We will now return to the narrative; and in due course discuss the condemnation of Galileo by the Inquisition sixteen years after the events just described.

It may be mentioned, as illustrating the feeling in Rome towards Galileo personally, that on the 11th March, 1616, he had an audience, lasting three-quarters of an hour, of Pope Paul V. He assured the Pope of the rectitude of his intentions, and complained of the persecutions of his adversaries. Paul V. answered very kindly, saying that both himself and the Cardinals of the Index had formed a high personal opinion of him, and did not believe his calumniators.

In the year 1620 there appeared a monitum of the Congregation of the Index, permitting the reading of the great work of Copernicus after certain specified corrections had been made.

Not long after this, in 1622, if I mistake not, Pope Paul V. died, and Galileo’s friend, Cardinal Barberini, succeeded him, taking the name of Urban VIII. Another of his friends, Monsignor Ciampoli, became secretary of briefs to the new Pope.

Our philosopher having ascertained that he would be well received, went to Rome in April, 1624, and was treated by the new Pope with all possible consideration. He had, in fact, several conversations with him; and we may well conjecture it was on these occasions that Urban VIII., discussing the Copernican theory, used some of those arguments which Galileo afterwards put in the mouth of Simplicio in his celebrated Dialogue, thereby deeply offending the Pope.

But there was, about this time, a sort of moderate reaction in favour of Galileo among the authorities at Rome. For instance, a work of his published since the decree of the Index, and entitled “Il Saggiatore,” in which he had favoured the theory of the Earth’s motion, was attacked, and an attempt was made to have it prohibited or at least corrected, but the attempt was a failure.

The reports of casual or unofficial conversations are always to be received with caution and with some qualification; yet at least they are “straws which show how the wind blows.”

Thus we are told that Cardinal Hohen-Zollern, in a conversation with the Pope (Urban VIII.) on the subject of Copernicus, endeavoured to show the necessity of proceeding with great circumspection on that point, to which it is said the Pope replied that the Church had not condemned and would not condemn that opinion as heretical, but only as temerarious. So again the Master of the Sacred Palace, himself resting neutral between Ptolemy and Copernicus, is reported to have said that there was no matter of faith in question, the great point being that one must not in any way mix up the Holy Scriptures with it.

We may suppose that when the Pope spoke of the opinion having been condemned as temerarious, what he meant was not that it had been explicitly censured as such—using the word in the technical sense which it bears when applied as a censure—for that it plainly had not been, but that the general effect of the prohibition issued by the Index was to stamp the mark of rashness upon it. This, I may observe, if it be the right interpretation, is quite consistent with the theory that the prohibition was of a disciplinary and a provisional character.

We have also another reputed conversation of the Pope with Campanella—resting on the authority of Prince Cesi, who related it to Father Castelli—and it is important if true. Campanella had said that certain Germans, ready to embrace the Catholic faith, had hesitated on account of the condemnation of Copernicus, to which Pope Urban VIII. had replied that this was not his intention, and if he had had the arrangement of matters the decree would never have been made. “Non fu mai nostra intenzione, e se fosse toccato a noi, non si sarebbe fatto quel decreto.”

As already remarked, we must not attach too great weight to reports of private conversations; but it is probable that some such scene took place as here represented, and, if it did, it is surely wholly incompatible with the idea that the decree was a decision in matters of faith. No Pope, no well-informed ecclesiastic of any rank, would express himself so in such a case; but it is quite consistent with what we might expect in a question of simple discipline.

It will now be convenient, before discussing the matter further, to resume the narrative, and to touch upon the questions connected with the condemnation of Galileo by the Inquisition, and his enforced abjuration. It is, indeed, these latter proceedings that have left so deep an impression upon the popular mind, though, strictly speaking, they were of less importance than the decree of the Index—of less importance, that is, to all others besides Galileo himself.

It seems that our philosopher overrated the effect of the reaction that had taken place in his favour, real though it was so far as it went. He thought he might now safely publish the work on which he had been labouring, and on which he probably relied as likely to influence the minds of learned men, ecclesiastical as well as lay, in the direction of Copernicanism.

He came in May in the year 1630 to Rome, and had a very long audience with the Pope, who treated him with great kindness and even increased a pension he had already bestowed upon him; but we do not know what passed as to other matters on this occasion. He had also an interview with Father Riccardi, who had now become Master of the Sacred Palace, with a view of obtaining authority to print his book. Father Riccardi upon this engaged Father Visconti, who was a professor of mathematics, to read the work and mark such passages as he thought necessary.

Father Visconti reported that there were some passages which required correction, and many points that he would like to discuss with the author. However, the Master of the Sacred Palace gave leave for the printing of the work, expressing at the same time a wish to see it once more himself; consequently it was arranged that Galileo should return to Rome in the autumn, in order to add the preface, and to insert in the body of the work certain passages, calculated to show that the question was being treated purely as a hypothesis.

