As a sequel to the story of Galileo, I think it may be interesting to inquire what the evidence, as we now have it, proves with regard to the truth of the Copernican theory, there being two opposite and contradictory errors on this subject, and these not merely popular errors, but shared to some extent by educated and otherwise learned men. But I must, before proceeding, remind my readers that I use the word Copernican simply to signify the system of modern astronomy, that in which the Sun is the centre round which the Earth and the other planets revolve, and not as meaning the precise theory of Copernicus, which (as I have said) was overthrown by Kepler, when he discovered that the planetary orbits were not circular but elliptical, the Sun, moreover, not being strictly in the centre, but in one of the foci of the orbit.

Now it is a plain fact, which all persons must perceive, that either the Earth revolves on its axis in twenty-four hours (more accurately 23 hours 56 mins. 5 secs.), or else that the whole of the celestial bodies are carried round the Earth in that same time. It is also a fact no less perceptible to careful observers, that either the Sun goes round the Earth in the course of a year, or else that the Earth goes round the Sun. The question is how these facts are to be accounted for.

The first of the two errors I have just mentioned is that which supposes the Copernican theory to have been directly and conclusively proved. This I imagine to be very common, and to arise from the elementary books learnt by schoolboys, which state (naturally enough) the modern theory of astronomy without the reasons that support it.

We need not dwell long on this point. Persons who have got this erroneous impression misunderstand the nature of the evidence. Some things in astronomy can be positively proved from observation, as, for instance, the existence of sun-spots. Many things in mechanics, chemistry, optics, and other branches of physical study can be demonstrated by experiment. The motion of the Earth round the Sun cannot, however, be so treated. It is inferred, and very rightly so, from the fact that it explains completely and easily all the observed phenomena, while, on the other hand, there are certain things which, as far as our present knowledge goes, cannot be explained in any other way; and the same argument applies to the rotation of the Earth on its axis. But though all this is perfectly clear so far, who can possibly say that as science progresses some explanation may not be hereafter found consistent with the antagonistic theory—consistent, let us say, with the system of Tycho BrahÉ, or some modification of it? I need not add that I consider the future discovery of such explanation as so improbable, that one may practically dismiss the idea, but I should be sorry to deny it as being conceivably possible.

The other, and opposite, error is that of certain well-meaning but ill-informed persons, who imagine that the Copernican theory is even now doubtful and liable to be overthrown—liable, I mean, in a real and practical sense, and not by distant contingencies, such as those at which I have just hinted, and which may be considered as shadowy and intangible. I do not suppose that amongst educated men there are many such scientific recusants; but at any rate it may be useful to give a short summary of the evidence on which the Copernican conclusion is based. In doing this I fear I shall tire the patience of my readers by partly repeating Galileo’s own arguments, which I have already quoted in discussing the Dialogue. This cannot easily be avoided, for much of his reasoning is so sound and so forcible, that after the lapse of more than two centuries we can add but little to it. On the other hand, there are grave mistakes that must be shunned; and, moreover, there have been discoveries made since the day when the Dialogue was written, of inestimable importance.

The best way of treating the question is to resume the history of astronomical research from the point where we dropped it; that is, at the time when Galileo first made known to the world the result of his observations.

It ought to be clearly understood that from the moment the telescope was turned on the heavens, the old system of astronomy was doomed, and nothing could finally have saved it. For a time prejudice and other more creditable feelings kept it floating on the sea of speculation, but such a state of things could not last; and the startling information that men like Galileo, Fabricius, and Scheiner imparted to the scientific world, could not fail to expel the old theory of the universe from the minds of men—at least, men of intellectual capacity—gradually and slowly, but yet most surely.

Now we have seen what the revelations were which the telescope at once displayed, even in its comparatively rude and imperfect state. There were the spots on the Sun, the satellites of Jupiter, the phases of Venus, the greater apparent size of the superior planets (Mars and the rest) when on the opposite side of the Earth from the Sun, this last phenomenon being quite inconsistent with the system of Ptolemy.

