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much rarer within the dense bodies of the sun, the stars, the planets, and the comets, than in the more empty celestial spaces between them, and to grow more and more dense as it recedes from the celestial bodies to still greater distances; by which means all of them, in his opinion, are forced towards each other by the excess of an elastic pressure.
It is possible, undoubtedly, to account for the effects of gravitation by an ethereal medium thus constituted; provided, as it is also necessary to suppose, that the corpuscles of such a medium are repelled by bodies of common matter with a force decreasing, like other repulsive forces, simply as the distances increase. Its density, under these circumstances, would be every where such as to produce the semblance of an attraction, varying like the attraction of gravitation. The hypothesis in connexion with the existence of a repulsive force in common matter has a great advantage in point of simplicity, and may perhaps hereafter be capable of proof, as indeed recent observations on some of the comets lead us to expect, though as Yet it can only be regarded, and was at first only offered, as an hypothesis.
M. Laplace, equally dissatisfied as Sir Isaac Newton with the idea of gravitation being an essential property of matter, passes away from the enquiry, with suitable modesty, to practical subjects of far higher importance, and which equally grow out of it, in whatever light it is contemplated. "Is this principle," says he, "a primordial law of nature? or is it a general effect of an unknown cause? Here we are arrested by our ignorance of the nature of the essential properties of matter, and
deprived of all hope of answering the question in a satisfactory manner. Instead, then, of forming hypotheses on the subject, let us content ourselves with examining more particularly the manner in which philosophers have made use of this most extraordinary power."*
There is one striking objection to Sir Isaac Newton's suggestion, which it has been thought difficult to repel. It is, that, though it may account for the attraction of gravitation, as a phænomenon common to matter in general, it by no means accounts for a variety of particular attractions which are found to take place between particular bodies, or bodies particularly circumstanced; and which, excepting in one or two instances, ought, perhaps, to be contemplated as modifications of gravitation. These, however, were not in Newton's original contemplation.
Upon these particular attractions, or modes of attraction, including homogeneous attraction, or the attraction of aggregation, heterogeneous attraction, or the attraction of capillary bodies, elective attraction, and those of magnetism and electricity, each of which is replete with phænomena of a most interesting and curious nature, I intended to have touched in the present lecture; but our limited hour is so nearly expired, that we must postpone the consideration of them as a study for our next meeting. Yet it is not possible to close the observations which have now been submitted, without testifying our gratitude to the memory of that transcendent genius whom the providence of the adorable Architect of
• Exposition du Système du Monde, liv. iv. ch. xv.
the universe at length gave to mankind six thousand years after its creation, to unravel its regular confusion, and reduce the apparent intricacy of its laws to that sublime and comprehensive simplicity which is the peerless proof of its divine original.
It has been said that the discovery of the universal law which binds the pebble to the earth, and the planets to the sun, which connects stars with stars, and operates through infinity, was the result of accident. Nothing can be more untrue or derogatory to the great discoverer himself. The earliest studies of Newton were the harbinger of his future fame his mighty mind, that comprehended every thing, was alive to every thing; the little and the great were equally the subjects of his restless researches and his attention to the fall of the apple was a mere link in the boundless chain of thought, with which he had already been long labouring to measure the phænomena of the universe.
Grounded, beyond all his contemporaries, in the sure principles of mathematics, it was at the age of twenty-two that he first applied the sterling treasure he had collected to a solution of the system of the world. The descent of heavy bodies, which he knew to be a very little less rapid on the summit of the loftiest mountains than on the lowest surface of the earth, suggested to him the idea that gravity might possibly extend to the moon; and that, combined with some projectile motion, it might be the cause of the moon's elliptic orbit round the earth: a suggestion in which he was instantly confirmed by observing, that all bodies in their fall describe conic sections of some modification or other. And he further conceived, that if the moon were retained in
her orbit by her gravity towards the earth, the planets must also, in all probability, be retained in their several orbits by their gravity towards the sun.
To verify this sublime conjecture, it was necessary to ascertain two new and elaborate positions: to determine the law of the progressive diminution of gravity, and to develope the cause of the curves or ellipses of bodies, when not descending vertically. Both these desiderata he accomplished by a series of reasonings and calculations equally ingenious in their origin and demonstrative in their result; and ascertained the truth of his principles by applying them, practically and alternately, to the phænomena of the heavens, and to a variety of terrestrial bodies.
The bold and beautiful theorem being at length discovered, and unequivocally established—a theorem equally applicable to the minutest corpuscles, and the hugest aggregations of matter-that all the particles of matter attract each other directly as their mass, and inversely as the square of their distance, he at once beheld the cause of those perturbations of motion to which the heavenly bodies are necessarily and so perpetually subject: it became manifest, that the planets and comets, reciprocally acting and acted upon, must deviate a little from that perfect ellipse which they would precisely follow if they had only to obey the action of the sun : it was manifest that the satellites of the different planets, exposed to the complicated action of the sun, and of each other, must evince a similar disturbance that the corpuscles which composed the different heavenly bodies in their formation, perpetually pressing towards one common centre, must necessarily have produced, in every instance, a
spherical mass that their rotatory motion must at the same time have rendered this spherical figure in some degree imperfect, and have flattened these masses at their poles; and, finally, that the particles of immense beds of water, as the ocean, easily separable as they are from each other, and unequally operated upon by the sun and the moon, must evince such oscillations as the ebbing and flowing of the tides.* The origin, progress, and perfection of these splendid conjectures, verifications, and established principles, were communicated in two distinct books, known to every one under the titles of his "Principia" and his "Optics;" - books which, though not actually inspired, would seem to fall but little short of inspiration, and have more contributed to exalt the intellect of man, and to display the perfections of the Deity, than any works upon which inspiration has not placed its direct and awful stamp.
* The reader may be referred, with great advantage, to the elegant popular illustrations of the principle of gravitation, and its effect in producing the perturbations of the planetary motions, and the phænomena of the tides, in Sir John Herschel's Treatise on Astronomy, recently published in the Cabinet Cyclopædia.-ED.