The same astronomer was perhaps the first person who conceived that there must be al ways a law capable of being expressed by arithmetic or. geometry, which connects such phenomena as have a physical dependence on one another. His conviction of this truth, and the delight which he appears to have experienced in the contemplation of such laws, led him to seek, with great eagerness, for the relation between the periodical times of the planets, and their distances from the sun. He seems, indeed, to have looked towards this object with such earnestness, that, while it was not attained, be regarded all his ether die. coveries as incomplete. He at last found, infinitely to his satids.otion, that in any two planets, the squares of the times of the revolution are as the cubes of their mean distances from the sun. This beautiful and simple law had a value beyond what Kepler could pos sibly conceive ; yet a sort of scientific instinct instructed him in its great importance. He has marked the year and the day when it became known to him ; it was on the 8th of May 1618 ; and perhaps philosophers will agree that there are few days in the seienti fic history of the world which deserve so well to be remembered.
These great discoveries, however, Were not much attended to by the astronomers of that period, or by those who immediately followed. They were but little considered. by Geseendi, --wthey were undervalued by Riccioli,--end were never mentioned by Descartes. It was an honour reserved for Newton to estimate them at their true value.
Indeed, the discoveries of Kepler were at first so far from being duly appreciated, that they were objected to not for being false, but for offering to astronomers, in the calcula tion of the place of a planet in its orbit, a problem too difficult to be resolved by elementary geometry. To cut the area of a semi-ellipsis in a given ratio by a line drawn through the focus, is the geometrical problem into which he showed that the above inquiry ultimately resolved. As if he had been answerable for the proceedings of nature, the difficulty of this question was considered as an argument against his theory, and he himself seems some what to base felt it as an objection, especially when he found that the best solution he-conk! obtain was. no more than an approximation. With all his power of invention, Kepler WAS a mathematician inferior to many of that period ; and though he displayed great ability in the management of this difficult investigation, his solution fell very far short of the simpli city which it was afterwards found capable of atWning.
In addition to all this, he rendered another very important service to the science si astronomy and to the system of Copernicus. Copernicus, it has been already men tioned, had supposed that a force was necessary to enable the earth to preserve the parallelism of its axis during its revolution round the sun. He imagined, therefore, that a third motion belonged to the earth, and that, besides turning on its axis and revolving round the sun, it had another movement by whiCh its axis was preserved always equally inclined to the ecliptic. Kepler was the first to observe that this third motion was quite superfluous, and that the parallelism of the earth's axis, in order to be ' preserved, required nothing but the absence of all force, as it necessarily proceeded from the inertia of matter, and its tendency to persevere in a state of uniform motion. Kepler
had a clear idea of the inertia of body ; he was the first who employed the term ; and, considering all motion as naturally rectilineal, he concluded that when a body moves in a curve, it is drawn or forced out of the straight line by the action of some cause, not resid ing in itself. Thus he prepared the way for physical astronomy, and in these ideas he was earlier than Descartes.
The discoveries of Kepler were secrets extorted from nature by the most profound and laborious research. The astronomical discoveries of Galileo, more brilliant and imposing, were made at a far less expense of intellectual labour. By this it is not meant to say that Galileo did not possess, and did not exert intellectual powers of the very highest order, but it was less in his astronomical discoveries that he had occasion to exert them, than in those which concerned the theory of motion. The telescope turned to the heavens for the first time, in the hands of a man far inferior to the Italian philosopher, must have un- • folded a series of wonders to astonish and delight the world.
It was in the year 1609 that the news of a discovery, made in Holland, reached Galileo, viz. that two glasses had been so combined, as greatly to magnify the objects seen through them. More was not told, and more was not necessary to awaken a mind abundantly alive to all that interested the progress either of science or of art. Galileo applied himself to try various combinations of lenses, and he quickly fell on one which made objects ap pear greater than when seen by-the naked eye, in the proportion of three to one. He soon improved on this construction, and found one which magnified thirty-two times, nearly as much as the kind of telescope he used is capable of. That telescope was formed of two lenses ; the lens next the object convex, the other concave ; the objects were presented upright, and magnified in their lineal dimensions in the proportion just as signed- • Having tried the effect of this combination on terrestrial objects, he next directed it to the moon. What the telescope discovers on the ever-varying face of that luminary, is now well known, and needs not to be described ; but the sensations which the view must have communicated to the philosopher who first beheld it, may be conceived more easily than expressed. To the immediate impression which they made upon the sense, to the wonder they excited in all who saw them, was added the proof, which, on reflection, they afforded, of the close resemblance between the earth and the celestial kidies, whose divine nature had been so long and so erroneously contrasted with the ponderous and opaque substance of our globe. The earth and the planets were now proved to be bodies of the same kind, -and views were entertained of the universe, more suitable to the simplicity and the magni ficence of nature.