Galileo and His Judges

by F. R. Wegg-Prosser

Supposing then the law of gravitation to be established by sufficient proof, we may now ask what must become of the old systems of astronomy? What must befall Ptolemy and even Tycho Brahé?

It is obvious that they could do nothing but collapse. If the law of gravitation were once admitted to be true, the idea of the Sun revolving round the Earth must be dismissed as impossible. Here it is right to remark that (assuming the law of universal gravitation) it is not, strictly and scientifically speaking, correct to say that any one heavenly body revolves round another, but that they both revolve round their common centre of gravity. In the case of the Earth and the Sun, so vastly superior is the mass of the latter that the centre of gravity is far away within his volume, and the disturbance exercised on him by the Earth is scarcely appreciable; so also, in the case of the Moon and the Earth, the centre of gravity is within the latter, but at a considerable distance from its own centre; and here there is a distinctly appreciable oscillation of the Earth, arising from this very cause, during each revolution of the Moon in her orbit. When two bodies are more nearly equal in mass, as is probably the case with the double stars that have been observed in recent times, then the two revolve round a centre of gravity lying between them, exterior to both of them. It is believed that this is actually the fact in the instance I am here alluding to of the double stars, and there is some reason for supposing that the curve in which they revolve is an ellipse. This, if true, would clearly indicate that the law of gravitation, as stated by Newton, extends not only through our own solar system, but over the whole material universe.

And there is one remarkable property of this mysterious agency which we term gravitation, and that is its instantaneous action even at the greatest distances. Light travels with an enormous and yet a finite velocity, so that it takes a few years to arrive at the Earth from even the nearest stars. The force of gravity knows no such limit, nor is its action retarded by even the minutest fraction of time.

Nor, again, is it impeded, as in the case of light, by any screen or obstacle of whatever nature. Furthermore, it does not lose anything of its intensity, as light does, by being diffused over a larger surface; it varies as the mass of the bodies concerned, but not in the least according to the extent of their surfaces. Given the same distance, no diffusion weakens its force.

Great as was the evidence adduced by Newton for the truth of his theory, there were some real difficulties in the way of its reception. I need not allude to these in detail; they are explained in treatises on physical astronomy for the benefit of those who are interested in the subject. Briefly, I may say that subsequent research and careful calculations have removed the difficulties, and thereby confirmed the already existing evidence.

Then, as regards terrestrial gravity, experiments have been made—notably at the mountain Schehallion, in Scotland—throwing additional light upon it, and indicating that not merely the Earth as a whole, but any great mass, such as a mountain, exercises an appreciable attractive force.

Newton seems to have expected that some further discovery would take place, at no distant period, as to the nature of this occult agency which operates so powerfully in the heavens and on the Earth. In one of his letters he strongly disclaims the opinion that gravity is essential to matter and inherent in it; he thinks it is “inconceivable that inanimate brute matter should, without the mediation of something else which is not material, operate on and affect other matter without mutual contact... that gravity should be innate, inherent, and essential to matter, so that one body may act upon another at a distance through a vacuum, without the mediation of anything else by and through which their action and force may be conveyed from one to another, is to me so great an absurdity that I believe no man who has in philosophical matters a competent faculty of thinking can ever fall into it.”

And yet we see that what he thought absurd is still apparently true, and that, great as was Newton’s sagacity in discovering and proving the effects of this great cosmical law, he failed when he came to speculate on the more remote causes of it. Since his time, other ingenious theorists have imagined hypotheses in the hopes of accounting for it; but their efforts have not met with any great success, and the last word of science on the subject is that the cause of gravitation remains undiscovered.

But if the attempt to trace the ultimate cause of the law of gravitation has been a failure, the proof of its operation in the physical universe has been a marvellous success, and that not only in the present day, when difficulties have been removed and fresh evidence has been added, but, to a certain extent, even in Newton’s own time, and especially here in his own country. Indeed, we cannot suppress a feeling of admiration when we contemplate the revolution in astronomy brought about by this quiet, unobtrusive man, who is said to have spent thirty-five years of his long life within the walls of Trinity College, Cambridge, of which he was a Fellow, and who, though twice elected to represent the University in Parliament, never opened his lips in the House of Commons. I may, perhaps, be here permitted to insert a passage from a work to which I have previously alluded, Whewell’s “History of the Inductive Sciences,” well worth quoting both for its eloquence and its truth. After recounting, with some detail, the circumstances of this great epoch in astronomical knowledge, he proceeds:

Such, then, is the great Newtonian induction of universal gravitation, and such its history. It is indisputably and incomparably the greatest scientific discovery ever made, whether we look at the advance which it involved, the extent of the truth disclosed, or the fundamental and satisfactory nature of this truth. As to the first point, we may observe that any one of the five steps into which we have separated the doctrine [these were, 1st, that the force attracting different planets to the sun, and, 2nd, the force attracting the same planet in different parts of its orbit, is as the inverse square of the distances; 3rd, that the earth exerts such a force on the moon, and that this is identical with terrestrial gravity; 4th, that there is a mutual attraction of the heavenly bodies on one another; 5th, that there exists a mutual attraction of all particles of matter throughout the universe] would of itself have been considered as an important advance, would have conferred distinction on the persons who made it, and the time to which it belonged. All the five steps made at once formed not a leap, but a flight; not an improvement merely, but a metamorphosis; not an epoch, but a termination. Astronomy passed at once from its boyhood to mature manhood. Again, with regard to the extent of the truth, we obtain as wide a generalisation as our physical knowledge admits when we learn that every particle of matter, in all times, places, and circumstances, attracts every other particle in the universe by one common law of action. And by saying that the truth was of a fundamental and satisfactory nature, I mean that it assigned, not a rule merely, but a cause, for the heavenly motions; and that kind of cause which most eminently and peculiarly we distinctly and thoroughly conceive, namely, mechanical force. Kepler’s laws were merely formal rules, governing the celestial motions according to the relations of space, time, and number; Newton’s was a causal law, referring these motions to mechanical reasons. It is no doubt conceivable that future discoveries may both extend and further explain Newton’s doctrines; may make gravitation a case of some wider law, and may disclose something of the way in which it operates—questions with which Newton himself struggled. But, in the meantime, few persons will dispute that, both in generality and profundity, both in width and depth, Newton’s theory is without a rival or neighbour.[27]