This hope was not disappointed. The planet, believed to be a fixed star, had been seen and observed no less than nineteen different times, by four different observers, through a period running back nearly one hundred years previous to the discovery of its planetary character by Herschel. These remote observations were of the greatest value as data for the determination of the elements of its elliptic orbit, and for the computation of the mean places, which might serve to predict its position in coming years.

A distinguished astronomer, M. Bouvard, of the Paris Academy of Sciences, about thirty years ago, undertook the analytic investigation of the movements of Uranus, and a computation of the exact tables. He was met, however, by difficulties which, in the state of knowledge as it then existed, with reference to this planet, were absolutely insurmountable. He found it quite impossible to obtain any orbit which would pass through the places of the planet determined after its discovery, and through these positions which had been fixed previous to that epoch. In this dilemma it became necessary to reject the old observations as less reliable than the new ones, and the learned computer leaves the problem for posterity to resolve, carefully abstaining from any absolute decision in the case.

His orbit, based upon the new or modern observations, and his tables being computed, it was hoped that the theoretic places of the planet would thereafter coincide with the observed places, and that all discrepancies which might not be fairly chargeable to errors of observation, would be removed. In this expectation, however, the astronomical world was disappointed; and while the tables of Bouvard failed absolutely to represent the ancient observations, in a few years they were

but little more truthful in giving the positions actually filled by the planet under the telescope.The discrepancies between the theoretic and actual places of the planet began to attract attention many years since. As early as 1838, Mr. Airy, Astronomer Royal of England, on a comparison of his own observations with the tables, found that the planet was out of the computed track, by a distance as great as the moon's distance from the earth, and that it was actually describing an orbit as great as that pointed out by theory. It seemed that this remote body was breaking away from the sun's control, or that it was operated upon by some unknown body deep sunk in space, and which thus far had escaped the scrutinising gaze of man.

These deviations became so palpable as to attract general attention, and various conjectures were made with reference to their probable cause.

Some were disposed to regard the law of gravitation as somewbat relaxed in its rigorous application to this remote body. Others thought the deviation attributable to the action of some large comet, which might sway the planet from its course; while a third set of philosophers conjectured the existence of a large satellite revolving about Uranus, and from whose attraction the planet was caused to swerve from the computed orbit. These conjectures were not sustained by any show of reasoning, and were of no scientific value.

Such was the condition of the problem when it was entertained by a young French astronomer, not quite unknown to fame in his own country, but comparatively at the beginning of his scientific career. The friend of Arago, Le Merrier's Cometary Investigations, and more especially his researches for the Motions of Mercury, had gained for him the confidence of this distin

guished savant, and Arago urged on his young associate the importance of the great problem presented in the perturbations of Uranus, and induced him to abandon other investigations, and concentrate all the energies of his genius on this profound and complex investigation.

The extraordinary powers of Leverrier as a mathematical astronomer had been so successfully displayed in his researches of the motions of Mercury, that it deserves a passing notice. The old tables of this planet, Leverrier believed to be defective. He therefore set about a thorough examination of its entire theory, and after a rigid scrutiny, deducted a new set of tables, from which the places of the planet might be predicted with greater precision.

The transit of Mercury across the sun's disc, which occurred on the 8th day of May, 1845, presented an admirable opportunity to test the truth of the new theory of the young astronomer. Most unhappily for his hopes, all observations in Paris were rendered impossible by the clouds, which covered the heavens during the entire day on which the transit took place. While the computer was sadly disappointed, I was more fortunate, for a pure and transparent atmosphere favoured this, the first astronomical observation I ever made. A slight reference to this occurrence may be pardoned. For three years I had been toiling to complete a most difficult and laborious enterprise, the erection of an astronomical observatory of the first class, in a country where none had ever existed. Amid difficulties and perplexities which none can ever know, the work had moved on, and at length I had the high satisfaction of seeing mounted one of the largest and most perfect instruments in the world. I had arranged

and adjusted its complex machinery--had computed the exact point on the sun's disc where the planet ought to make its first contact-had determined the instant of contact by the old tables, and by the new ones of Leverrier, and with feelings which must be experienced to be realized, five minutes before the computed time of contact, I took my post at the telescope to watch the coming of the expected planet. After waiting what seemed almost an age, I called to my friend how much time was yet to pass, and found but one minute out of five had rolled heavily away. The watch was again resumed. Long and patiently did I hold my place, but again was forced to call out, how speeds the time? and was answered that there was yet wanting two minutes of the computed time of contact.-With steadfast eye, and a throbbing heart, the vigil was resumed, and after waiting what seemed an age, I caught the dark break which the black body of the planet made on the bright disc of the sun. Now! I exclaimed; and within sixteen seconds of the computed time did the planet touch the solar disc, at the precise point at which theory had indicated the first contact would occur.

The planet was followed across the disc of the sun, round and sharp, and black, and every observation confirmed the superior accuracy of the new tables of Leverrier. While the old tables were out fully a minute and a half in the various contacts, those of Leverrier were in error by only about sixteen seconds as a mean.

The great success of this investigation encouraged the young astronomer to accept the difficult task which Arago proposed for his accomplishment, and he earnestly set about preparing the way for a full discussion of the grand problem of the perturbations of Uranus. The importance of the subject

demanded the greatest caution, and having determined to rely solely on his own efforts, he at once rejected all that had been previously done, and commenced the problem at the very beginning. New analytic theories were formed; elaborate investigations of the planets Jupiter and Saturn, as disturbing bodies, were made, and an entire clearing up of all possible causes of disturbance in the known bodies of the system was labouriously and successfully accomplished, and the indefatigable mathematician finally reached a point where he could say, here are residual perturbations which are not to be accounted for by any known existing body, and their explanation is to be sought beyond the present ascertained limits of the solar system.

As early as the 10th of November, 1845, M. Leverrier presented a memoir to the Royal Academy of Sciences in Paris, in which he determined the exact perturbations of Jupiter and Saturn on Uranus. This was followed by a memoir, read before the academy on the 1st of June, 1846, in which he demonstrates that it is impossible to render an exact account of the perturbations of Uranus in any other way than by admitting the existence of a new planet exterior to the orbit of Uranus, and whose heliocentric longitude he fixes at 325° on the 1st of January, 1847. On the 30th of August, 1846, a third memoir was presented to the academy, in which the elements of the orbit of the unknown planet are fixed, together with its mass and actual position, with greater accuracy, giving on the 1st of January, 1847, 326° 32′ for its heliocentric longitude. Finally, on the 5th of October, 1847, a fourth memoir was read, relative to the determina- · tion of the plane of the orbit of the constructive planet.

It is quite impossible to convey, in popular form, the least idea of the proud analytic reasoning em