Two untoward events, however, now occurred: one was the death of Prince Cesi, a powerful and devoted friend of Galileo, which took place on the 1st May; and the other was the outbreak of the plague at Florence, a circumstance which interrupted communications, and caused delays resulting in mistakes and misunderstandings. With a view of having the Dialogue printed at Florence, it was arranged that the revision required by the ecclesiastical authorities should take place there instead of at Rome. Father Hyacinthe Stephani, a Dominican, who acted as reviser, marked several passages in the work, thinking that they should be explained before the final permission for publication was conceded.

Then followed mutual delays: the author was tardy in sending to Rome the corrections to which he had in principle agreed, and the Master of the Sacred Palace was late in sending to Florence the preface and the conclusion, so the impatient philosopher began to print his book. The plague still continued, and the result was that communications were still interrupted.

The Inquisitor of Florence however received from Rome the power to approve officially the copy of Galileo’s work that would be submitted to him, with instructions specially added by Father Riccardi that he must bear in mind the wishes of the Pope to the following effect: The title of the work must indicate that it dealt only with the mathematical question connected with Copernicanism, also that the Copernican opinion must not be put forward as a positive truth, but merely as a hypothesis, and this without alluding to the interpretation of Scripture; moreover, that it should be stated that the work was only written to show that if the decree (i.e. of 1616) was made at Rome, nevertheless the authorities knew all the reasons against it that could be urged, and were not ignorant of one of them—an idea conformable to the words of the preface and the conclusion, which he would send from Rome corrected. With this precaution, it was intimated the book would meet with no obstacle at Rome, and thus satisfaction might be given to the author, and also to the Grand Duke of Tuscany, who had shown himself to be so eager in the matter.

This remarkable letter points towards a conclusion which has been drawn by some writers, that the preface to the Dialogue was written for Galileo by Father Riccardi or some other person, and was not his own composition; for the above is precisely what was said in the preface as it afterwards appeared, and it seems to me almost incredible that Galileo should have spontaneously written any such words, exposing him to the charge, which has really been made against him, of transparent irony, thereby giving offence in the very quarters where conciliation was desirable.

And it must be remarked that when Father Riccardi on the 19th July of this year sent the preface to Florence, he allowed Galileo the liberty of making verbal alterations only; so that whether he composed it or only revised it, it is Father Riccardi rather than the author of the Dialogue who must be held responsible for the contents, and the same remark applies at least partially to the conclusion also, it having been specially revised by the same hand.

The preface is addressed to the discreet reader, and the words to which I have just alluded are as follows: “Some years ago, a wholesome edict was promulgated in Rome which, in order to check the dangerous scandals of the present age, imposed an opportune silence upon the Pythagorean opinion of the motion of the earth. There were not wanting some who rashly asserted that that decree resulted, not from judicious examination, but from ill-informed passion; and there were heard complaints that Consultors, wholly inexperienced in astronomical observations, ought not to be allowed, with a hasty prohibition, to clip the wings of speculative intellects. My zeal could not keep silence on hearing the temerity of the complaints so made. As one fully informed of that most prudent decision, I judged it right to appear publicly in the theatre of the world, as a witness of pure truth. I happened then to be present in Rome; I had not only audiences, but approbations from the most eminent prelates of that Court, and it was not without my own previous information that the publication of that decree then followed.” The author goes on to say that he wished to show to foreign nations how much was known in Italy, and particularly in Rome, on this subject; and that from this climate there proceed not only dogmas for the salvation of the soul, but ingenious devices for the delight of the mind.

This last clause certainly savours of bitter irony, and probably did not proceed from Father Riccardi’s pen. He then states that for the purpose in hand he had taken the Copernican part in the Dialogue as a pure mathematical hypothesis, endeavouring by every artifice to represent it as superior, not to that of the stability of the Earth absolutely speaking, but to the doctrine as defended by the Peripatetics, to whom he alludes with some contempt.

He adds that he will treat of three principal heads: under the first he would show that all our experience was insufficient to prove conclusively the motion of the Earth, but that it adapted itself equally to either theory; he hoped also to produce many observations unknown to antiquity. In the second place, the celestial phenomena would be examined, by which the Copernican hypothesis would be so reinforced as if it ought to come out of the contest absolutely victorious. In the third place he would propound his theory about the tides: “proporrÒ una fantasia ingegnosa,” he says. He had long been of opinion that the unknown problem of the tides would receive some light on the assumption of the Earth’s motion. Other persons had adopted his statement on this point as if it had been their own; he therefore thought it desirable to expound it himself. He hints, too, that the willingness to admit the stability of the Earth, and to take the contrary side solely for mathematical caprice, is partly based on piety, religion, the knowledge of the Divine omnipotence, and the consciousness of human weakness.

