214 
POPULAR SCIENCE REVIEW. 
ington, and it was soon employed upon the difficult task of 
solving the problem as to the exact periods of tbe Uranian 
satellites. This was very satisfactorily effected, and with dis- 
tinct and conclusive favour to Mr. Lassell, whose observations, 
were fully corroborated. Only four satellites could be distin- 
guished by the American observers, and their periods, as com- 
puted from a valuable series of measures, agreed with those 
previously derived at Malta. In Appendix I. to the Washington 
Observations for 1873, Prof. Newcomb gave a valuable summary 
of results, — the first obtained, be it noted, with that splendid 
instrument which soon afterwards, in 1877, revealed the satel- 
lites of Mars— which included the elements of the satellites of 
Uranus as follows : — 
Satellite. 
Mean Longitude 
Epoch 1871, 
Dec. 31, W.M.T. 
Radius of 
Orbit. 
Period of 
Revolution in days. 
I. Ariel 
21-83 
13-78 
2-52038 
II. Umbriel 
136-52 
19-20 
4-14418 
III. Titania 
229-93 
31-48 
7-70590 
IY. Oberon 
154-83 .. 
42-10 
13-46327 
Speaking of the comparative brightness of the satellites. 
Prof. Newcomb says : — ‘ The greater proximity of the inner 
satellites to the planet, makes it difficult to compare them 
photometrically with the outer ones, as actual feebleness of light 
cannot be distinguished from difficulty of seeing arising from 
the proximity of the planet. However, that Umbriel is intrinsi- 
cally fainter than Titania is evinced by the fact that although 
the least distance of the latter is somewhat less than the greatest 
distance of the former, there is never any difficulty in seeing it 
in that position. From their relative aspects in these respec- 
tive positions, I judge Umbriel to be about half as bright as 
Titania . Ariel must be brighter than Umbriel because I have 
never seen the latter unless it was farther from the planet than 
the former at its maximum distance I think I may 
say with considerable certainty that there is no satellite within 
2' of the planet and outside of Oberon, having one-third the 
brilliancy of the latter ; and therefore that none of Sir William 
HerscheTs supposed outer satellites can have any real existence. 
The distances of the four known satellites increase in so regular 
a way that it can hardly be supposed that any others exist be- 
tween them. Of what may be inside of Ariel it is impossible to 
speak with certainty, since in the state of atmosphere which 
prevails during our winter, all the satellites named disappear 
at 10" from the planet/ 
Prof. Newcomb mentions that no systematic search for new 
satellites was undertaken because it must have interfered with 
the fullness and accuracy of the micrometer measures of the 
