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remarked three bright stars which were invisible to the naked eye. He carelessly noted their position with reference to the planet, for he believed them to be fixed stars, and of no special interest, except to point out the change in Jupiter's place. On the following night, « induced,” as he says,“ by he knew not what cause," he again directed his attention to the same planet. The three bright stars of the preceding evening were still within the field of his telescope, but their positions with reference to each other were entirely changed, and such was the change, that the orbitual motion of Jupiter could in no way account for it. Astonished and perplexed, the eager astronomer awaits the coming of the following night to resolve this mysterious exhibition. Clouds disappoint his hopes, and he is obliged to curb his impatience. The fourth night was fair, the examination was resumed, and again the bright attendants of Jupiter had changed

his suspicions were confirmed-he no longer hesitated, and pronounced these bright stars to be moons, revolving about the great planet as their centre of motion. A few nights perfected the discovery; the fourth satellite was detected, and this astounding discovery was announced to the world.

No revelation could have been more important or more opportune than that of the satellites of Jupiter. The advocates of the Copernican theory hailed it with intense delight; while the sturdy followers of Ptolemy stoutly maintained the utter absurdity of such pretended discoveries, and urged as a sort of unanswera. ble argument, that as there were but seven openings in the head-two ears, two eyes, two nostrils and the mouth, there could be in the heavens but seven planets

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The more rational, however, saw the earth, by this discovery, robbed of its pretended dignity. It commanded the attendance of but one moon, while Jupiter received the homage of no less than four bright attendants. The delighted Copernicans saw in Jupiter as a central orb and in the orderly revolution of his satellites, a miniature of the sun and his planets, hung up in the heavens, and there placed to demonstrate to all coming generations, the truth of the new doctrines.

Another discovery soon followed, which it is said the sagacity of Copernicus foresaw would sooner or later be revealed to human vision. It had been urged by the Ptolemaists, that in case Venus revolved about the sun, as was asserted by Copernicus, and reflected to us the light of that luminary, then must she imitate exactly the phases of the moon; when on the side opposite to the sun, turning towards us her illuminated hemisphere she ought to appear round like the moon, while the crescent shape should appear on reaching the point in her revolution which placed her between the sun and the eye of the observer. As these changes were invisible to the naked eye, the objection was urged with a force which no argument could meet. Indeed it was unanswerable, and in case the telescope should fail to reveal these changes in Venus, the fate of the Copernican theory was forever sealed.

The position of Venus in her orbit was computedthe crescent phase due to that position determined the telescope applied, and the eye was greeted with an exquisite miniature of the new moon. There was the planet, and there was the crescent shape long predicted by Copernicus, received by him and his followers as a matter of faith, now become a matter of sight. The doctrines of Copernicus thus received not only confirmation, but so far as Venus was concerned, a proof so positive that no skepticism could resist. It is not my design to follow the dis coveries of the Florentine philosopher among the planetary orbs. These will be resumed hereafter, when we come to examine more particularly the physical constitution of the planets. I have merely adverted to those discoveries, which became specially important in the discussions between the partizans of the old and new astronomy.

Admitting the doctrines of Copernicus, and uniting to them the great discoveries of Kepler, let us examine the condition of astronomical science, ascertain precisely the point the mind had reached, and the nature of the investigations which next demanded its attention. From the first of Kepler's laws, the figure of the planetary orbits became known, and the magnitude of the ellipse described by any planet was easily determined. By observing the greatest and least distances of any planet from the sun, the sum of these distances gave the longer axis of the orbit; and knowing this important line, and the focus, it became a simple matter to construct the entire orbit. The line joining the planet with the sun, while the planet occupied its shortest br perihelion distance, gave the position of the axis of the orbit in space, and its plane being determined by its inclination to that of the ecliptic, nothing remained to fix in space the figure, magnitude and position of the planetary orbits. The next point was to pursue and predict the ments of these revolving bodies. This was readily accomplished. A series of observations soon revealed the time occupied by any planet in performing ono complete revolution in its elliptic orbit. Knowing thus the periodic time, and the position of the planet in its orbit at any given epoch, the second law of Kepler furnished the key to its future movements; its velocity in all parts of its orbit became known, and the mind swift and true followed the flying world in its rapid flight through space. It even went further; anticipated its changes, and predicted its positions, with a degree of certainty only limited by the accuracy with which the elements of its orbit had been determined.

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The third of Kepler's laws, exhibiting the proportion between the periodic times and the mean distances of the planets from the sun, united all these isolated and wandering orbs into one great family. Their periods of revolution were readily determined by observation, and an accurate determination of the distance from the sun of a single planet in the group, gave at once the distances of all the remaining ones. The increased accuracy of the means of observation would render more perfect each successive measure of the earth's distance from the sun, and it seemed now that the mind might stop and rest from its arduous toil; that scarcely any thing remained to be done. The solar system was conquered, and the fixed stars defied the utmost efforts of human power.

How widely does this view differ from the true one. In fact, the true investigation had not even commenced. A height had indeed been gained, from whence alone the true nature of the next great problem became visible, and standing upon this eminence the mind boldly propounds the following questions:

Why should the orbits of the planets and satellites be ellipses, rather than any other curve? What power compelled them to pursue their prescribed paths with undeviating accuracy? What cause produced their accelerated motion when coming round to those parts of their orbits nearer to the sun ? What power held planet and satellite steady in their swift career, producing the most exquisite harmony of motion, and a uniformity of results as steady as the march of time?

Here I may be asked, do not such questions border on presumption ? Are not such inquisitorial examinations touching on the domain of God's inscrutable providences, and would it not be wiser to stop and rest satisfied with the answer to all these questions, that God, who built the universe, governs and sustains it by his power and wisdom? Doubtless this answer is true, and in its truth man humbly finds his highest encouragement to attempt the resolution of the sublime questions already propounded for examination. Let us admit that the divine will produces a!! motion, speeds the earth in its rapid flight about the sun, guides the planets, and their revolving moons, and poises the sun himself in empty space, as the great centre of life and light and heat to his attendant worlds. Is it not reasonable to believe that the will of the Omnipotent is exerted according to some uniform system, that this system is law, and that this law is within the reach of man? To encourage this view the simple laws of motion had been already revealed, and as these must exert a controlling influence in our future examinations, we proceed to unfold them.

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