He had thought it well to cast these thoughts into the form of a dialogue, which gave a certain amount of freedom to digressions.

He then introduces the personages who sustain the discussion, and who are supposed to meet at Venice at the palace of one of their number, Sagredo by name.

This preface, if one may judge by internal evidence, was probably the joint composition of Galileo and Father Riccardi, the former having written the original draft, the latter having altered the draft and supplemented it with important additions.

The body of the Dialogue—which I suspect that many persons who consider themselves competent to give an opinion on the Galileo case have not so much as even seen—is divided into four portions, each being supposed to be one day’s dialogue. The interlocutors are Salviati, Sagredo, and Simplicio. Great offence was taken at the rÔle attributed to this last-named personage—the true doctrine put into the mouth of a simpleton! It has been said that Pope Urban VIII. considered it as an insult directed against himself, because, in conversation with Galileo, he had used some of the very arguments employed by Simplicio. This, however, may have happened without the author intending thereby to offer any personal affront to His Holiness; some character was bound to appear on the anti-Copernican side, and it was inevitable that the arguments that Galileo had heard, whether from ignorant or enlightened antagonists, should be put into the mouth of such character. The name Simplicio is of course not meant as a compliment; moreover, he is made to say some very unwise things, and is occasionally treated with a sort of polite contempt by the scientific and mathematical Salviati; and yet he is not at all a simpleton in our sense of the word, he is a devoted follower of Aristotle, whom he constantly quotes, and is in fact a type—probably exaggerated—of the school of the Peripatetics, as they were, and still are, called; he does not know much of geometry or arithmetic, and so is at no small disadvantage when arguing with Salviati, but he is far from being a mere fool. Our author, in his preface, introduces Salviati and Sagredo—the former a Florentine, the latter a Venetian—as real personages, deceased friends of his own, though this may be a mere conventional form of expression; but he expressly states that Simplicio is not the true name of the “buon Peripatetico.”

The friends are supposed to meet in the palace of Sagredo, at Venice, as before stated.

The first day’s dialogue deals with a good deal of what one may term preliminary matter: that bodies have three dimensions and no more; that circular motion is the most perfect and the most natural; showing by this that Galileo had not at that time arrived at a true comprehension of the first law of motion, as we now hold it. The motion of weights on an inclined plane finds also a place in the discussion; and so does what we now term the law of accelerating force, which Galileo had grasped so well as to be able to explain how the velocity increases by infinitely small steps gradually, and not, as it were, by sudden jumps.

Much of the matter disputed on—as, for example, whether the heavenly bodies being incorruptible differ in that respect from the Earth, liable as it is to corruption and decay—which seems to us either erroneous in conception or irrelevant to the question at issue, or both—arose out of the old Aristotelian philosophy; and in those days a dissertation which neglected points of this kind would have been looked upon probably with contempt, as evading subjects that it ought to have grappled with. The distinction between natural and artificial motion, which occurs repeatedly in the Dialogue, is an instance of an utterly mistaken notion, having its origin in Aristotle, who, great philosopher though he was in other ways, failed in his investigations of physical science, partly from being misled by verbal fallacies.7

Another point that our author endeavours to establish in the first day’s dialogue is that the Moon is not a polished surface, as Simplicio and others thought, but much like our own Earth, with mountains and plains and seas—this last being a mistake, as subsequent observation has shown. The solar spots are also discussed, and so, incidentally, is the question whether the heavenly bodies are inhabited, the affirmative opinion finding little favour with any one.

During the second day the great subject is the revolution of the Earth on its axis; and Salviati urges forcibly the improbability of the motion of the whole celestial sphere round the Earth in twenty-four hours, including such a number of vast bodies, and with such an immense velocity, while one single body (the Earth), turning round on itself, would produce the same effect. He argues also that if you believe in this motion of the celestial sphere, you must suppose the planets to be moving in two opposite directions at the same time, the diurnal one from east to west, and the annual one from west to east—using the word annual in its extended sense, as applied to the periodical revolutions of all the planets. To this Simplicio makes the sapient answer that Aristotle proves that circular motions are not contrary to each other; upon which the third interlocutor, Sagredo, asks him whether when two knights meet one another in the open field, or two fleets at sea—in the latter case sinking each other—such motions can be called contrary? This Simplicio is obliged to admit; he uses, however, another argument, which did not seem so absurd in the then existing state of science, namely, that there may be another sphere beyond that of the stars, and itself starless, to which belongs the property of the diurnal revolution, and that this sphere may carry along with it the inferior spheres, these latter participating in its movement. Ideas such as these were part of the pre-telescopic notions of astronomy. Simplicio’s argument is in reply to some powerful reasons drawn from the motions of the planets, the nearer revolving in a shorter, and the more remote in a longer period; it being extremely unlikely that they would be all whirled round the Earth in one day; and also from considerations connected with the stars.

