Dr. Lamont on the Mass and Satellites of Uranus, 523 



was observed only once, and the observation is of consequence 

 omitted, as being of no use in the present inquiry. 



The measures were obtained by using a parallel wire-micrometer 

 of Fraunhofer's construction, having the wires, not the field, illu- 

 minated. Instead of the lamps usually employed, a light placed at 

 a distance was reflected on the wires by a small mirror. Dr. Lamont 

 remarks, that the use of a mirror is greatly to be preferred to lamps, 

 because, in addition to its being more convenient, it affords in the 

 measurement of faint objects a peculiar advantage, in enabling the 

 observer to direct the illumination to any part of the wires, and with 

 any degree of intensity that may be required. 



But, however carefully the illumination may be managed, it would 

 be impossible to bisect with a wire any of the satellites of Uranus, 

 and, accordingly. Dr. Lamont had recourse to another method, 

 which he has frequently adopted in similar cases. Placing the fixed 

 wire so as to bisect the disc of the planet, he moved the micrometer 

 until the satellite appeared exactly in the middle of the space be- 

 tween both wires. The measure being repeated on the opposite 

 side of the fixed wire, in order to eliminate the zero point, the 

 diflference of the two readings gave the quadruple distance of the 

 satellite. 



The table of observations given by the author contains the 

 sidereal time of each observation ; the mean Paris time, including 

 aberration ; the observed angles of position ; the observed distances, 

 and the number of measures taken at each observation. The 

 number of observations of the second satellite was 11, and of the 

 fourth, 15. 



Although the observations furnish sufficient proof of the elliptic 

 motion of the satellites, any attempt to investigate the elliptic ele- 

 ments from the few data obtained in the present unfavourable situ- 

 ation of the orbits, would be unavailing. Dr. Lamont, therefore, 

 assumes the satellites to move in circular orbits, in a plane having, 

 as computed by Sir W. Herschel, an inclination of lOP 2' to the 

 ecliptic, the longitude of the ascending node being 165° 30'; and 

 on this hypothesis proceeds to compute from the observations the 

 distances and times of revolution of the two satellites. The results 

 of the computation are as follows : 



Distance. Periodic Time. 



Second satellite 31"-35 8^705886 



Fourth ditto 40 '07 13 ,463263 



Having found the distances and periods of revolution, it remains 

 to compute the value of the planet's mass. It is found, however, 

 that the values derived from both satellites exhibit a considerable 

 difference, as might indeed be expected, when it is considered that 



spectively to t\ie first and second of Sir W. Herschel. [An abstract of Sir 

 John F. W. Herschers investigation of the motions of the same two satel- 

 lites of Uranus, was given in Lond. and Edinb. Phil. Mag^ vol. iv. p. 381. 

 Sir John observes, that he had, at the period of his communication, no evi- 

 dence of the existence of any other satellites of this planet. — Edit,] 



