PORTION OF THE COSMOS. THE PLANETS. 393 



nised with certainty ( 641 ) the first of Neptune's satellites, in 

 his great 20 -foot reflector, with an aperture of 24 inches. 

 Otto Struve ( 642 ), at Pulkowa (llth Sept. to 20th Dec. 

 1 847), and Bond ( 643 ), the Director of the Astronomical 

 Observatory of Cambridge, U.S. (16th Sept. 1847), con- 

 firmed LasselFs discovery. The Pulkowa observations gave 

 the satellite's period of revolution, 5 days, 21 hours, 7 min. j 

 the inclination of its orbit to the ecliptic, 34 7'; its dis- 

 tance from the centre of the primary planet, 54000 German 

 (216000 English) geographical miles ; and its mass, lT to- 6 - 

 Three years later (14th August, 1850) Lassell discovered a 

 second satellite of Neptune, to which he applied magnifying 

 powers of 628 ( 644 ). This last discovery has not yet, I 

 believe, been confirmed by any other observer. 