It took a long time to disabuse the human mind of the antiquated opinion that the stars and planets were set in vast movable spheres, as lamps might be set in a large revolving cupola.

One of the objections made at that time against the axial rotation of the Earth was that, if it were really the case, any weight dropped from a high tower would fall some way to the west of the tower, on account of the latter having been carried on eastward by the revolution of the Earth during the few seconds the weight takes in falling,8 and that such a result was contrary to experience. In those days, when even the first law of motion had been barely guessed at, the second law, that of the action of combined forces on any body, was of course not generally understood; and a considerable debate as to this point occurs in this same day’s dialogue. Simplicio has the hardihood to assert that if a stone be let fall from the mast of a vessel, the vessel being in motion, it falls behind the mast. Salviati, after making a foolish distinction—in accordance, however, with the philosophical ideas then prevalent—between the natural motion of the Earth on its axis, and the artificial motion of the vessel, asks Simplicio if he has ever tried the experiment, which, of course, he had not. He then tells him, and most truly so, that the experiment, if made, would show a very different result, and that the stone would fall at the foot of the mast, whether the vessel were in motion or not. Further on, Simplicio maintained that a projectile thrown from the hand, according to Aristotle’s argument, is carried on by the air, itself set in motion by the hand of the projector; and if the stone let fall from the mast of a ship falls at the foot of the mast, it must be the effect of the air. So again he imagines that a ball dropped from the hand of a man, riding fast on horseback, falls some way behind, and does not partake of the horse’s speed. Salviati, however, tells him that he deceives himself, and that experience would teach him the contrary.

Various difficulties are discussed in this dialogue well known to the disputants of that day. It being questioned why a projectile shot from a gun point-blank towards the east does not fall above the mark aimed at; or shot westwards fall below it? How it is that birds, when flying, are not left behind by the revolving Earth, since they at any rate are completely detached from the ground above which they are soaring? Why it is that light objects do not fly off at a tangent?

One sees throughout the power of the master-mind of Galileo. He knew many things in mechanics which no subsequent research or experiment has ever corrected; but here and there, as may naturally be supposed, he is at fault. It must ever be remembered that a dialogue, though a convenient form of argument in some respects, does not always give one a clear insight into the author’s real convictions. You are not sure whether he quite agrees with any of the spokesmen, and, indeed, Galileo, in his defence before the Inquisition, practically assumes that he did not so agree. It is, however, a good form of discussion for a man whose opinions are intended to be expressed in a tentative shape, and perhaps Galileo’s mind was in a state congenial to such expression. But, at any rate, it makes it rather more difficult to do justice to the author, as one is never sure what he intends to be taken as the expression of his own deliberate belief; indeed, whatever may have been the amount of indecision in which in this case our author’s mind was involved, it is scarcely possible, notwithstanding his disclaimer, to ignore the fact of his strong Copernican opinions.

I think one may say that Galileo did not, at the time when he wrote the dialogue, know the gravity of the air. I say at that time, because it is quite possible that he knew it before his death, since he lived some ten or twelve years after writing this work. It is maintained that he knew it because there is extant a letter from Baliani, the date of which I believe to be about 1631, in which the latter expresses his acknowledgments to Galileo for having taught him this truth. May it not, however, be that what is here meant is the pressure of the air? If any one thinks Galileo understood at that time the principle of the gravity of the atmosphere, I refer him to the second day’s dialogue. He was aware, no doubt, that the air was carried round by the Earth in its diurnal motion, but why it was so carried round I do not think he quite understood; indeed, as may well be supposed, he did not clearly understand what gravity was; it was a mysterious force, drawing heavy bodies towards the centre of the Earth, a force to which we, indeed, give the name of gravity, but of the essence of which we know nothing, as, in fact, we know nothing of the nature of the force that moves the heavenly bodies. This passage is remarkable because it looks as if Galileo half suspected that the force which acted on the Moon and the planets might be akin to that which attracted terrestrial objects towards the centre of the Earth. If he really had arrived at such a conclusion, he would have anticipated the great discovery made thirty or forty years later. I think, however, that he only wished to illustrate the one by the other, and that the allusion means no more. I give, however, the passage in a note,9 so that any reader may form his own judgment; and I may add that according to an opinion commonly held by the Copernican school of that age, the adherence of the atmosphere to the Earth as it revolved was the effect of friction.

Our philosopher, wise as he was, had not freed himself from the antiquated notion that some bodies were essentially heavy and others light, which latter had no tendency to descend; not thereby meaning comparatively light substances, but such as were absolutely free from the action of gravity; the fact not being then understood that it is only the resistance of the air that prevents the smallest feather from falling to the ground as quickly as a cannon-shot.

