PLANETARY SYSTEMS. 75 



or secondary planets and myriads of comets, three of which, 

 known as the " planetary comets," do not pass beyond the 

 narrow limits of the orbits described by the principal planets. 

 We may, with no inconsiderable degree of probability, include 

 within the domain of our Sun, in the immediate sphere of its 

 central force, a rotating ring of vaporous matter, lying pro- 

 bably between the orbits of Venus and Mars, but certainly 



nomy. This startling discrepancy, which seemed only to gain additional 

 weight from every attempt made by M. Bouvard to correct his calcula- 

 tions, led Leverrier, after a careful modification of the tables of Bouvard, 

 to establish the proposition that there was "a formal incompatibility, be- 

 tween the observed motions of Uranus and the hypothesis that he was 

 acted on only by the Sun and known planets, according to the law of 

 universal gravitation." Pursuing this idea, Leverrier arrived at the 

 conclusion that the disturbing cause must be a planet, and finally, after 

 an amount of labour that seems perfectly overwhelming, he, on the 31st 

 of August, 1846, laid before the French Institute a paper, in which he 

 indicated the exact spot in the heavens where this new planetary body 

 would be found, giving the following data for its various elements : 

 mean distance from the Sun, 36'154 times that of the Earth; period of 

 revolution, 217'387 years; mean long., Jan. 1st, 1847, 318 47'; mass, 

 P J 7 ; heliocentric long., Jan. 1, 1847, 326 32'. Essential difficulties 

 still intervened, however, and as the remoteness of the planet rendered 

 it improbable that its disc would be discernible by any telescopic instru- 

 ment, no other means remained for detecting the suspected body but its 

 planetary motion, which could only be ascertained by mapping, after 

 every observation, the quarter of the heavens scanned, and by a com- 

 parison of the various maps. Fortunately for the verification of Le- 

 verrier's predictions, Dr. Bremiker had just completed a map of the 

 precise region in which it was expected the new planet would appear, 

 this being one of a series of maps made for the Academy of Berlin, of 

 the small starg along the entire zodiac. By means of this valuable 

 assistance, Dr. Galle, of the Berlin Observatory, was led, on the 25ih of 

 September, 1846, by the discovery of a star of the eighth magnitude, not 

 recorded in Dr. Bremiker's map, to make the first observat on of the 

 planet predicted by Leverrier. By a singular coincidence, Mr. Adams 

 of Cambridge, had predicted the appearance of the planet simultaneously 

 with M. Leverrier ; but by the concurrence of several circumstances much 

 to be regretted, the world at large were not made acquainted with Mr. 

 Adams' valuable discovery until subsequently to the period at which 

 Leverrier published his observations. As the data of Leverrier and 

 Adams stand at present there is a discrepancy between the predicted 

 and the true distance, and in some other elements of the planet ; 

 it remains, therefore, for these or future astronomers to reconcile theory 

 with fact, or perhaps, as in the case of Uranus, to make the new planet 

 the means of leading to yet greater discoveries. It would appear from 



