Galileo and His Judges

by F. R. Wegg-Prosser

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.