One consequence of all this was that the less enlightened men of the old school indulged in a violent antipathy to the new-fangled instrument, which threatened to overthrow their time-honoured traditions, and simply refused to believe in the telescope and its results. Thus the principal professor of philosophy at Padua, when invited by Galileo to look through his glass at the Moon and the planets, pertinaciously refused to do so. Simplicio, who, of course, represents in the Dialogue the prejudices of men of this stamp, admits (as we have seen) his feelings on this subject, and his suspicions that the new discoveries were to be attributed to optical errors. He was willing to be corrected if mistaken, but such had hitherto been his opinion.

It was not, however, to be expected that men of sound sense would allow themselves to be misled for any length of time by fallacies such as these. Continued observations carefully made are sure to correct mere optical errors, and after a reasonable interval it must have been evident that the phenomena discerned through the telescope were facts that had to be dealt with—not phantoms to be ignored.

Thus, when it was found that the planet Venus presented to the eye phases such as the Moon does, instead of always appearing like a round body, it became evident that she revolved, not as Ptolemy supposed, round the Earth, but round the Sun, an inference subsequently confirmed by the observation of her transits over the Sun’s disc.

This being so, the adherents of Ptolemy had to meet this difficulty: here was a planet much nearer to the Earth than to the Sun,24 and yet revolving round the latter in preference to the former. There was clearly, then, some attractive force belonging to the Sun (whatever its nature might be), greater than that of the Earth, which Venus obeyed; the same was true of Mercury, with the difference that this planet was much nearer to the Sun. Then as regards the superior planets, Mars, Jupiter, Saturn, the probability that the Sun was the great central power that controlled their movements was a very strong one. There is but little to add on these topics to Galileo’s own forcible argument in the third day’s dialogue; he is, however, inaccurate in his figures, and states that Mars appears sixty times as large when in opposition to the Sun, as at conjunction. More recent observations have shown that he appears rather more than thirty times as large when at his nearest point to the Earth, than he does when near his conjunction with the Sun, and consequently at his farthest point from the Earth; but this variation is quite sufficient for the argument, and proves incontestably that if Mars revolves round the Earth as in any way the centre of his orbit, it must be in an ellipse of so great eccentricity as no one could reasonably imagine him to do; indeed, the anti-Copernicans of Galileo’s day knew nothing of the elliptic motions of the planets; neither, as we have seen, did Galileo himself.

The same argument, drawn from the apparent size of the planet at different periods, applies also to Jupiter and Saturn—the other exterior planets were discovered much later—only not so strikingly as in the case of Mars. The improbability, if we once admit that all the planets revolve round the Sun, that the Earth, occupying the position it does, should be at rest, while the Sun, controlling the motions of the planets (vast bodies, some of them), circled, nevertheless, round the Earth; the improbability, I say, of this is so great as to be almost overwhelming; at any rate, unless the difficulties of the counter hypothesis were shown to be insurmountable, which, as we know, is far from being the case. It was of course possible, without going the lengths of the Paduan professor, and setting oneself against the telescope altogether, to admit the facts but deny the inferences; to grant, for instance, that Mars appeared to have a diameter more than six times as great in one position as in another, and to attribute it, as I hinted just now, to some extraordinary eccentricity in his orbit round the Earth; but it is not wise to look through a telescope with the eyes of the body open and the eyes of the mind closed; and generally it is but right to be guided by clear and distinct probabilities when discussing questions of natural philosophy on scientific grounds—and it is of these alone that I am at the present moment speaking.

It must be borne in mind distinctly that the discovery of the moon-like phases of Venus, showing her to revolve round the Sun, was simply conclusive as against the old system of Ptolemy, which had so long been the received system of astronomy. The theory of Tycho BrahÉ, or some modification of it, was the only one that could henceforth be adopted. But when you dethrone an ancient theory which has for centuries held an almost undisputed sway, you have to reconsider your whole position, and compromises such as that of Tycho are not always adequate to the emergency.

