Galileo and His Judges

by F. R. Wegg-Prosser

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?