COMETS. 181 



(September 11th to December 20th, 1847), and Bond,* the 

 director of the observatory at Cambridge (U. S.), (September 

 16th, 1817). The Pulkowa observations gave : the period 

 of rotation of Neptune's satellite, 5d. 21h. 7m. ; the inclina- 

 tion of its orbit to the plane of the ecliptic, 34° 1' ; the dis- 

 tance from the center of the primary, 210,000 geographic- 

 al miles ; the ?nass, T - i \- u ^. Three years afterward (August 

 14th, 1850), Lassell discovered a second satellite, for the ex- 

 amination of which he employed a magnifying power of 628. t 

 This last discovery has not, I believe, been confirmed by other 

 observers. 



III. 



THE COMETS. 



The comets, which Xenocrates and Theon of Alexandria 

 call light-clouds, and which, according to an old Chaldean 

 belief, Apollonius Myndius considered to " ascend periodically 

 from great distances in long-regulated orbits," though subject 

 to the attractive force of the central body, form a peculiar 

 and separate group in the solar regions. They are distin- 

 guished from the planets, properly so called, not merely by 

 the eccentricity of their orbits, and, what is still more import- 

 ant, their intersection of the planetary orbits ; they also pre- 

 sent a variability of figure, a change of outline, which in some 

 instances has been observable during the space of a few hours , 

 as, for example, in the Comet of 1744, so accurately described 

 by Hensius, and at the last appearance of Halley's Comet in 

 1835. Before Encke had discovered the existence of inte- 

 rior comets of short periods of revolution, whose orbits were 

 inclosed within those of the planets, dogmatic speculations, 

 founded upon false analogies as to the increasing eccentricity, 

 magnitude, and decreasing density in proportion to the dis- 

 tance from the Sun, led to the opinion that planetary bodies 

 of enormous volume would be discovered beyond Saturn, re- 

 volving in eccentric orbits, and " forming an intermediate 

 group between planets and comets, and, indeed, that the last 

 exterior planet ought to be called a comet, since perhaps its 

 orbit intersected that of Saturn, the planet next to it."$ Such 



* W. C. Bond, in the Proceedings of the American Academy of Arts and 

 Sciences, vol. ii., p. 137 and 140. 



t Schum., Astr. Nachr., No. 729, p. 143. 



t " By means of a series of intermediate members," says Immanuel 