But these considerations formed only a part of this complicated controversy. The anti-Copernicans of the seventeenth century would not even admit the revolution of the Earth on its own axis, and were consequently forced to hold that the whole of the heavenly bodies were carried round this our globe in twenty-four hours. In ancient times, when men knew little or nothing of the sizes and distances of the Sun, the planets, or the stars, such a belief was quite reasonable and natural; they thought the stars were set as if they were jewels in a hollow sphere, which was turned round its poles each day. But the astronomers of Galileo’s day knew something far more accurate than this; he himself, as we observed in the Dialogue, greatly under-estimated the distance and the size of the Sun, and had but a very imperfect idea of the enormous interval that separates us from the stars; yet he evidently perceived the improbability of all these vast and remote bodies revolving with an almost inconceivable velocity round the Earth every twenty-four hours. And what must be our judgment on such a subject, seeing that we know the Sun’s mean distance to be about 92,000,000 miles, more than nineteen times as much as Galileo’s estimate? And yet some of the planets are farther and much farther from us than the Sun. Then as regards the stars, a Centauri, the nearest of them, is calculated to be more than 20,000,000,000,000 miles distant; but this calculation supposes the truth of the Copernican theory, and that we may not seem to argue in a circle, we will not use it, but content ourselves with saying that, from certain reasons about which there can be no mistake, we are sure that the distance of the stars is very considerably greater than even the remotest planet in our own system, which is Neptune. Now, this planet’s distance from the Sun is computed at 2,775,000,000 miles, and if, indeed, he is carried daily round the Earth in a circle, it must be with a velocity exceeding that of light; the stars, therefore, with a velocity far greater still. Now, nothing with which we are acquainted moves with so great a speed as light—or, as some men call it, radiant energy, meaning thereby to include heat as well as light in the term—a speed estimated at 186,000 miles in a second of time. Are we then to believe that the stars are carried in a circle round the Earth every day at a velocity much exceeding even this? It seems almost enough to ask such a question without pausing for the answer. The simple rotation of the Earth on its own axis explains all the phenomena without resorting to such extreme suppositions as those just mentioned.

It is remarkable that no one of any note—at least, in modern times, for I am not so sure about the ancients—ever appears to have suggested the intermediate theory of the Earth revolving on its axis, and yet remaining stationary as regards any motion of translation. With our present knowledge of astronomy we could not entertain such an opinion, though in the early part of the seventeenth century it might have been considered plausible. Since, however, it has not been maintained by any noteworthy author, we need not further discuss it.

The reader will bear in mind what has already been said on this branch of the subject in the second day’s dialogue,25 and it is not necessary to repeat it in detail. It may, however, be useful to mention a few experiments of a later date, which have tended to confirm the truth of the Earth’s diurnal revolution.

Before the close of the seventeenth century it was observed that a diminution of gravity occurred at, and near, the equator. This was proved by the vibration of the pendulum, an experiment associated chiefly with the name of Richer; and it has, if I mistake not, been since then carefully tested by spring balances. This phenomenon is owing partly to the spheroidal figure of the Earth—itself the result of the rotation on the axis—but principally to the centrifugal tendency being greater at the equator, from the higher velocity of rotation.

I have already alluded to the trade winds, and the argument to be drawn from them, which I think a sound and strong one; but I need not dwell on it further.

It is, however, well worth remembering that in our own day another proof has been given, which has been generally allowed to be an important one. It is the result of an experiment of Foucault, and is simply this: if a pendulum, with a heavy weight attached to it, be made to oscillate in a plane due north and south, say in the latitude of Paris, the pendulum, after a time, and supposing it to continue in movement long enough for the purpose of observation, will oscillate in a direction slightly north-east and south-west. Now the pendulum moves naturally always in the same direction, backwards and forwards, as originally started, and if the Earth were shaped like a cylinder no change would be detected; but the spherical form of the Earth, as it rotates on its axis, here makes the whole difference; the floor of the room where the pendulum vibrates is carried round the axis of rotation, as everything else is, but the plane of oscillation remaining the same—or parallel to the original one—it no longer points north and south. At the equator this phenomenon would disappear, and in the southern hemisphere it would be the other way: that is, the pendulum would vibrate north-west and south-east.

