68 Royal Astronomical Society. 



served and the mounting of his telescope* sufficiently account for 

 discrepancies in Sir William's results. The measures in distance 

 were, with his means, exceedingly difficult : hence we have only two 

 such measures of the interior of the two principal satellites, and none 

 at all of the supplementary satellites. There are, however, several 

 measures of the exterior principal satellite II. 



In the memoir of 1788, Sir William gives 44"*23 as the greatest 

 elongation of II. from Uranus at his mean distance from the earth, 

 and calculates the similar greatest elongation of I. to he 33", i. e. 

 from the periodic time and Kepler's law. He considers these to be 

 approximate only. In the memoir of 1815 he uses in round numbers, 

 and for numerical convenience, 48" and 36" for the elongations ; 

 and he adds a list of selected observations from which more certain 

 values may be deduced ; but it does not appear that he performed 

 this calculation. The other elements of the orbits of the two satel- 

 lites are pretty exact ; the position of the planet was very favourable 

 for these determinations. In 1797 the apparent orbits of the satel- 

 lites were nearly right lines. 



As to the supplementary satellites and their orbits, it would appear 

 from both Memoirs that Sir William did not affect to speak of them 

 with confidence. Though he had no doubt of their existence gene- 

 rally, yet he avowed that "to determine their numbers and situation 

 would probably require an increase of illuminating power." The 

 precision with which their periodic times, &c. are stated by various 

 authorities must not be supposed to rest on any certain foundation, 

 but on Sir William's approximations and shrewd guesses. 



Sir John Herschel directed his attention to the satellites of Uranus 

 in 1828-1832, and the results are to be found in our Memoirs, 

 vol. viii. The situation of Uranus was unfavourable, the planet 

 having considerable southern declination. From his own observa- 

 tions, combined with those of his father, Sir John deduced the fol- 

 lowing periodic times : — 



I. =8 d 16 b 56" 31 s -3 II. = 13 d ll h 7'" 12 8 6 ; 



but " of other satellites," he says, " than these two I have no evi- 

 dence." 



M. Lamont, Director of the Observatory of Munich, commenced 

 his observations of the satellites of Uranus in 1837. His memoir is 

 printed in our Transactions, vol. xi. They relate to the principal 

 satellites, I. and II. M. Lamont believes that he once saw what he 

 takes to be 4, the most distant of the supplementary satellites. 



The mass of Uranus has been found to be, — 



— — — , the Sun = J , by Sir W. Herschel's observations. 



bv M. Lamont. 



24605 



1 by Bcnvard, from the perturbations of other 



17918 planets by Uranus. 



* This was a 20-foot reflector with the front view, and Sir William's usual 

 mounting. 



