226 Ellipses of the Third and Fourth Satellites. [Mahch, 



precaution — the same observer — the same weather — be used, 

 found, or employed, at the various stations both at the immer- 

 sion, and subsequent emersion, that the results so far from 

 meriting obloquy, will probably be far more accurate than 

 any two eclipses of either the first or second satellites — and if 

 what be stated prove true of the fourth satellite, it cannot be 

 otherwise of the third. 



But in the instance we have dwelt upon, only two observers 

 were concerned ; further observations therefore should be re- 

 curred to — those then whose stations relatively to Green- 

 wich are well settled — whose instruments are well mounted, and 

 whose time is well known, will do an essential service to prac- 

 tical astronomy, by observing the immersions and subsequent 

 emersions of the third and fourth satellites whenever they are 

 visible — they should be at their telescopes some eight or ten 

 minutes before the phenomena are expected ; should take every 

 possible care that the immersion and emersion be observed 

 under similar circumstances — and should note at the time, how 

 far the weather at the emersion coincided with that in which 

 the immersion was observed ; nor will the relative splendour of 

 such other satellites, as may be visible at the two periods, allow 

 an accustomed eye much difficulty in deciding. 



I have dwelt upon this subject perhaps longer than some will 

 say, its importance warrants — two marked instances of the 

 injurious effects of the opinions generally entertained against 

 observations of the fourth satellite have however recently presented 

 themselves — One individual many years an accurate and assiduous 

 observer assures me, that in consequence of prejudice, he never 

 looked out for an eclipse of the fourth satellite in his life. — And 

 another astronomer did not observe the very eclipse here so 

 often referred to, "because of sentiments he entertained founded 

 on Delambre's statements." — Nor are the individuals referred to 

 of any mean importance — the latter is well known as the author 

 of many useful astronomical publications ; whilst to the former, 

 practical astronomy owes greater obligations, than to any person 

 in existence. 



To remind observers a table is subjoined giving in sidereal 

 and mean time the predicted immersions and emersions of the 

 third and fourth satellites during the next two months, and should 

 the weather prove favourable, I have little doubt the result 

 will show; that the opinions generally entertained of the inutility 

 of observations of eclipses of the Third and Fourth satellites, 

 originate in PREJUDICE, and terminate in ERROR. 



Sidereal Time. Mean Time. 



March 2. Immersion third satellite . . 8 h 



Emersion 11 



April 8. Immersion fourth satellite . . 9 



Emersion 12 



It is almost needless to say that where the satellite is lost at 

 the immersion, it may be looked for at the emersion. 



James South. 