Another mistake into which he falls is that of maintaining, in answer to the argument that the diurnal rotation of the Earth would cause objects to fly off from the surface at a tangent, that no amount of velocity of rotation would be sufficient for such a result to follow; whereas, it is well known to modern students of mechanics that if a certain very high velocity of rotation were reached, the centrifugal force would overcome that of gravity, and objects would be projected from the surface of the Earth in the direction of the tangent at that point.

Some irrelevant arguments occur, of which, no doubt, many were employed at that time on both sides; I think it was the late Professor de Morgan who (in an article written for a popular periodical) made a list of these; and it must in all fairness be said, that this circumstance ought to be taken into account, as palliating the apparent obstinacy of the anti-Copernican party in denying the motion of the Earth. The argument drawn from the tides is, of course, the most striking instance of these scientific fallacies; but it was by no means the only one; in this particular dialogue there is another, which is worth noticing because it confirms what I have just said as to Galileo knowing nothing of the doctrine of universal gravitation. He puts into the mouth of Salviati the argument that bodies which emit light, as do the Sun and fixed stars, are essentially different from those which, like the Earth and planets, have no such property—a distinction which modern astronomy does not endorse—and that, as the Earth in this respect resembles the planets, and the planets are undoubtedly moving, so probably the Earth also is like them in motion, whilst the Sun and the stars remain at rest. It is obvious that ideas of this kind, however plausible they may seem, are utterly at variance with the theory of universal gravitation, according to which, even if the Sun were a dark, cold body and the Earth glowing with heat and light, the Earth would revolve about the Sun just as it does now, provided the mass of the two bodies remained the same as at present.

Another suggestion, and a rather amusing one, on the opposition side, was that all things in motion require occasional rest, as we see to be the fact with animals; therefore the Earth, if it were constantly moving, would stand in need of rest—an argument, I suppose, which needs no very elaborate answer.

In the third day’s dialogue a question is raised, and sifted at great length, as to whether a certain newly observed star in the constellation Cassiopeia was in the firmament among the distant fixed stars, or “sublunar,” i.e. nearer to the Earth than the Moon. This star was probably the same as the very remarkable one first observed by Tycho BrahÉ in 1572, which attained a brilliancy so extraordinary, that it is said to have been equal to the planet Venus, and to have been visible to good eyes in full daylight; in about a month’s time it appeared to grow smaller, and gradually faded away until it disappeared entirely—about six months after it was first discovered. This was some years before the invention of the telescope, and the observations were deprived of any assistance they might have gained from that source. The star was one of the most noteworthy of all the variable stars on record.

There followed upon the mention of this star, a dissertation on the method of finding the distances of the heavenly bodies by parallax. The principle of this method was, as we may suppose, well known to Galileo; but he probably did not allow sufficiently for the great difficulty in taking accurate observations, especially with the imperfect instruments then in use; I say sufficiently, because that there were such errors he knew, and he insists on the fact in the Dialogue.

Much discourse is spent on the distance of this new star; the apparent reason of which is that it had created some sensation among the astronomers of that day, and therefore the subject received an attention out of proportion to its real importance—I mean importance so far as the Copernican controversy was concerned.

The conversation is then brought back to the objections made by contemporary philosophers to the Copernican system. Aristotle’s idea of the universe was that of a vast sphere, or number of concentric hollow spheres, with the Earth in the centre; if that were shown to be probably untrue, his system broke down.10 Coming, however, to our own immediate portion of the universe, the question is now raised whether the Earth or the Sun is the centre of revolution. Galileo, by the mouth of Salviati, explains forcibly the argument for the Sun being so. That Mercury and Venus revolve round the Sun he takes for certain; the phases of Venus, which he had himself observed, proved it as regards that planet; and the fact of neither of these bodies ever being seen far apart from the Sun, greatly strengthened the conclusion in respect of both of them. A transit of Mercury over the Sun’s disc had, in fact, been observed in the year 1631, by Gassendi; but Galileo was doubtless not aware of it when he wrote the Dialogue.

It being clear then that Venus and Mercury revolve round the Sun, Galileo shows what strong ground there is for inferring that the superior planets, Mars, Jupiter, and Saturn (the others not being then known), do so also; this he judges from the greater size of these latter, and particularly of Mars, when in opposition than when in conjunction; whence we may conclude that the Earth, which as well as the Sun is contained within their orbits, is not in the centre of them, or nearly so. It is remarkable that Galileo treats all the planets as revolving in circles, though one would think he must at that time have been aware of Kepler’s discovery—that they move in ellipses. He makes Simplicio grant these last-mentioned points, which is curious; and he also explains how the telescope showed phenomena, such as the phases of Venus, which were unknown to Copernicus. Simplicio has hitherto had no confidence in this new instrument, and following in the footsteps of his friends the Peripatetic philosophers, has supposed the appearances in question to be optical illusions arising from the lenses used; he will, however, gladly be corrected if in error. Simplicio’s mathematical acquirements are not very great, and it is necessary to explain to him that the areas of circles vary in proportion, not to their diameters simply but to the squares of the diameters, a point which arises in reference to the false judgment formed by the naked eye as to the size of the celestial bodies, an error which is corrected by the telescope. Then to those who made it a difficulty that the Earth should move round the Sun, not alone, but accompanied by the Moon, Salviati is made to reply that Jupiter revolves round the Sun accompanied by four moons.

