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 in 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

greater 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 for 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.

But the hour of trial arrives. The balls to be employed in the experiment are carefully weighed and scrutinized 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 assembles round the base-the signal is given the balls are dropped at the same instant, and swift descending, at the same moment they strike the earth. Again and again is the experiment repeated, with uniform results. Galileo's triumph was complete.Not a shadow of doubt remained; but far from receiving, as he had hoped, the warm congratulations of honest conviction-private interest, the loss of place, and the mortification of confessing false teaching, proved too strong for the candour of his adversaries.-They clung to their former opinions with the tenacity of despair, and assailed

the now proud and haughty Galileo with the bitter feelings of disappointment and hate.

The war was now openly declared, and waged with a fierceness which seems to have excited the mind of the young philosopher to the most extraordinary efforts. Driven from Pisa, by the numbers and influence of his enemies, no suffering or danger could drive from his mind the great truths which his researches by experiment were constantly revealing. His spirit was unbroken, and in retiring from the unequal contest, he hurled back defiance into the face of his conquered, though triumphant persecutors.

The mechanical investigations of Galileo, conducted with clearness and precision, soon led to the most important discoveries. He detected the law of falling bodies, and showed that the spaces described were proportional to the squares of the times; that is, if a body fell ten feet in one second of time, it would fall four times as far in two seconds, nine times as far in three seconds, and so on for any number of seconds. He studied with success the subject of the composition of forces, and demonstrated this remarkable proposition, which lies at the very foundation of all modern philosophy. It may be thus stated :-If a body receive an impulse, which singly would cause it to move thirty feet in a second, on the line of the direction of the impulse, and at the same instant another impulse be communicated in a different direction from the first, and which, if acting alone, would cause the body to move on the line of direction of the second impulse forty feet in one second, under the joint action of these two impulses the body will move in a direction easily determined from those of the impulsive forces, and will fly with a velocity of fifty feet in the first second of time.

Such is the universal prevalence of this beautiful proposition, that no falling, flying, or moving body, whether it be the rifle ball, the cannon shot, or the circling planet, is free from its imperious sway. Strike the knowledge of this great truth from existence, and the magnificent structure which modern science has reared falls in ruins at a single blow. It is founded in the simple but invariable laws of motion, and while these endure, this elegant discovery of the Florentine philosopher will remain as a monument to his sagacity and penetration.

Possessed of such rare qualities for philosophic research, so free from prejudice, and withal, so candid, we cannot but inquire with interest, how the mind of Galileo stood affected towards the new astronomical doctrines of Copernicus. He had early adopted and taught the Ptolemaic system, and his conversion is so remarkable, and is so characteristic of the man, that it cannot be omitted. A disciple of Copernicus visited the city of Galileo, and delivered several public lectures to crowded audiences on the new doctrines. Galileo, regarding the whole subject as a species of solemn folly, would not attend. Subsequently, however, in conversing with one who had adopted these new doctrines, the Copernican sustained his views with such a show of reason, that Galileo now regretted that he had heedlessly lost the opportunity of attending the lectures. To make amends, he sought every opportunity to converse with the Copernicans, and remarking that they, like himself, had all once been Ptolemaists, and that from the doctrines of Copernicus no one had ever subsequently become a follower of the old philosophy, he resolved to examine the subject with the most serious attention. The result may be readily anticipated; the conversion was sudden and thorough; the old

astronomy was abandoned, and the new convert became the great champion by whose ardour and unconquerable zeal the strongholds of antiquated systems were to be destroyed, and a new and truthful one founded.

Thus far the career of Galileo in science had been successful and brilliant. He was rapidly rising in reputation and influence, when a fortunate accident revealed to the world, the applications of a principle in optics fraught with consequences, which it is impossible to estimate. Galileo was informed that Jansen, of Holland, had contrived an instrument possessing the extraordinary property of causing distant cbjects, viewed through it, to appear as distinctly as when brought near to the eye. The extensive knowledge which Galileo possessed of optics, immediately gave him the command of the important principle on which the new instrument had been constructed. He saw at once the high value of such an instrument in his astronomical researches, and with his own hands commenced its construction.

After incredible pains, he finally succeeded in constructing a telescope, by whose aid the power of the eye was increased thirty fold. It is impossible to conceive the intense interest with which the philosopher directed for the first time his wonderful tube to the inspection of the heavens. When we reflect that with the aid of this magical instrument the observer was about to sweep out through space, and to approach the moon, and planets, and stars, to within a distance only onethirtieth of their actual distance; that their size was to increase thirty fold, and their distinctness in the same ratio, it is not surprising that these wonders should have excited the most extravagant enthusiasm. Galileo commenced by an examination of the moon.

Here he beheld, to his inex