[620] Vol. vi., p. 50.
[621] Montucla, ii., p. 297.
Progress of Copernican system. 32. The progress, however, of the Copernican theory in Europe, if it may not actually be dated from its condemnation at Rome, was certainly not at all slower after that time. Gassendi rather cautiously took that side; the Cartesians brought a powerful reinforcement; Bouillaud and several other astronomers of note avowed themselves favourable to a doctrine which, though in Italy it lay under the ban of the papal power, was readily saved on this side of the Alps by some of the salutary distinctions long in use to evade that authority.[622] But in the middle of the seventeenth century, and long afterwards, there were mathematicians of no small reputation, who struggled staunchly for the immobility of the earth; and except so far as Cartesian theories might have come in vogue, we have no reason to believe that any persons unacquainted with astronomy, either in this country or on the continent, had embraced the system of Copernicus. Hume has censured Bacon for rejecting it; but if Bacon had not done so, he would have anticipated the rest of his countrymen by a full quarter of a century.
[622] Id., p. 50.
Descartes denies general gravitation. 33. Descartes, in his new theory of the solar system, aspired to explain the secret springs of nature, while Kepler and Galileo had merely showed their effects. By what force the heavenly bodies were impelled, by what law they were guided, was certainly a very different question from that of the orbit they described or the period of their revolution. Kepler had evidently some notion of that universally mutual gravitation which Hooke saw more clearly, and Newton established on the basis of his geometry.[623] But Descartes rejected this with contempt. “For,” he says “to conceive this we must not only suppose that every portion of matter in the universe is animated, and animated by several different souls which do not obstruct one another, but that those souls are intelligent and even divine; that they may know what is going on in the most remote places, without any messenger to give them notice, and that they may exert their powers there.”[624] Kepler, who took the world for a single animal, a leviathan that roared in caverns and breathed in the ocean tides, might have found it difficult to answer this, which would have seemed no objection at all to Campanella. If Descartes himself had been more patient towards opinions which he had not formed in his own mind, that constant divine agency, to which he was, on other occasions, apt to resort, could not but have suggested a sufficient explanation of the gravity of matter, without endowing it with self-agency. He had, however, fallen upon a complicated and original scheme; the most celebrated, perhaps, though not the most admirable, of the novelties which Descartes brought into philosophy.
[623] “If the earth and moon,” he says, “were not retained in their orbits, they would fall one on another, the moon moving about 33/34 of the way, the earth the rest, supposing them equally dense.” By this attraction of the moon he accounts for tides. He compares the attraction of the planets towards the sun to that of heavy bodies towards the earth.
[624] Vol.ix., p. 560.
Cartesian theory of the world. 34. In a letter to Mersenne, January 9th, 1639, he shortly states that notion of the material universe, which he afterwards published in the Principia Philosophiæ. “I will tell you,” he says, “that I conceive, or rather I can demonstrate, that besides the matter which composes terrestrial bodies, there are two other kinds; one very subtle, of which the parts are round or nearly round like grains of sand, and this not only occupies the pores of terrestrial bodies, but constitutes the substance of all the heavens; the other incomparably more subtle, the parts of which are so small and move with such velocity, that they have no determinate figure, but readily take at every instant that which is required to fill all the little intervals which the other does not occupy.”[625] To this hypothesis of a double æther he was driven by his aversion to admit any vacuum in nature; the rotundity of the former corpuscles having been produced, as he fancied, by their continual circular motions, which had rubbed off their angles. This seems at present rather a clumsy hypothesis, but it is literally that which Descartes presented to the world.
[625] Vol. viii., p. 73.
35. After having thus filled the universe with different sorts of matter, he supposes that the subtler particles, formed by the perpetual rubbing off of the angles of the larger in their progress towards sphericity, increased by degrees till there was a superfluity that was not required to fill up the intervals; and this, flowing towards the centre of the system, became the sun, a very subtle and liquid body, while in like manner, the fixed stars were formed in other systems. Round these centres the whole mass is whirled in a number of distinct vortices, each of which carries along with it a planet. The centrifugal motion impels every particle in these vortices of each instant to fly off from the sun in a straight line; but it is retained by the pressure of those which have already escaped and form a denser sphere beyond it. Light is no more than the effect of particles seeking to escape from the centre, and pressing one on another, though perhaps without actual motion.[626] The planetary vortices contain sometimes smaller vortices, in which the satellites are whirled round their principal.