He eays, "It is not necessary that hypotheses should be true or even probable ; it is sufficient that they lead to results of calculation which agree with observations." He points out the admitted defects, and admitted unlikelihood, of several points of the Ptolemaic system; requires that the new hypothesis should be admitted on tho same footing as the ancient ones, and ends thus—" Neither lot any one, so far as hypotheses are concerned, expect anything certain from astro nomy, since that science can afford nothing of the kind; lest, in case he should adopt for truth things feigned for another purpose, he should leave this study more foolish than he came." With such safeguards, headed by the urgent request of a cardinal, and dedicated, probably by permission, to the pope, the work was ushered into the world, of which it was the ultimate destiny to help largely in overthrowing submission to authority in matters of science, whether to the doctrines of the Greeks or to the reputed interpretation of the sacred writings. The title-page is as follows :— Copernicus contsins an expression of recommendation to buy it in the title-pago, being the only instance of the kind we know. The second ecitsion, edited by Reeticus, was published at Basel, 1566, end is little esteemed ; the third, edited by Sluice was printed at Amsterdam in 1617, and again In 1610, with notes : it is the most correct of the three. This stuns Muter, in his • Tabules Frisicse,' Alcmaar, 1611, has reduced the hypotheses of Copernicus to the form of tables.
Wo now come to the description of the Copernican system, by which we mean, the system actually promulgated by Copernicus, and not the Keplerian, Gallilean, Newtonian, linlleian, Laplacian, &e., system which bears the name. We have before remarked plunk, Teeno], that it is customary to call all existing notions on the system of the world, Copernican. This matters little, considered as a mere method of expression ; but it becomes of consequence when, by means of it, a degree of lustre is thrown on the speculations of Copernicus, which, properly considered, they do not need, and, critically examiued, they will not bear. We are accustomed to see Copernicus represented as a man so far in advance of his age, that in the main points of his system nothing has been added and nothing subtracted. The plates in our elementary works show, uudor the heading of the ' Copernican system,' planets, satellites, and comets, all with orbits such as the latest discoveries have assigned. We shall therefore exhibit the 'Copernican system' as far as wo can in the words of its author (translated, of course), and at greater length than would have been necessary had no misconception prevailed. It will hardly be supposed that we intend an absolute depreciation of the merits of one of the most original talents that over existed. Copernicus was a mathema tician of the first order, a siueere lover of truth, a mind free from trammels to an extent which was then almost unknown, and which we should have deemed almost incredible had we not had the proof before us. It is no easy thing for us to conceive the state of a head furnished in youth with theories upon all things in heaven or earth, proved in as many words by the assertion that Aristotle sail it.
The work De Revolutionibus,' &c., consists of an introductory dedication to Paul III., and six books. In the former Copernicus distinctly informs us that, being discontented with the complexity of the prevailing systems, he closely examined all the writings of the ancients, to see if ho could fiud anythiug better. lie found the testi mony of Cicero and Plutarch, us to the opinions of 7e1icetne, Pythagoras, and Philolaus. He thereupon claims for himself the same licence, adverts to his hypothesis as purely fictitious, and says, Quamvis absurda opinio videbatur, tames quia sciebarn aliis ante me concessarn libertatem, ut quoslibet fingerent circulos ad demonstrandum pheno mena astrorum. Existimavimihi quoque facile permitti, ut experireas, an posito terra) aliquo motu firrniores dernonstrationese, quhm Mortice essent, inveniri in revolutions orbium ccelestium peanut." With
regard to the word demonstratiou, it must be particularly remembered that at this period the term, as applied to astronomy, always meant a showing how the thing would happen if the supposition were true, not a proof of the supposition itself. Thus, in this sense, the suppo sition of a daily motion of tho sun round the earth may be made to yield a perfect demonstration of the phenomena of day and night ; and it would be legitimate to say that of two hypotheses, one false and one true, the false one might give truer demonstrations of the celestial motions than the other.
The first book contains the propositions—I. That the universe is spherical ; which is proved by old reasons, such as that a sphere is the most perfect figure, &c. 2. That the earth is spherical; for which lie gives the same reasons as are now given. 3. That the earth and sea make one globe. 4. That the motions of all the heavenly bodies must be either uniform and circular, or compounded of uniform and circular motions. Nothing but circular motion, he asserts, could reproduce phenomena periodically; and he maintains that a simple body must move circularly. 5. He examines the questions whether tho earth can have an axical and an orbital motion, and satisfactorily shows that, supposing the distance of the fixed stars to be immense, there is no astronomical reason to the contrary. 6. He gives what he imagines to be a proof that the sphere of the fixed stars is immensely distant. It must be observed that he has no notion of a universe of stare unequally distributed throughout space. 7 and 8. lie examiuos and argues against the reasons of the ancients for placing the earth in the centre of the universe, by considerations which are as purely imaginary as those against which he was contending. lie says that circular motion must bo that of a whole, rectilinear motion that of a part separated from its whole; and from this assumption he deduces the falling of a body to the earth. That rectilinear and circular motiou can exist together is, according to him, a thing of the same kind as tho notion of a horse existing with that of an animal. He is through out possessed by the opinion that there must be a 'centrum mundi,' or fixed point in the middle of the universe, which however he con siders to he the sun, not tho earth. It is needless to any that tho 'centrum mundi' forma no part of the Newtonian system. 9. He contends for the possibility of the earth having several motions. 10. He establishes the order of the planets, remarks that it Ls impossible to explain the motion of Mercury and Venue upon the supposition of the earth being their centre, and observes that the motion of the other planets round the sun is perfectly possible, consistently with The taste of what we now should call the puff in the title-page is doubtleta that of (Wander, to whom it is slue that the great work of that of the earth, if the radii of their orbits be made large enough. He draws a diagram of the system in the manner now usual, and concludes with the following words, which must be considered as the first announcement of the system ;—" Proinde non pudet nos fateri hoc totum, quod lung prmcingit, ac centrum terse per orbem ilium magnum inter cmteras errantes stellas annuft revolutions circa solem transire, et circa ipsum ease centrum mundi : quo etiam sole immobili permanente, quicquid de motu soils apparet, hoc potihs in mobilitate term verilicari," &c. It must be observed that he lays down a sphere for the fixed stars so distinctly, that his commentator Muler finds it necessary to remind the reader that be does not name the spheres of the planets. But we shall presently see that he could not divest himself of the idea that the primitive motions of the planets were such as would be caused by their being fixed in immense crystal spheres which revolve round the sun.