URANUS. 175 



ness of this relation extends to ji^ of the longer periods. 

 This unnoticed result was communicated to me by Sir John 

 Herschel in a letter as long back as 1845. The four satel- 

 lites of Jupiter present a certain regularity in their distances, 

 forming very nearly the series 3, 6, 12. The distance of the 

 second from the first, expressed in diameters of Jupiter, is 

 3-6 ; the distance of the third from the second, 5*7 ; that of 

 the fourth from the third, 11-6. Moreover, Fries and Chal- 

 lis have endeavored to prove the so-called law of Titius in all 

 •satellite systems, even in that of Uranus.^ 



Uranus. 



The acknowledged existence of this planet, the great dis- 

 covery of William Herschel, has not only increased the num- 

 ber of the principal planets known for thousands of years, and 

 more than doubled the diameter of the solar regions — it has 

 also, after the lapse of sixty-five years, led to the discovery of 

 Neptune, through the disturbances which it underwent from 

 the influence of the latter. Uranus was discovered accident- 

 ally (13th March, 1781), during the examination of a small 

 group of stars in Gemini, by its small disk, which, with mag- 

 nifying powers of 460 and 932, increased far more consider- 

 ably than was the case with other adjacent stars. The saga- 

 cious discoverer, so thoroughly acquainted with all optical phe- 

 nomena., also observed that the luminous intensity decreased 

 considerably in proportion as stronger magnifying powers 

 were employed, while in the fixed stars (6th and 7th magni- 

 tude) it remained nearer the same. 



When Herschel first announced the existence of Uranus, 

 he called it a comet,-f and it was only by the united labors of 

 Saron, Lexell, Laplace, and Mechain, which were consider- 

 ably facilitated by the discovery made by the meritorious 

 Bode, in 1784, of the previous observations of the planet by 

 Tobias Mayer (1756) and Flamstead (1690), that the ellip- 

 tical orbit of Uranus and the whole of its planetary elements 

 were determined with admirable celerity. According to Han- 

 sen, the mean distance of Uranus from the Sun is 1,918,239, 

 or 1585 million geographical miles; his period of sidereal 

 revolution 84y. 5d. 19h. 41m. 36s.; the inclination of his 



orbit to the ecliptic, 0° 46' 28" ; his apparent diameter at 



•> 



* Fries, Vorlesungen iiher die Stemhunde, 1833, p. 325 ; Challis, in the 

 Transact, of the Cambridge Philos. Society, vol. iii., p. 171. 



t William Herschel, Account of a Comet in the Philos. Transact, for 

 I781,vol, Ixxi., p. 492. 



