as the world had done for 2000 years, was driven to adopt the same expedients which characterised the theories of Ptolemy: the eccentric and epicycle were fastened upon the new system of astronomy. Yet another difficulty embarrassed the mind of Copernicus. In giving to the earth a rotation on its axis once in twenty-four hours, he explained the apparent revolution of the starry heavens. This axis of rotation, it was readily seen, ́must ever remain parallel to itself in the annual revolution of the earth in its orbit. Being in this way carried round such a vast circumference, the prolongation of the axis ought to pierce the northern heavens in a series of points which would form a eurve so large as not to escape detection. But no such curve appeared; the north pole of the heavens, scrutinised with the most delicate instruments, preserved its position immoveably throughout the entire revolution of the earth in its orbit; and to escape from this difficulty there was no alternative but to admit that the distance of the sphere of the fixed stars was so great that the diameter of the earth's orbit, equal to 200,000,000 of miles, was absolutely nothing, when compared with that mighty distance.

Under these circumstances, it is not wonderful that Copernicus should have promulged his system with extreme diffidence, and only after long delay; indeed, his great work, setting forth his doctrines, was never read by its author in print, and only reached him in time to cheer his dying moments.

We cannot, then, be surprised that the new system was received with doubt and distrust, or rather that it was for a long while absolutely rejected. The progress of truth is ever slow, while error moves with rapid pace. The reason is obvious: error is seized by a class of minds, which asks no evidence ; while the searchers for truth adopt it only after the most deliberate examination.

But the revolution had been commenced. A few bold minds were struck with the simplicity and beauty of the conjectures of Copernicus; and when the exigencies of the age demand genius, it seems to rise spontaneously. The mind had persevered in a system founded in reason, and which nothing short of this very perseverance could have demonstrated to be erroneous. Like the traveller who is uncertain which of two roads to take, he reflects, reasons, and decides, and, even if his choice be a wrong one, it would be folly to stop before fully convinced that he had chosen erroneously.

But the mind is once again in the path of truth; and after wandering twenty long centuries in darkness, which grew deeper and deeper, the change from darkness to light gives vigour to its movements, and its future achievements are destined to be rapid and glorious.

Here let us pause for a moment on the boundary which divides ancient from modern science, and glance at the collateral circumstances which were found to modify and retard the investigations which had commenced. The old doctrines of philosophy and astronomy had become intimately interwoven with human society. Ptolemy, and Plato, and Aristotle, were regarded with a sort of reverential awe. Even the Church, not following, but leading the world in this profound respect for ancient philosophy, pronounced the doctrines of Ptolemy in accordance with the revelations of Scripture, and girdled them with the fires of persecution, through which alone their sacredness could be attacked. Thus entrenched and defended by prejudice, by society, and by religion, none but the most daring spirit would enter the conflict against such unequal odds. Conscious of these difficulties, Copernicus had wisely avoided collision, and gave his doctrines to the world with such caution as not to provoke attack. But this armed neutrality could not long endure. If the new doctrine were founded in error, left to itself it would never advance, and would soon quietly sink into oblivion. On the contrary, should it prove to be based upon truth, no power could arrest its progress, or stay its development. The contest must come, sooner or later, and demanded in those who should battle for the truth the rarest qualities.

He

Copernicus had merely commenced the examination of his bold conjecture. A lifetime was too short to accomplish more. had transferred the centre of motion from the earth to the sun, and rested the truth of his hypothesis on a diminished complexity in the celestial phenomena. In case the true centre had been found, it now remained to determine the exact curves in which the planets revolved, the laws regulating their motion, and the nature of the bond which it was now suspected united the planetary worlds into one great system. The resolution of these` profound questions was reserved for Kepler, who has, without flattery, been termed the Legislator of the heavens, and who has earned the reputation of being first in fact and first in genius among modern astronomers. He united, in the most perfect

manner, all the qualifications of a great discoverer. Ardent, enthusiastic, and subtle, he pursued his investigations with a

Kepler.

keen and restless activity. Patient, laborious, and determined, difficulties shrunk at his approach and obstacles melted before him. Unprejudiced and pious, he sought for truth in the name and invoking ever the guidance of the great Author of truth. If his theories were not actually deduced from facts, when formed, no test was too severe, and nothing short of a rigid coincidence with fact could satisfy the exacting mind of this wonderful genius. Realising fully the difficulty and importance of the researches before him, once comno limit, and the fertility of

his imagination was utterly inexhaustible.

Such was the man to whom the interests of science at this critical juncture were committed. Having adopted as an hypothesis, the central position of the sun, and the revolution of the earth and planets around this centre, he determined to discover the true nature of the planetary orbits, and find, if possible, some single curve which would explain the orbitual motions of the celestial bodies. To accomplish this difficult enterprise, Kepler wisely determined to confine his efforts and investigations to one single planet, and Mars was selected as the subject for experiment. He commenced by a rigorous comparison between the observed places of the planet, and those given by the best tables which could be computed by the circular theory. Sometimes the predicated and observed places agreed well with each other, and hope whispered that the true theory had been found; but pursuing the planet onward in its sweep around the sun, it would begin to diverge from its theoretic track, its distance would increase, until it became evident that the theory was false, and must be abandoned.