Again the greater simplicity of the Copernican theory, in accounting for the planetary motions, as they appear to us, is expounded by the same personage.

Galileo occasionally makes the interlocutors allude to himself as “il nostro amico comune,” “il nostro Accademico Linceo,” etc., and thus claims credit for having been the first to discover the solar spots, a credit which ought not to belong exclusively to him, as Fabricius and the Jesuit Father Scheiner saw the spots at about the same time.

An argument is here attempted to be drawn in favour of the Earth’s annual motion from the apparent course of the Sun-spots, and the curves they sometimes describe (as viewed from hence), owing to the inclination of the Sun’s axis to an axis perpendicular to the plane of the ecliptic—an inclination of about 7°; there is nothing, however, at all conclusive in such argument, because the appearances in question result from the different relative positions of the Earth and Sun at different seasons of the year, and would be the same whichever of the two bodies were in motion.

There follows some conversation arising from one of the anti-Copernican books of that day; one of the difficulties suggested, being the vast distance at which you must suppose the fixed stars to be placed, if Copernicus be right. We who are accustomed to the idea of these immense distances, can scarcely understand the prejudices of the philosophers of that age against admitting them. And it is worth noting that Galileo takes for granted, while answering these theoretical objections, the calculation of his predecessors—that the distance of the Sun is that of 1,208 semi-diameters of the Earth, that is something more than 4,800,000 miles, about one-nineteenth part of what we now know it to be. So also he supposes the size of the Sun to be much less than what is really the case. He was also under the erroneous impression, arising doubtless from the imperfection of the instruments he used, that the stars really had an apparent diameter, though less than Tycho BrahÉ and other astronomers had supposed, and estimates the angular diameter of a star of the first magnitude at about 5; consequently he imagined the stars to be much nearer than is actually the fact. It is well known to modern observers, that the apparent size of a star is the effect of an optical illusion, and that greatly as the stars vary in brightness, they present no appreciable diameter at all to the eye; not even those classed as being of the first magnitude.

Another and more weighty objection to Copernicus is, however, urged by the mouth of Simplicio, and it is this—if the Earth really makes an annual revolution round the Sun, why do not the fixed stars, viewed as they must be at different seasons of the year from points so widely distant, change their apparent positions in the heavens? We have just seen that the true distance of the Sun was not known at that time;—if it had been known, and if the men of that age had been aware that the diameter of the Earth’s orbit was about 184,000,000 miles in length, the objection would have been still more forcible. But the modern answer to it is conclusive: the stars, or rather a certain number of them, do actually undergo a small displacement in their apparent position every year, or in the technical language of astronomy, they have an annual parallax, a fact which not merely disposes of the objection, but actually confirms the truth of the Copernican theory.

Galileo’s reply (by the mouth of Salviati) is to the effect that the followers of Ptolemy admit that it takes 36,000 years to effect a complete revolution of the starry sphere; then, judging from the planets, the length of time required for the orbit is in proportion to the distance, and we suppose the distance of the starry sphere to be, on such assumption, 10,800 semi-diameters of the Earth’s orbit (or Sun’s orbit, as they called it). At so great a distance as that, the change of position caused by the Earth’s annual motion round the Sun would not be appreciable.

The principle of this reply is of course quite sound, and we, who know the stars to be considerably farther from us than the above estimate supposes, can well understand that the vast majority of them have no annual parallax whatever, that the finest instruments can discover.

To further objections drawn from the enormous distances of the stars, and the difficulty of perceiving the use which such remote bodies can be to the Earth, it is replied that such speculations are useless and presumptuous, and also that words like small, very small, immense, etc., are relative rather than absolute.

Some pains are taken in the course of the dialogue to explain how the stars, according to their different positions, would be affected by annual parallax, supposing such to be discoverable, and assuming the motion of the Earth. And a minute explanation is also given, on this latter assumption, of the length of day and night varying in different latitudes according to the seasons; illustrating the fact that details which appear to us elementary and are taught to schoolboys, were strange to the minds even of educated and learned men in those days.

