Before proceeding further it will help ns here to observe, that Copernicus does not in the smallest degree attempt to answer the mechanical objections to the earth's motion, which were urged with success against his system till the time of Galileo. The laws of motion, as then explained, and as admitted by Copernicus himself, were alto gether insufficient to explain why, if the e firth moved, a stone should fall directly under the point from which it is dropped. No explanation of such difficulties is given by Copernicus, nor can we find (nor did Delambra find) that they are even alluded to as an elemeut of the question. If the mechanics of Copernicus had been true, the system of Copernicus would have been physically impossible. Now this is an essential element in the character of a discovery, which is materially altered if that which is advanced as true be advanced on false grounds. It is true that fire burns, and it is true that two and two make four ; but it is false that two and two make four because fire burns. We give no credit to the Pythagoreans, if it be true, as asserted, that they placed the sun io the centre of the planets because they thought file the most excellent of all things. We may consider the omission of Copernicus in two different lights. The first is, that he saw the mechanical difficulties, but was so struck by the simplicity of his astronomical system that he thought it more probable than the mechanics of his day, and suspected that future researches would produce laws of motion which shonld allow of the possibility of his system; and thinking thus, he judged it more wisely not to enter upon the mechanical question, so as thereby to shock two sets of received opinions at once. This would do honour to his sagacity ; but unfortunately, the single sentence below alluded to, the equestrian sireile, prevents us from supposing that if lie considered the subject mechanically at all, he was other than satisfied with his own conclu sion, "Cum ergo marls circularis sit universorum, partium vero etiam rectus, dicers possumus manere cam recto circularem, skirt cum arra animal." The word in italics must be a misprint for equo, as remarked by Muler. The latter distinctly points out that this is meant to explain the difficulty of a falling stone, and adds, 'Sententios hujus veritas dependet ex hypothesi Coperniciana ;" that is, the truth of the proof depends upon that of the thing proved. He should have said (and possibly did mean), that upon the truth of this sentence the Copernican hypothesis depends, so far as it was proved by Copernicus himself. Our readers now must begin to have an idea how great an injustice has been done to those who found better reasons for the co-existence of rectilinear and circular motion, by the attachment of the name of Copernicus to the present cosmical system.
The second method by which we may suppose Copernicus to have reconciled difficulties, is the actual assertion made both by himself and °slander, that the hypothesis of terrestrial motion was nothing but an hypothesis, valuable only so far as it explained pheuomeua, and not considered with reference to absolute truth or falsehood. It is usual to consider the expressions in question entirely as a concession to general opuioo, and as iutended either to avoid the Inquisition, or to induce those to look into the book who would otherwise have put it aside as anti-Mosaical and heretical. And though there may be some truth in this, we are on the whole inclined to suspect that the hypothetical hypothesis, as we may term it, really did bias the mind of Copernicus much more than has been supposed. We do not at all
concede that the interference of ecclesiastical power was as likely in the case of the Prussian priest of 1540, as in that of the Italian lay man of 1633. Nothing is more common than to view the middle ages as a whole, without noticing the ebbs and floods of power and opinion. The epoch contained between the last Lateran Council and that of Trent, in which the work of Copernicus was written, printed, and published, was sufficiently occupied by diocesan councils, both against Luther, and on the reformations in discipline, of which the necessity began to be perceived. It appears to us far the most likely that the mind of Copernicus must have balanced between the two views we have described, and probably must have inclined different ways at different times.
We now come to the brightest jewel in the crown of Copernicus, the method in which he explained, for the first time and with bril liant success (so far as demonstration went, as before described), the variation of the seasons, the precession of the equinoxes (book i. cap. 2, book iii., and book vL cap. 35), and the stations and retrogradations of the planets. The latter point is fully made out, and in the manner now adopted, so far as the qualities of the phenomena are con cerned: we shall presently see the method of rectifying the quantities. With regard to the variation of the seasons, Copernicus explains it rightly, from the continual parallelism of the earth's axis. But he cannot obtain this parallelism from his mechanics. He imagines that if the globe of the earth move round the sun, and also round its own axis, the axis of rotation must always preserve the same inclivation to the line joining the centre of the earth and sun : just as when a ball fastened by a string is made to spin, and a conical motion is simultaneously given to the string and ball. It is most evident that he got this idea from the solid crystal spheres. If the earth's axis were fixed in an immense sphere, with which it turned round the sun, and if in the first instance the axis produced would pass through the axis of the sphere, the complete phenomenon of Copernicus would be produced. The earth's axis would then describe a cone yearly. To produce parallelism, Copernicus imagines what we may call an anticonical motion, namely, that the earth's axis is itself endued with such motion, independent of its motion in the sphere, as would, did it act alone, carry the axis round the same cone in a year, but the contrary way. The effect of the two motions is to destroy each other, and the axis remains parallel iu all its positions. Then, by supposing the anticonical motion to be a little greater than the direct conical motion, by 50" in a year, he produces the phenomenon of the pre cession of the equinoxes. If we consider that even Newton himself, in tracing the effect of the forces which cause the precession, is thought to have misconceived his own laws of motion, it is not at this part of the mechanics of Copernicus that we need express surprise : and this explanation of the cause of the seasons and of the precession, together with that of the stations, &e of the pleads, must always place him among cosmical discoverers of the first order of sagacity.