524 COSMOS. 



Between the first four satellites nearest to Saturn a 

 remarkable relation of commensu/r ability in the period of revo- 

 lution presents itself. The period of the third satellite 

 (Tetliys) is double that of the first (Mimas}-, that of the 

 fourth (Dione) double that of the second (Enceladus). The 

 closeness of this relation extends to -^ 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 satellites 

 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 Challia 

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

 satellite systems, even in that of Uranus.* 



UBANTJS. 



The acknowledged existence of this planet, the great dis- 

 covery of William Herschel, has not only increased the 

 number 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 dis- 

 covery of Neptune, through the disturbances which it under- 

 went from the influence of the latter. Uranus was discovered 

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

 small group of stars in Gemini by its small disc, which with 

 magnifying powers of 460 and 932 increased far more con- 

 siderably than was the case with other adjacent stars. The 

 sagacious discoverer, so thoroughly acquainted with all optical 



91 Fries, Vorlesungen uber die Sternkunde, 1833, p. 325; 

 Challis, in the Transact, of the Cambridge Philos. Society, 

 vol. iii. p. 171. 