One remark, arising from the questions connected with stellar parallax, is most striking, as showing how far Galileo was advanced in his knowledge of pure mathematics as well as of mechanics and astronomy. Salviati is made to say that the circumference of an infinite circle is identical with a straight line: “sono l’istessa cosa.” This idea, familiar though it be to modern mathematicians, is one that we should not have expected to find enunciated in the early part of the seventeenth century; even the intelligent Sagredo cannot understand or believe it, and it is not further discussed; but the fact of its being here stated is especially noteworthy.11

Another (less felicitous) guess is hazarded by the same interlocutor Salviati, who, as I have already remarked, appears to be the one that most nearly represents the author’s own mind,—to account for the Earth keeping her axis pointed (approximately, that is to say) in the same direction during each annual revolution round the Sun. Salviati suggests that it may be due to some magnetic influence, and that the interior of the Earth may be a vast loadstone. This is strange, because it is evident from what immediately preceded, that the author was aware of the true reason, which in fact he illustrates by the well-known experiment of a light ball floating in a bucket of water, to which a revolving motion is imparted. It seems, however, that a work by William Gilbert on the subject of magnetism had had some influence on the scientific thought of the period, and that Galileo had considered it worthy of his attention. The writer had maintained the probability of this theory, of the Earth’s interior being an enormous loadstone—not an unnatural idea in the then-existing state of science—and Galileo was evidently somewhat fascinated by the hypothesis. Magnetism was attracting the notice of the philosophers of that day, and the property of the needle, which is termed the dip, had been recently discovered.

There is not much else worthy of special mention in the third day’s dialogue; which in fact, as a whole, is not equal to that of the second day.

The fourth day is mainly devoted to the argument drawn from the tides. It was in handling this branch of the subject that Galileo’s great sagacity and power of discernment seem to have deserted him. It is a curious thing that the inhabitant of a Mediterranean country, who, for all that one knows, never saw a really great tide in his life, should have seized upon this topic, and so utterly misused and perverted it.

If, instead of living in Italy, he had resided at an English seaport, he would probably have never fallen into the mistakes he thus made. In the Mediterranean there are currents, arising from other causes, which he, however, attributed to tidal action; but for the most part there is little, if any, appreciable ebb and flow of the tides, scarcely any perceptible rise and fall of the sea, a fact which he particularly notices. But in some few places, and notably at Venice, there is a sensible tide, so it is said, causing a difference of a few feet between high and low water.

Now Galileo was under the impression that the ebb and flow took each about six hours, following the ordinary solar day; whereas, if he had observed the phenomenon on the shores of any sea, where the tidal wave of the ocean made its full force to be felt, or again, at the mouth of a great tidal river, he never could have failed to perceive that the rise and fall of the water follow approximately the lunar, and not the solar day, the former being fifty minutes longer than the latter. It must of course be understood that the theory of the tides was first investigated fully and scientifically by the same great genius to whom we owe the theory of universal gravitation; and Galileo, who lived half a century earlier, may well be excused for not having grasped it. But it had long been known that the Sun and Moon had an influence upon the tides, and as I have just stated, any one who watched the movements of the sea from day to day, and from week to week, at a place where there is a great rise and fall—as for instance, in the Bristol Channel—could not fail to perceive that the Moon had the principal share in the work, however unable he might be to comprehend the theory. Besides which, the theory, however obvious to us (at least in its main outlines), was not by any means so intelligible to the men of Galileo’s age. They might just guess that the Sun exercised some attractive influence over the Earth, and the Earth again over the Moon, but they did not know that the Moon attracted the Earth exactly in the same way, though with far inferior potency, owing to her much smaller mass; and consequently they were not aware of the Moon’s power to raise the great tidal wave in the ocean, to which are due the remarkable phenomena so familiar to the inhabitants of the English coasts.

Galileo would have been wise if he had not touched on a point which he neither understood in theory, nor had properly acquainted himself with by practical observation. Good causes are often damaged by bad arguments, and such was the case on this occasion.12 There was, however, something ingenious in his argument. If you take a basin of water, and move it along quite smoothly and evenly, no great commotion in the water takes place; but suppose some stoppage or jerk to occur, the result will be, as we know, very different. Now the Earth has two motions, one round its axis in twenty-four hours, and the other round the Sun in one year; every point, then, on the Earth’s surface moves through space more rapidly while on that side of the globe which is turned away from the Sun, than on that side which by the diurnal revolution is turned round in the contrary direction. Here, then, with the sea lying in its vast basin, and revolving with other things on the surface of the Earth from west to east every day, and thus accelerated in its motion through space during twelve hours and retarded during the other twelve hours, you have on a large scale the same result that a basin, half full of water, held in your hands and checked by some retarding obstacle, gives you on a very small and minute scale. Strange indeed it is that a man who was acquainted with the laws of motion sufficiently to know that anything thrown or dropped in a vessel or a vehicle, partook of the motion of the latter and followed its course (so long as it remained within the vehicle) just as if the whole were at rest—that he should have failed to perceive that the ocean, lying in its bed in that mighty vehicle the Earth, would be carried round in the daily rotation with an uniform velocity, unless interfered with by the attraction of other bodies. Simplicio, who for once is right, puts the difficulty, that if the sea behaved in the way supposed, the air would do so in the same way: the reply to which is that the air being thin and light is less adherent to the Earth than the water which is heavier, and does not accommodate itself to the Earth’s movements as water does; further, that where the air is not hemmed in, as it were, by mountains and other inequalities on the Earth’s surface, it really is partially left behind by the diurnal rotation, and in the neighbourhood of the tropics, where the effect is chiefly felt, a constant wind blows accordingly from east to west. Our philosopher had evidently heard of the trade winds, though he had not acquired an accurate knowledge of their course or of their origin. It is undoubtedly true that they do help strongly to prove the revolution of the Earth, because they arise from cold currents of air flowing in from the north and from the south respectively towards the tropics, to supply the place of the atmosphere rarefied by the sun’s heat, and consequently ascending, as is the case in those regions. Then these cold currents, coming from latitudes where there is a less velocity of rotation, tend to preserve that velocity and lag behind the Earth as it revolves, so that they have the effect of north-easterly winds in the northern hemisphere, and south-easterly in the southern hemisphere. Galileo’s imperfect information prevented him from using this important argument.

