the sky, and watch the eccentric motions of these anomalous objects, without a feeling of dread. The movements of the planets inspire confidence. They are ever visible, and true to their appointed times while the comet, erratic in its course, bursts suddenly and unannounced upon the sight, and no science can predict in the outset its uncertain track-whether it may plunge into the sun, or dash against one of the planetary systems, or even come into collision with our own earth, is equally uncertain, until after a sufficient number of observations shall have been made to render the computation of the elements of its orbit possible.

Previous to the discovery of the law of universal gravitation, comets were looked upon as anomalous bodies, of whose motions it was quite impossible to take any account. By some philosophers they were regarded as meteors kindled into a blaze in the earth's atmosphere, and when once extinguished they were lost forever. Others looked upon them as permanent bodies, revolving in orbits far above the moon, and reäppearing at the end of long but certain intervals. When, however, it was discovered that, under the influence of gravitation, any revolving world might describe either of the four curves, the circle, ellipse, parabola or hyperbola, it at once became manifest that the eccentric movements of the comets might be perfectly represented by giving to them orbits of the parabolic or hyperbolic form, the sun being located in the focus of the curve. According to this theory, the comet would become visible in its approach to its perihelion, or nearest distance from the sun,-would here blaze with uncommon splendor, and in its recess to the re

mote parts of its orbit, would gradually fade from the sight, relaxing its speed, and performing a large proportion of its vast curve far beyond the reach of human vision.

Such was the theory of Newton, and such were the fair deductions from the great law of nature which he had revealed to the world. He awaited with deep interest the appearance of some brilliant comet, whose career he might trace, in the full confidence that observation would confirm the truth of his bold hypothesis. Fortunately, his impatience was soon gratified. In the year 1680 a most wonderful comet made its appearance, which, by its splendor and swiftness, excited the deepest interest throughout the world. It came from the regions of space immediately above the ecliptic, and plunging downwards with amazing velocity, in a direction almost perpendicular to this plane, it appeared to direct its flight in such manner that it must inevitably plunge directly into the sun. This was not, however, the case. Increasing its velocity as it approached the sun, it swept round this body with the speed of a million of miles an hour approaching the sun to within a distance of its surface of a sixth part of the sun's radius. It then commenced its recess, throwing off a train of light which extended to the enormous distance of 100,000,000 of miles. With the swiftness of thought almost, it swept away from the sun, and was gradually lost in the distant regions of space whence it came, and has never since been seen. Such were the general characteristics of the body to whose rapid motions Newton attempted to apply the law of universal gravitation.

Its positions were marked with all the accuracy which the instruments then in use permitted, and it was found that a parabolic curve could be constructed which would embrace all the places of the comet.— The great eccentricity of its orbit, and its vast period, amounting to nearly six hundred years, gave to the comet great interest, but rendered it an unfit object for successful analytic researche. The great English astronomer, Halley, had studied it with the closest care, and with a rigid application of Newton's theory, he reached results quite as satisfactory as the circumstances of the case rendered possible.

Fortunately, in 1682, another comet made its appearance, to the study of which Halley devoted himself with a zeal and success which has justly stamped his name on this remarkable body; and as our limits forbid an extensive investigation of the history and theory of comets, I propose to examine this one with that degree of detail which may convey some idea of the limits of human knowledge in this complicated department of science.

At the suggestion of Newton, Halley had searched all ancient and modern records, for the purpose of rescuing any historical details touching the appearance and aspect of comets, from the primitive ages down to his own time. On the appearance of the comet of 1682, he observed its positions with great care, and with wonderful pains computed the elements of its orbit. He found it moving in a plane but little inclined to the ecliptic, and in an ellipse of very great elongation. In its aphelion, it receded from the sun to the enormous distance of 3,400,000,000 of miles.He discovered that the nature of its orbit was such

as to warrant the belief that the comet would return at regular intervals of about seventy-five years, and recurring to his historical table of comets, he found it possible to trace it back with certainty several hundred years, and with probability even to the time of the birth of Mithridates, one hundred and thirty years before Christ. At this, its first recorded appearance, its magnitude must have been far beyond anything subsequently seen, as its splendor is said to have surpassed that of the sun.

In the years 248, 324, and 399 of the Christian era, remarkable comets are recorded to have appeared, and the equality of interval corresponds well with Halley's comet. In the year 1006, it presented a frightful aspect, exhibiting an immense curved tail in the form of a scythe. In 1456, its appearance spread consternation through all Europe, and led to most extravagant acts on the part of the reigning pontiff, who actually instituted a form of prayer against the baleful influence of the comet, and thus increased the terrors of the ignorant and superstitious. The comet appeared with certainty in 1531, and again in 1607, and from an examination of all the facts, and with full confidence in his computations, Halley ventured the bold prediction that this same comet would reäppear about the close of 1758, or the beginning of 1759.

This was certainly the most extraordinary prediction ever made, and the distinguished philosopher, knowing that he could not live to witness the verification of this prophetic announcement, expresses the hope that when the comet shall return, true to his computed period, posterity will do him the justice to remember that this first prediction was made by an

Englishman. In the age when these investigation were made, the theory of comets was in its infancy, and it is believed by those competent to form a just opinion, that Halley was the only man living who could have computed the orbit of his comet.

As the period approached for the verification of this extraordinary announcement, the greatest interest was manifested among astronomers, and efforts were made to predict its coming with greater accuracy, by computing the disturbing effects of the larger planets within the sphere of whose influence the comet might pass. This was a new and difficult branch of astronomical science, and it would be impossible to convey the least idea of the enormous labor which was gone through by Clairaut and Lalande, in computing the perturbations of this comet through a period of two revolutions, or one hundred and fifty years.

"During six months," says Lalande, "we calculated from morning till night, sometimes even at meals,— the consequence of which was that I contracted an illness which changed my constitution during the remainder of my life. The assistance rendered by Madame Lepaute was such, that without her we never should have dared to undertake the enormous labor, in which it was necessary to calculate the distance of each of the two planets, Jupiter and Saturn, from the comet, separately for every degree, for one hundred and fifty years."

Amid all these difficulties, the computers toiled on, until finally, the period coming on rapidly for the comet's return, they were forced to neglect some minor irregularities, and Clairaut announced that the comet would be retarded one hundred days by the in