The Essays of Adam Smith

by Adam Smith

The same principle, the attraction of the Sun, which thus accounts for the motions of the Nodes, connects, too, another very perplexing irregularity in the appearances of the Moon; the perpetual variation in the inclination of her orbit to that of the Earth.

As the Moon revolves in an ellipse, which has the centre of the 381 Earth in one of its foci, the longer axis of its orbit is called the Line of its Apsides. This line is found, by observation, not to be always directed towards the same points of the Firmament, but to revolve forwards from west to east, so as to pass through all the points of the Ecliptic, and to complete its period in about nine years; another irregularity, which had very much perplexed Astronomers, but which the theory of gravity sufficiently accounted for.

The Earth had hitherto been regarded as perfectly globular, probably for the same reason which had made men imagine, that the orbits of the Planets must necessarily be perfectly circular. But Sir Isaac Newton, from mechanical principles, concluded, that, as the parts of the Earth must be more agitated by her diurnal revolution at the Equator, than at the Poles, they must necessarily be somewhat elevated at the first, and flattened at the second. The observation, that the oscillations of pendulums were slower at the Equator than at the Poles, seeming to demonstrate, that gravity was stronger at the Poles, and weaker at the Equator, proved, he thought, that the Equator was further from the centre than the Poles. All the measures, however, which had hitherto been made of the Earth, seemed to show the contrary, that it was drawn out towards the Poles, and flattened towards the Equator. Newton, however, preferred his mechanical computations to the former measures of Geographers and Astronomers; and in this he was confirmed by the observations of Astronomers on the figure of Jupiter, whose diameter at the Pole seems to be to his diameter at the Equator, as twelve to thirteen; a much greater inequality than could be supposed to take place betwixt the correspondent diameters of the Earth, but which was exactly proportioned to the superior bulk of Jupiter, and the superior rapidity with which he performs his diurnal revolutions. The observations of Astronomers at Lapland and Peru have fully confirmed Sir Isaac’s system, and have not only demonstrated, that the figure of the Earth is, in general, such as he supposed it; but that the proportion of its axis to the diameter of its Equator is almost precisely such as he had computed it. And of all the proofs that have ever been adduced of the diurnal revolution of the Earth, this perhaps is the most solid and most satisfactory.

Hipparchus, by comparing his own observations with those of some former Astronomers, had found that the equinoctial points were not always opposite to the same part of the Heavens, but that they advanced gradually eastward by so slow a motion, as to be scarce sensible in one hundred years, and which would require thirty-six thousand to make a complete revolution of the Equinoxes, and to carry them successively through all the different points of the Ecliptic. More accurate observations discovered that this procession of the Equinoxes was not so slow as Hipparchus had imagined it, and that it required somewhat less than twenty-six thousand years to give them a complete 382 revolution. While the ancient system of Astronomy, which represented the Earth as the immovable centre of the universe, took place, this appearance was necessarily accounted for, by supposing that the Firmament, besides its rapid diurnal revolution round the poles of the Equator, had likewise a slow periodical one round those of the Ecliptic. And when the system of Hipparchus was by the schoolmen united with the solid Spheres of Aristotle, they placed a new crystalline Sphere above the Firmament, in order to join this motion to the rest. In the Copernican system, this appearance had hitherto been connected with the other parts of that hypothesis, by supposing a small revolution in the Earth’s axis from east to west. Sir Isaac Newton connected this motion by the same principle of gravity, by which he had united all the others, and showed, how the elevation of the parts of the Earth at the Equator must, by the attraction of the Sun, produce the same retrograde motion of the Nodes of the Ecliptic, which it produced of the Nodes of the Moon. He computed the quantity of motion which could arise from this action of the Sun, and his calculations here too corresponded with the observations of Astronomers.

Comets have hitherto, of all the appearances in the Heavens, been the least attended to by Astronomers. The rarity and inconstancy of their appearance, seemed to separate them entirely from the constant, regular, and uniform objects in the Heavens, and to make them resemble more the inconstant, transitory, and accidental phenomena of those regions that are in the neighbourhood of the Earth. Aristotle, Eudoxus, Hipparchus, Ptolemy, and Purbach, therefore, had all degraded them below the Moon, and ranked them among the meteors of the upper regions of the air. The observations of Tycho Brahe demonstrated, that they ascended into the celestial regions, and were often higher than Venus or the Sun. Des Cartes, at random, supposed them to be always higher than even the orbit of Saturn; and seems, by the superior elevation he thus bestowed upon them, to have been willing to compensate that unjust degradation which they had suffered for so many ages before. The observations of some later Astronomers demonstrated, that they too revolved about the Sun, and might therefore be parts of the Solar System. Newton accordingly applied his mechanical principle of gravity to explain the motions of these bodies. That they described equal areas in equal times, had been discovered by the observations of some later Astronomers; and Newton endeavoured to show how from this principle, and those observations, the nature and position of their several orbits might be ascertained, and their periodic times determined. His followers have, from his principles, ventured even to predict the returns of several of them, particularly of one which is to make its appearance in 1758.[1] We must wait for that time 383 before we can determine, whether his philosophy corresponds as happily to this part of the system as to all the others. In the meantime, however, the ductility of this principle, which applied itself so happily to these, the most irregular of all the celestial appearances, and which has introduced such complete coherence into the motions of all the Heavenly Bodies, has served not a little to recommend it to the imaginations of mankind.