The same thing is exemplified by the small machine called the gyroscope, where a heavy disc, so adjusted as to revolve freely in any given direction, independently of the frame in which it is placed, will continue, when once set in rapid motion, to spin in the same plane, directed, for instance, to any one star that happens at the time to be due north or due south of us, while the frame moves round it with the rotation of the Earth.

I think, then, on the whole, we may say that those persons who, in the present state of our knowledge on the subject, are not convinced that the Earth revolves on its own axis, would not be satisfied by any evidence whatever.

Returning now to the general question of Copernicanism, we find that for some time after the trial of Galileo, things remained much in statu quo; unless we except the observation of the transit of Venus, in 1639; but, as that eventful seventeenth century was drawing to its close, there came on the scene some thoughtful and able astronomers, who could not only utilise the knowledge of their predecessors, but could also guess, with more or less accuracy, what that law—hitherto unknown—might be, which governed the planets and our own Earth in their movements. It was about this time that the Royal Society was founded in London, and a stimulus was thus given to investigation and to experiment. The third law of Kepler, which states that in all the planetary orbits the square of the periodic time of revolution is in a constant proportion to the cube of the mean distance, suggested the existence of another law, not yet discovered, a law of attraction, on which this itself depended. Among the astronomers of that day three names deserve special mention, Wren, Hooke, and Halley, because each of them guessed with some accuracy at the true doctrine—as it is now known to be—that the planets are attracted to the Sun by a force which acts inversely as the square of the distance. Hooke, in particular, deserves the credit of having applied this law to the path of a projectile, under certain circumstances, as well as to the planetary orbits; but though he thus lighted upon true conclusions, he appears to have been deficient in mathematical skill, and therefore unable to verify his results. It is, however, only just to the memory of Horrox, who was carried off by an early death, to mention that the true theory of the identity of terrestrial and astronomical gravity had occurred to his mind; if he had lived twenty or thirty years longer, he might have survived in history as the discoverer of the great problem.

Be this as it may, there now arose another man greater than his predecessors, and greater than all his contemporaries; he also was an Englishman, by name Isaac Newton. What others guessed, or concluded on insufficient evidence, became, in his powerful hands, clear and well-grounded truths, proved, so far as such things could be proved, by rigid mathematical reasoning, and established on a solid basis, which time has not shaken, and which subsequent investigation has confirmed. Others had supposed the existence of the law of attraction by which the Sun acted on the planets; many persons had understood the existence of terrestrial gravitation. Newton showed that these two are identical; and, moreover, that every particle of matter attracts every other particle mutually, and according to the one universal law, that of the inverse square of the distance; so that a vast planet revolving round the Sun obeys the same law as a pebble dropped from one’s hand to the Earth. The popular story of his having been suddenly led to this conclusion by the sight of an apple falling is apparently fabulous; and what really occurred is this: he sat alone one day in a garden, and fell into a speculation (as men of scientific mind are apt to do) on the power of gravity, that is, of gravity as we feel it here on the Earth. Then it struck him that however high you ascend, even on the loftiest mountains, no sensible diminution in this remarkable force takes place; so, he said to himself: why not as high as the Moon? If so, perhaps she is retained in her orbit by this very power. And again if so, what then? To which question his active mind gave the just and true answer, that it was probably one and the same force that acted at the surface of the Earth, at the distance of the Moon, and finally, as regulating the action of the Sun on the planets.

It seems that there was an error, which it is unnecessary to explain in detail, in Newton’s first calculations; but that when, after a lapse of time and with the error corrected, he again returned to them, he found the motion of the Moon to be exactly accounted for by his theory.

