Mars; till finally full conviction burst upon his mind: he had won the goal, the struggle of seventeen long years was ended, God was vindicated, and the philosopher, in the wild excitement of his glorious triumph, exclaims:

"Nothing holds me. I will indulge my sacred fury! If you forgive me, I rejoice; if you are angry, I can bear it. The die is cast. The book is written, to be read either now, or by posterity, I care not which. It may well wait a century for a reader, since God has waited six thousand years for an observer!"

More than two hundred years have rolled away since Kepler announced his great discoveries. Science has marched forward with swift and resistless energy. The secrets of the universe have been yielded up under the inquisitorial investigations of god-like intellect. The domain of the mind has been extended wider and wider. One planet after another has been added to our system; even the profound abyss which separates us from the fixed stars has been passed, and thousands of rolling suns have been descried, swiftly flying or majestically sweeping through the thronged regions of space. But the laws of Kepler bind them all; satellite and primary, planet and sun, sun and system, all with one accord proclaim, in silent majesty, the triumph of the hero philosopher.

LECTURE IV.

DISCOVERY OF THE GREAT LAWS OF MOTION AND GRAVITATION.

HE remarkable discoveries which had rewarded the researches of Kepler, confirmed in the most perfect manner the doctrines of Copernicus, flowing as they did from his prominent hypothesis, the central position of the sun. Having reached to the true laws of the planetary motions, the whole current of astronomical research was changed. New methods were demanded, and more delicate means of observation must be brought into use before the data could be furnished for new discoveries.

Henceforward astronomy could only advance by the aid of kindred sciences. Mathematics, optics, and, above all, mechanical philosophy, were to become the instruments of future conquests.

The philosophy of Aristotle, though very far from deserving it, wielded quite as extensive an influence over the age as did the astronomy of Ptolemy. It appears, indeed, that the followers of Aristotle regarded their master as absolutely infallible, and gave to his doctrines a credence so firm, that even the clearest experiments, the most undeniable evidence of the senses, were sooner to be doubted than the doctrines of the divine Greek. To attack and destroy a system so deeply rooted in the prejudices of the age, required a mind of extraordinary courage and power, a

mind deeply imbued with the love of truth, quick in its perceptions, logical in argument, and firm in the hour of trial.

Such a mind was that of the great Florentine philosopher, Galileo Galilei, the senior, friend, and contemporary of Kepler. Indeed, the exigencies of the age seem to have given birth to three men, whose peculiar constitutions fitted them for separate spheres, each of the highest order, each in some measure independent, and yet all combining in the accomplishment of the great scientific revolution. While Tycho, the noble Dane, immured within the narrow limits of his little island, watching from his sentinel-towers the motions of the stars, noting with patient and laborious continuity the revolutions of the sun, moon, and planets, was accumulating the materials which were

Aristotle.

to furnish the keen and inquisitive mind of Kepler with the means of achieving his great triumphs, Galileo, with a giant hand, was shaking to their foundations the philosophical theories of Aristotle, and startling the world with his grand mechanical discoveries. But for the observations of Tycho, Kepler's laws could not have been revealed; but for the magic tube of Galileo, these laws had been the ne plus of astronomical science. Thus do we witness the rare spectacle of

three exalted intellects contemporaneously putting forth their diverse talents in the accomplishment of one grand object. The Dane, the German, and the Italian, divided by language and by country, united in the pursuit of science and of truth.

Called to Pisa to discharge the duties of a philosophical teacher, Galileo was not long in detecting the extravagant philosophical errors of Aristotle, which had been implicitly received for more than twenty centuries. He continued to teach the text of his old master; but it was only to expose its unsound and false

philosophy to his wondering and incredulous pupils. A desecration so monstrous could not long escape exposure and punishment. Indeed the Florentine made no secret of his teachings. The Aristotelians made common cause against the young philosophical heretic, and he was warned to desist from his heresy. Galileo gave for answer to his opponents, that he was ready to relinquish his new views the moment they were shown by experiment to be false; on the other hand, he demanded of them equal candour, and proposed to refer the matter of controversy to the tribunal of experiment.

Aristotle, in discussing the laws of falling bodies, affirmed the principle, that the velocity acquired by any falling body was in the direct proportion of its weight; and if two bodies of unequal weight commenced their descent from the same height, at the same moment, the heavier would move as many times swifter than the lighter as its weight exceeded that of the smaller body. Galileo doubted the truth of this principle, and on subjecting it

to the test of experiment, he saw instantly that its variation from fact was as wide as it could be. The obvious character of this experiment, its freedom from all chances of deception, and the importance of the principle involved, induced the young philosopher to select it as the test, and to challenge his opponents to a public demonstration of the truth or falsehood of their old system of philosophy. The challenge was accepted. The leaning

tower of Pisa presented the most convenient position for the performance of these experiments, on which Galileo so confidently relied for triumphant demonstration of the error of Aristotle; and thither, on the appointed day, the disputants repaired, each party perhaps with equal confidence. It was a great crisis in the history of human knowledge. On the one side stood the assembled wisdom of the universities, revered for age and science, venerable, dignified, united, and commanding. Around them thronged the multitude, and about them clustered the associations of centuries. On the other, there stood an obscure young man, with no retinue of followers, without reputation, or influence, or station. But his courage was equal to the occasion; confident in the power of truth, his form is erect, and his eye sparkles with excitement.

Galileo.

But the hour of trial arrives. The balls to be employed in the experiments are carefully weighed and scrutinised to detect deception. The parties are satisfied. The one ball is exactly twice the weight of the other. The followers of Aristotle maintain that when the balls are dropped from the top of the tower, the heavy one will reach the ground in exactly half the time employed by the lighter ball. Galileo asserts that the weights of the balls do not affect their velocities, and that the times of descent will be equal; and here the disputants join issue. The balls are conveyed to the summit of the lofty tower. The crowd