[¹] It must be observed, that the whole of this Essay was written previous to the date here mentioned; and that the return of the comet happened agreeably to the prediction.

But of all the attempts of the Newtonian philosophy, that which would appear to be the most above the reach of human reason and experience, is the attempt to compute the weights and densities of the Sun, and of the several Planets. An attempt, however, which was indispensably necessary to complete the coherence of the Newtonian system. The power of attraction which, according to the theory of gravity, each body possesses, is in proportion to the quantity of matter contained in that body. But the periodic time in which one body, at a given distance, revolves round another that attracts it, is shorter in proportion as this power is greater, and consequently as the quantity of matter in the attracting body. If the densities of Jupiter and Saturn were the same with that of the Earth, the periodic times of their several Satellites would be shorter than by observation they are found to be. Because the quantity of matter, and consequently the attracting power of each of them, would be as the cubes of their diameters. By comparing the bulks of those Planets, and the periodic times of their Satellites, it is found that, upon the hypothesis of gravity, the density of Jupiter must be greater than that of Saturn, and the density of the Earth greater than that of Jupiter. This seems to establish it as a law in the system, that the nearer the several Planets approach to the Sun, the density of their matter is the greater: a constitution of things which seems to be the most advantageous of any that could have been established; as water of the same density with that of our Earth, would freeze under the Equator of Saturn, and boil under that of Mercury.

Such is the system of Sir Isaac Newton, a system whose parts are all more strictly connected together, than those of any other philosophical hypothesis. Allow his principle, the universality of gravity, and that it decreases as the squares of the distance increase, and all the appearances, which he joins together by it, necessarily follow. Neither is their connection merely a general and loose connection, as that of most other systems, in which either these appearances, or some such like appearances, might indifferently have been expected. It is everywhere the most precise and particular that can be imagined, and ascertains the time, the place, the quantity, the duration of each individual phenomenon, to be exactly such as, by observation, they have been determined to be. Neither are the principles of union, which it employs, such as the imagination can find any difficulty in going along with. The gravity of matter is, of all its qualities, after its inertness, 384 that which is most familiar to us. We never act upon it without having occasion to observe this property. The law too, by which it is supposed to diminish as it recedes from its centre, is the same which takes place in all other qualities which are propagated in rays from a centre, in light, and in every thing else of the same kind. It is such, that we not only find that it does take place in all such qualities, but we are necessarily determined to conceive that, from the nature of the thing, it must take place. The opposition which was made in France, and in some other foreign nations, to the prevalence of this system, did not arise from any difficulty which mankind naturally felt in conceiving gravity as an original and primary mover in the constitution of the universe. The Cartesian system, which had prevailed so generally before it, had accustomed mankind to conceive motion as never beginning, but in consequence of impulse, and had connected the descent of heavy bodies, near the surface of the Earth, and the other Planets, by this more general bond of union; and it was the attachment the world had conceived for this account of things, which indisposed them to that of Sir Isaac Newton. His system, however, now prevails over all opposition, and has advanced to the acquisition of the most universal empire that was ever established in philosophy. His principles, it must be acknowledged, have a degree of firmness and solidity that we should in vain look for in any other system. The most sceptical cannot avoid feeling this. They not only connect together most perfectly all the phenomena of the Heavens, which had been observed before his time; but those also which the persevering industry and more perfect instruments of later Astronomers have made known to us have been either easily and immediately explained by the application of his principles, or have been explained in consequence of more laborious and accurate calculations from these principles, than had been instituted before. And even we, while we have been endeavouring to represent all philosophical systems as mere inventions of the imagination, to connect together the otherwise disjointed and discordant phenomena of Nature, have insensibly been drawn in, to make use of language expressing the connecting principles of this one, as if they were the real chains which Nature makes use of to bind together her several operations. Can we wonder then, that it should have gained the general and complete approbation of mankind, and that it should now be considered, not as an attempt to connect in the imagination the phenomena of the Heavens, but as the greatest discovery that ever was made by man, the discovery of an immense chain of the most important and sublime truths, all closely connected together, by one capital fact, of the reality of which we have daily experience.

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