system of Des Cartes, however, though it connected together the real motions of the heavenly bodies according to the system of Copernicus, more happily than had been done before, did so only when they were considered in the gross; but did not apply to them, when they were regarded in the detail. Des Cartes, as was said before, had never himself observed the Heavens with any particular application. Though he was not ignorant, therefore, of any of the observations which had been made before his time, he seems to have paid them no great degree of attention; which, probably, proceeded from his own inexperience in the study of Astronomy. So far, therefore, from accommodating his system to all the minute irregularities, which Kepler had ascertained in the movements of the Planets; or from showing, particularly, how these irregularities, and no other, should arise from it, he contented himself with observing, that perfect uniformity could not 378 be expected in their motions, from the nature of the causes which produced them; that certain irregularities might take place in them, for a great number of successive revolutions, and afterwards gave way to others of a different kind: a remark which, happily, relieved him from the necessity of applying his system to the observations of Kepler, and the other Astronomers.

But when the observations of Cassini had established the authority of those laws, which Kepler had first discovered in the system, the philosophy of Des Cartes, which could afford no reason why such particular laws should be observed, might continue to amuse the learned in other sciences, but could no longer satisfy those that were skilled in Astronomy. Sir Isaac Newton first attempted to give a physical account of the motions of the Planets, which should accommodate itself to all the constant irregularities which astronomers had ever observed in their motions. The physical connection, by which Des Cartes had endeavoured to bind together the movements of the Planets, was the laws of impulse; of all the orders of succession, those which are most familiar to the imagination; as they all flow from the inertness of matter. After this quality, there is no other with which we are so well acquainted as that of gravity. We never act upon matter, but we have occasion to observe it. The superior genius and sagacity of Sir Isaac Newton, therefore, made the most happy, and, we may now say, the greatest and most admirable improvement that was ever made in philosophy, when he discovered, that he could join together the movements of the Planets by so familiar a principle of connection, which completely removed all the difficulties the imagination had hitherto felt in attending to them. He demonstrated, that, if the Planets were supposed to gravitate towards the Sun, and to one another, and at the same time to have had a projecting force originally impressed upon them, the primary ones might all describe ellipses in one of the foci of which that great luminary was placed; and the secondary ones might describe figures of the same kind round their respective primaries, without being disturbed by the continual motion of the centres of their revolutions. That if the force, which retained each of them in their orbits, was like that of gravity, and directed towards the Sun, they would, each of them, describe equal areas in equal times. That if this attractive power of the Sun, like all other qualities which are diffused in rays from a centre, diminished in the same proportion as the squares of the distances increased, their motions would be swiftest when nearest the Sun, and slowest when farthest off from him, in the same proportion in which, by observation, they are discovered to be; and that upon the same supposition, of this gradual diminution of their respective gravities, their periodic times would bear the same proportion to their distances, which Kepler and Cassini had established betwixt them. Having thus shown, that gravity might be the 379 connecting principle which joined together the movements of the Planets, he endeavoured next to prove that it really was so. Experience shows us, what is the power of gravity near the surface of the Earth. That it is such as to make a body fall, in the first second of its descent, through about fifteen Parisian feet. The Moon is about sixty semidiameters of the Earth distant from its surface. If gravity, therefore, was supposed to diminish, as the squares of the distance increase, a body, at the Moon, would fall towards the Earth in a minute; that is, in sixty seconds, through the same space, which it falls near its surface in one second. But the arch which the Moon describes in a minute, falls, by observation, about fifteen Parisian feet below the tangent drawn at the beginning of it. So far, therefore, the Moon may be conceived as constantly falling towards the Earth.

The system of Sir Isaac Newton corresponded to many other irregularities which Astronomers had observed in the Heavens. It assigned a reason, why the centres of the revolutions of the Planets were not precisely in the centre of the Sun, but in the common centre of gravity of the Sun and the Planets. From the mutual attraction of the Planets, it gave a reason for some other irregularities in their motions; irregularities, which are quite sensible in those of Jupiter and Saturn, when those Planets are nearly in conjunction with one another. But of all the irregularities in the Heavens, those of the Moon had hitherto given the greatest perplexity to Astronomers; and the system of Sir Isaac Newton corresponded, if possible, yet more accurately with them than with any of the other Planets. The Moon, when either in conjunction, or in opposition to the Sun, appears furthest from the Earth, and nearest to it when in her quarters. According to the system of that philosopher, when she is in conjunction with the Sun, she is nearer the Sun than the Earth is; consequently, more attracted to him, and, therefore, more separated from the Earth. On the contrary, when in opposition to the Sun, she is further from the Sun than the Earth. The Earth, therefore, is more attracted to the Sun: and consequently, in this case, too, further separated from the Moon. But, on the other hand, when the Moon is in her quarters, the Earth and the Moon, being both at equal distance from the Sun, are equally attracted to him. They would not, upon this account alone, therefore, be brought nearer to one another. As it is not in parallel lines however that they are attracted towards the Sun, but in lines which meet in his centre, they are, thereby, still further approached to one another. Sir Isaac Newton computed the difference of the forces with which the Moon and the Earth ought, in all those different situations, according to his theory, to be impelled towards one another; and found, that the different degrees of their approaches, as they had been observed by Astronomers, corresponded exactly to his computations. As the attraction of the Sun, in the conjunctions and oppositions, diminishes the gravity of 380 the Moon towards the Earth, and, consequently, makes her necessarily extend her orbit, and, therefore, require a longer periodical time to finish it. But, when the Moon and the Earth are in that part of the orbit which is nearest the Sun, this attraction of the Sun will be the greatest; consequently, the gravity of the Moon towards the Earth will there be most diminished; her orbit be most extended; and her periodic time be, therefore, the longest. This is, also, agreeable to experience, and in the very same proportion, in which, by computation, from these principles, it might be expected.

The orbit of the Moon is not precisely in the same Plane with that of the Earth; but makes a very small angle with it. The points of intersection with those two Planes, are called, the Nodes of the Moon. These Nodes of the Moon are in continual motion, and in eighteen or nineteen years, revolve backwards, from east to west, through all the different points of the Ecliptic. For the Moon, after having finished her periodical revolution, generally intersects the orbit of the Earth somewhat behind the point where she had intersected it before. But, though the motion of the Nodes is thus generally retrograde, it is not always so, but is sometimes direct, and sometimes they appear even stationary; the Moon generally intersects the Plane of the Earth’s orbit behind the point where she had intersected it in her former revolution; but she sometimes intersects it before that point, and sometimes in the very same point. It is the situation of those Nodes which determines the times of Eclipses, and their motions had, upon this account, at all times, been particularly attended to by Astronomers. Nothing, however, had perplexed them more, than to account for these so inconsistent motions, and, at the same time, preserve their so much sought-for regularity in the revolutions of the Moon. For they had no other means of connecting the appearances together than by supposing the motions which produced them, to be, in reality, perfectly regular and equable. The history of Astronomy, therefore, gives an account of a greater number of theories invented for connecting together the motions of the Moon, than for connecting together those of all the other heavenly bodies taken together. The theory of gravity, connected together, in the most accurate manner, by the different actions of the Sun and the Earth, all those irregular motions; and it appears, by calculation, that the time, the quantity, and the duration of those direct and retrograde motions of the Nodes, as well as of their stationary appearances, might be expected to be exactly such, as the observations of Astronomers have determined them.