History of the Discovery of the Planet Neptune. 203 
It is considered certain that Neptune is attended by a satellite. 
Mr. Lassell states that on the 10th of October, 1846, he observed 
a faint star distant from the planet about three diameters. On 
the 11th and 30th of November and also Dec. 3d, he saw a small 
star having about the same appearance ; and he considered it prob- 
able that the star was a satellite. On the 7th of July, 1847, he 
again saw the supposed satellite, and on the following evening, the 
planet and satellite had both changed their position with reference 
to the neighboring stars. On the 22d, 25th and 26th of the 
same month, the planet appeared attended by a satellite, and on 
the Ist of August he obtained the fullest evidence of the verity 
of the satellite, in being able clearly to ascertain that during the 
two hours he watched the planet, it had carried the satellite along 
with it in its orbital motion. On the 20th of September last, Mr. 
Lassell announced that he had obtained twenty observations of 
the satellite during the current year, and from them all he had 
concluded that the period of the satellite was five days, twent 
hours, fifty minutes, and forty-five seconds; and he thinks this 
result cannot be in error beyond a small number of minutes. 
The radius of its orbit is about eighteen seconds, which gives 
about 250,000 miles for the distance of the satellite from the 
planet. ‘The satellite is always more brilliant in the S. W. than 
in the N. E. part of its orbit, presenting in this respect a striking 
analogy with the outer satellite of Saturn. In the former posi- 
tion Mr. Lassell found it easy to observe; in the latter extremely 
difficult. 
The conclusions of Mr. Lassell have been confirmed by the ob- 
servations of Mr. Bond at Cambridge. Mr. Bond believes that 
e saw the satellite in August last, and in October he obtained 
Satisfactory measures of its distance and angle of position by 
means of a micrometer with illuminated wires. 
The preceding results enable us to compute the mass of Nep- 
tune, which is found to be about ;;1,;,5 part of the sun. | 
Mr. Bond states that he has at times been quite confident of 
Seeing a second satellite, but has never yet been able to obtain 
Successive measures of its distance from the primary. : 
The grand question still remains untouched, Will the new 
Srconp Series, Vol. V, No. 14.—March, 1848. 