Again, in dealing with the complicated problem of the action of the heavenly bodies one upon the other, that is, when the disturbing force, for instance, of a third body is brought to bear on the motions of two others, although Hooke and others had as a conjecture put forth the existence of such mutual action, yet Newton was the first who thoroughly grappled with it.

The mutual attraction of matter, so far as things terrestrial are concerned, had occurred to the inquiring intellect of Francis Bacon; but it was left for Newton to propound it as the great principle that governs the physical universe.

Now let us see how all this bears on the truth of the Copernican system. Newton proved—and I may add that the improved methods of mathematics which have been adopted since his day make the proofs more simple and easy—that if any body moves in an ellipse, or indeed, in one of the other conic sections, the law of force, tending to the focus, is that of the inverse square of the distance.26 Conversely, he proved that a body under the action of a central force, varying in intensity as the inverse square of the distance, will move in a conic section.

Then if the Moon moved in an ellipse, as it was easy to perceive that she did, and if her motion corresponded precisely with what it would be on the theory of universal gravitation; if also, as seemed evident, the planets revolved in ellipses, then the inference that the law of gravitation, as stated by Newton, was true became irresistible; not susceptible, as before stated, of direct and absolute proof, but established conclusively by a sound and legitimate induction.

What I have just stated shows that Kepler’s first law corresponds with Newton’s discovery; but the same is true of the two other laws. It would of course be out of place here to go minutely into all the evidence which can be gathered in support of the doctrine of universal gravitation. I may briefly state that all of Kepler’s laws are simply explicable by that hypothesis, and that the evidence derives additional confirmation from the following curious fact: observation shows that Kepler’s laws, though approximately true, are not strictly and accurately so; if the planets were mere particles revolving round the Sun, they would then be quite rigidly true, but the planets have a certain mass (though very small compared to the Sun) and so do in some measure attract the Sun as well as being attracted by him, and they, moreover, exercise a disturbing influence on each other. These perturbations, however, have been calculated, and the result is that they agree with what ought reasonably to be expected, supposing the theory of universal gravitation to be true. This confirmatory proof has been acquired, I need not add, since the time of Newton by the labours of astronomers, Laplace and others, who have succeeded him, and who have had the advantage of that more manageable method of mathematical calculation to which I have just alluded.

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

The effect of all this on the Copernican system and the evidence on which it rested, was to raise that system from a simple though strong probability, a question on which at any rate something might be said for and against it, to a probability of almost overwhelming force; for it not only showed how the heavenly bodies moved, but it explained the cause of their motions, and in a word furnished the key that unlocked the arcana of Nature. When you came to know not only how the Moon and the planets moved, but the law which regulated their movements, and when you found that all fitted into one harmonious whole (at least with some minor exceptions), it was not easy to refuse assent to a theory supported by such powerful evidence.

Yet in saying this we are perhaps rather viewing the question from our present standpoint, than as a contemporary would have done. As a matter of fact, Newton’s hypothesis, though eagerly received in England, met with a long opposition on the Continent, and particularly in France, where Descartes’ theory of vortices reigned supreme for many years. It must not be supposed that these Cartesian philosophers were anti-Copernicans; far otherwise, only they accounted for the celestial motions in a different way from Newton, and, as every one now admits, in a wrong way.

I have already remarked that there were some apparent difficulties in the application of the law of universal gravitation to all the heavenly bodies, and that these have been removed by subsequent calculation. One of these difficulties, if indeed it could be so called (for it hardly amounted to that), has been solved within living memory. It was noticed that the planet Uranus showed signs of perturbation from some unknown reason; and even the work I have just quoted, “Whewell’s History of the Inductive Sciences,” published in 1847, contains the following sentence: “Uranus still deviates from his tabular place, and the cause remains yet to be discovered.” Two astronomers, one French and one English, Le Verrier and Adams, found out the cause by discovering the existence, each independently of the other, of an exterior planet revolving in an orbit more distant by far than that of Uranus; to this planet the name of Neptune has been given, and his existence is one more confirmatory proof of the theory of gravitation.

