The Essays of Adam Smith · Adam Smith
THE HISTORY OF ASTRONOMY.
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 arc 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.
Ideas discussed in this chapter
Text from Project Gutenberg, public domain in the United States.