However, to return to the tides. He had to account for other phenomena, besides the daily rise and fall, namely, for the much greater rise and fall which take place soon after new and full moon, and which are known as the spring-tides. Unable to deny that these were in some way due to lunar influence, he took refuge in the supposition that the Moon, when at the full, retarded the motion of the Earth in its orbit, since as the two travel together round the Sun at those particular times, they form, as it were, a lengthened pendulum, longer than at other times by the semi-diameter of the lunar orbit; and therefore (like any other pendulum) must vibrate more slowly. I should say that he does not appear to have been aware of the existence of two spring-tides in each lunation, and therefore only tries to account for one; and it is obvious that this method of explaining them is not only utterly inadequate, but even absurd. The Moon truly enough exercises a certain disturbing influence on the orbital motion of the Earth, but that has nothing to do with the spring-tides.

There remained the necessity of accounting for the annual, or, more properly, semi-annual increase of the ebb and flow of the sea. Galileo suggests that this arises from the angle made by the plane of the equator with the ecliptic at the equinoxes, owing to which there would not be the same counteraction exercised by the Earth’s motion in its orbit on the waters of the ocean at those periods as there would at the solstices. But it seems that this would rather tend to diminish the tides than to increase them, as, indeed, would be the case as regards the last-mentioned explanation with respect to the ordinary spring-tides. What really does happen at the equinoxes is, that the Sun and the full or new Moon being at those times vertical to the equator (or nearly so), they have a greater attractive force than at other spring-tides over the vast expanse of the ocean, and the tides are consequently greater. There is also another increase which sometimes occurs when the Moon happens to be at its least distance from the Earth at the time of spring-tides, but that was unknown to Galileo. He touches, however, and very properly so, on the great modifications in the tides caused by various gulfs, by the forms of the great continents, and the shapes of different seas—modifications, in fact, which are well known to be almost innumerable, and have been learnt only by careful observation and experience.

One of the worst features of this Dialogue is the contempt which the author shows for those opinions on the subject which differ from his own; and it is difficult to suppress a feeling of disgust when he alludes in this way to Kepler, who had partly guessed the true cause of the tides, and of whom he otherwise speaks in terms of respect.13

If a man of science, when he wishes to publish to the world a discovery or a hypothesis, adopts the form of a dialogue as a method of stating his case, he ought in all reason to do full justice to the antagonistic side, and state his opponent’s case as well as his own. I fear that Galileo failed to do this, not only in this particular dialogue, but also to some extent in those of the three preceding days. Simplicio, as I said above, is not a fool, but as a personage in a scientific argument he is lamentably deficient.

Simplicio at the end of the Dialogue urges that God could, in His infinite power, cause the tides by some other means than those suggested by Salviati, to which true and pious (though, perhaps, rather irrelevant) argument the latter respectfully and devoutly assents.

The concluding sentences are said, as I have remarked elsewhere, to have been recast or retouched by Father Riccardi.

It is worth noticing that there is a passage in the fourth day’s dialogue, in which the author alludes to the fact of the Sun being apparently longer by about nine days in passing along the ecliptic from the spring to the autumn equinox, than in passing from the autumnal to the vernal; that is to say, of the northern hemisphere having so much longer summer than winter, and he treats it as one of the recondite problems of astronomy not as yet understood. This is an additional proof that for some reason or another he had not made himself acquainted with Kepler’s researches; for as soon as it became known that the planets move, not in circles, but in ellipses, with the Sun in one of the foci, it was obvious that there would be in every case (though in some more than others) this inequality to which allusion has been made, and the Earth, if a planet, would be subject to the same rule as the rest.

Such, then, is a somewhat imperfect prÉcis of this famous work of Galileo, which owes its importance to the historical circumstances connected with its publication quite as much, to say the least of it, as to its own intrinsic merit.