The Copernican system had been built up and consolidated by Newton’s great discovery; but another piece of evidence, of a most important character, was added by the investigations of Bradley, Professor of Astronomy at Oxford, and afterwards Astronomer Royal; this careful observer, while engaged in endeavouring to detect such an apparent motion of the fixed stars (so called) as would indicate an annual parallax, noticed that another motion existed different from that which the annual parallax would produce, and for which he could not account; the apparent orbits described by the stars observed depended on the distance of the stars from the pole of the ecliptic; the phenomenon was different from anything hitherto discovered, and one or two modes of explanation were tried in vain. Accident, however, turned Bradley’s thoughts in the right direction; he was one day in a boat on the Thames, and observed that the vane on the mast gave a different apparent direction to the wind, according as the boat sailed in different courses. Here, then, was the solution of the difficulty: it was already known from RÖmer’s investigations that light moved with a finite velocity, and if so it would naturally produce the same effect as that observed in the boat, or to take an illustration very commonly given, like that which any one finds when moving along rapidly in a shower of rain, in which latter case the rain seems to fall not in the direction it has when one is at rest, but in a direction compounded of that and the one opposite to the person’s line of motion.

Bradley soon drew the correct conclusion, that light acted in precisely the same way upon the Earth as it moved in its orbit, and that the apparent annual displacement of the stars, as detected by him, arose from this sole cause. All the great astronomers who followed him have agreed with his conclusions, and the phenomenon in question, which is called the aberration of light, has conferred a lasting fame on its discoverer. And the remarkable point about it is this, that not only does it give a fresh illustration to the Copernican theory, but it is one of the very few scientific facts that cannot (so far as our knowledge of the subject goes) be explained in any other way. It is, therefore, generally considered as a critical test of the truth of the system.

There are two other phenomena, on which however I do not propose to dwell at any length, known as precession and nutation, which it is not easy to explain otherwise than by the modern theory of astronomy and the principle of gravitation; the latter of these two owed its discovery to Bradley, and the former to Hipparchus, who could not have been aware of its real cause, though he had observed the fact of its occurrence.

But passing on from these, I may call attention to one most remarkable result of modern scientific research, connected with the stars. In Galileo’s day, it was a drawback to the Copernican theory that none of the stars showed the smallest annual parallax; in popular language, none of them seemed to undergo any change of place, however small, when observed at opposite points of the Earth’s orbit, or as the opponents would have said, the Earth’s imagined orbit. A displacement of this kind, I need hardly repeat, must not be confounded with that other motion which Bradley observed and explained. This was one of Tycho BrahÉ’s reasons for rejecting the Copernican system, and it was one of the best arguments used by the opponents of Galileo. As the enormous distance of the stars from the Earth was, as we have already seen, at that time unknown, the celestial distances generally being under-estimated even by the best astronomers, the argument had an apparent force, which no one now would attribute to it. Galileo himself had some hope of overcoming the difficulty by discovering some annual displacement in certain stars, but it is needless to add that his instruments were unequal to such a task. Subsequent observers tried various methods, but without any real success until the present century, when Bessel and other observers found that a star called 61 Cygni had a certain annual parallax; and not long afterwards, Henderson, making his observations at the Cape of Good Hope on a conspicuous star in the constellation of the Centaur, a constellation belonging to the southern hemisphere, found at length that this star, which in fact is a double star, and known as a Centauri, had a parallax of nearly 1; subsequent calculations show it to be probably rather less, that is to say about 0·91. This means that it is more than twenty billions of miles distant, and that light takes more than three years to travel from a Centauri to the earth. It is, however, believed to be much the nearest of all the stars, no other coming within double of the distance.

Now it is difficult to evade the conclusion which naturally follows from these results, that the Earth really does move in an annual orbit round the Sun. It is no part of my present task to give a list of the stars of which the parallax has been found, but I may say there are several others besides the two I have named; and I know of no method of accounting for the fact in any way but by the annual motion of the Earth, unless we suppose some instrumental error to have occurred. There have been so many of these in times past that it may seem rash to exclude such a possibility, but, considering the perfection of modern scientific instruments, it is in the highest degree improbable; and we may fairly reckon the parallaxes of the stars as a strong confirmation of the already strong evidence in favour of the Copernican theory—a theory which, as we have seen, was, from a purely scientific point of view, very probable in the days of Galileo, overwhelmingly probable after the great discovery of Newton, and at the present time, with all the light that subsequent research and observation have thrown on it, scarcely short of a moral certainty.

I may repeat once more that it has not, indeed, that absolute physical certainty, arising from direct experiment, which has been obtained in other scientific investigations; but, allowing for this faint element of instability, we may fairly say that no truth of natural philosophy stands on a firmer basis.

And for Galileo, who lived before the day when, as Whewell says, “Astronomy passed from boyhood to mature manhood,” we may fairly say that, after we have censured his faults and his errors, after we have ascertained that he was not a hero or a “martyr of science,” we must still recognise the fact that he was one of the greatest natural philosophers of his day, pre-eminent in astronomy, in mechanics, in mathematics. To his honour also be it added, that his religious faith, and his respect for the Church and her authority, so far as we can judge, never failed. Whatever his defects may have been—want of prudence, want of candour, want of consideration for others—we can easily perceive that he would never have been willingly drawn into any controversy intended to provoke antagonism between Religion and Science.

In the present age, unhappily, there have been men who have taken the other course, and have contributed their share towards exciting antagonism, heedless of the consequences. Some have done this unwittingly, arguing on the side of religion, but without a proper supply of sound scientific information; others, on the opposite side, have shown so bitterly hostile a spirit to Revelation, if not even to Natural Religion, as to render it more than ever difficult to re-establish that concord between the two studies, that of the supernatural and that of the physical, which should never have been interrupted.

This, however, is so wide a subject that I must not be led into it. Yet I may briefly remark that two of the greatest lights of the Catholic Church, men whose teaching and whose writings have exercised an undying influence, have both, either by words explicitly, or implicitly by their example, contributed to encourage a sound knowledge of natural philosophy, and in harmony with Christian theology.

They both lived when physical science was in its infancy, though at intervals of nearly 800 years apart. St. Augustine, who flourished towards the latter part of that period dominated by the corrupt civilisation of ancient Rome, amongst his voluminous works devoted one treatise to the interpretation of the Book of Genesis, “De Genesi ad Litteram;” and he takes the opportunity of cautioning those whom he addresses against the risk of exciting the ridicule of unbelievers by a mistaken adherence to a rigidly literal interpretation of Holy Scripture. He was, I believe, one of the first that interpreted the six days of Creation in the non-literal sense, though his particular theory is not one in accordance with modern scientific opinion. I allude to him not for the details of natural philosophy, but as enunciating a principle, which some subsequent authors have not followed as they might have done.

St. Thomas Aquinas lived in those middle ages of which he was one of the most brilliant ornaments. The power of his intellect is admitted by those who have little sympathy with his teaching; his literary industry is a standing marvel; and I have already observed that besides the theological and metaphysical works on which he expended so much labour, he wrote a treatise on the astronomy of Aristotle. It may be said this is no very great matter, but I mention it as illustrating the breadth of mind of this great saint and theologian, who could spare time for a study of physical science without neglecting the more solemn duties of his calling. His active mind was alive to every source from whence wisdom and learning could be imbibed; and if he had lived in the age of Galileo, I have sometimes fancied that he would have thrown some oil on the troubled waters, would have counselled prudence to the adventurous astronomer, patience and forbearance to his antagonists. But it is of no avail to indulge in speculations such as these. Each age of the world has its difficulties, moral and intellectual, and we can neither hurry the stream of human thought onwards nor drive it backwards.

So again it is with the dispositions of individuals; if Galileo had been gifted with the calm, dignified reserve of Newton, instead of being the vivacious, loquacious Italian that he in fact was, he might have lived and died in peace.

And now, if I may be permitted to recur once more to the subject of gravitation, I have a word to say as to the lesson which this great all-pervading law seems to teach. It has nothing to do with any question of revealed Religion; but does it not bear the unmistakable signs of the action of an all-wise, an all-powerful Creator? It may possibly be the result of some other, though unknown, law; and even then it brings us back to the same point. The result in nature remains the same, and that result is written in characters that cannot be ignored. Mathematicians have occupied themselves in making suppositions as to the effects of imaginary laws of gravity, some of which might, no doubt, ensure sufficient order and regularity to maintain this world, and the countless worlds that people space, while others would cause hopeless confusion. The striking thing is that the existing law perfectly answers its purpose.

Only let us imagine that no law of attraction acted upon matter at all, nor any force of whatever kind—what would be the result? There would be no coherence, no abode for human or animal life—nothing but chaos and anarchy.

If, then, we contrast this imagined picture with the one actually before us, we are, I think, forcibly led to the conclusion that the physical universe owes its origin, its existence, its harmony to an Omnipotent Being, unseen, yet not unknown, intangible to the senses, ever present to the intelligence.

And now, in order to avoid misapprehension, I venture to restate briefly the propositions I have sought to establish.

I have maintained that the Catholic Church has a right to lay her restraining hand on the speculations of Natural Science, just as much as she has in the case of other speculative inquiries. Those who do not believe in her prerogatives will, of course, deny such right in toto; but I contend that if you grant the existence of this right at all, you cannot exclude Physical Science from its operation.

On the other hand, in the particular case of Galileo, I have not attempted to defend all the proceedings of the Cardinals of the Index and the Cardinals of the Inquisition. For it must be remembered it was no gentle rebuke with which the Copernican system and the individual Galileo were visited; no such light condemnation as that of placing on the Index of prohibited books all Copernican works as being inopportune, or again, that of a caution to Galileo to be more prudent, was deemed adequate to the emergency—if, indeed, any one even thought of them.

So with the facts of the history before us, I think any sweeping defence of the proceedings in question would be unnecessary from an ecclesiastical point of view, and from a scientific point of view untenable.

Moreover, I must add, as an indispensable premiss to the conclusion just stated, I have also maintained that the censures pronounced by the Cardinals on both occasions were not dogmatic decisions, such as Catholic theologians hold to be infallible; but disciplinary enactments, varying with the changing characters of different ages.

Then again, referring to the scientific questions involved, we may see that Astronomy, considered historically, is divided into three periods—the ancient one before the invention of the telescope, that is, up to the time of Galileo; the intermediate one, when the telescope was in use but the law of universal gravitation as yet unknown—from Galileo until the publication of the “Principia” of Newton; and the modern one, from Newton downwards. During the first period it seemed highly probable to the whole world, with the exception of a few gifted intellects, that this Earth was the centre of the Universe, and that all the heavenly bodies revolved round it; during the second period, when the telescope had shed a light so powerful and so brilliant upon astronomical research that men could not absolutely close their eyes to it even if they wished, the balance of probability passed into the opposite scale, and the more intelligent men of science guessed at the truth, however indistinctly. But some elements of uncertainty remained; and this circumstance, taken in connection with the irrelevant arguments so much in vogue at that time, must in all fairness be allowed as an excuse for the many good men, ecclesiastics and others, who opposed the Copernican doctrine. After the great step made by Newton it was no longer a question of balancing probabilities, for the weights were almost all transferred to one scale, and the probabilities of the truth of the Heliocentric System (to give it for once its accurate name) became overwhelming. The subsequent investigations of Bradley and others have gone further still, and have converted this strong, overpowering probability into something approaching indefinitely near to a moral certainty.

Beyond this we cannot reasonably expect to go; physical certainty is not to be attained when we have to traverse the vast distances of celestial space, and human infirmity must be content to recognise the boundary beyond which it may not pass, the limit imposed on finite minds by the Infinite.

THE END.

CHARLES DICKENS AND EVANS, CRYSTAL PALACE PRESS