Hinwatava Mounrarins: 993 
the greatest accuracy, short of trigonometrical certainty, and the differ- 
ence of longitude being so small, while the climate is equally favorable, 
there was a likelihood of finding a corresponding observation for every 
one made here. It was with these.ideas, that a list of a number of the 
immersions and emersions of Jupiter’s satellites, was forwarded to the Com- 
pany’s Astronomer Mr. Gotpinauam, who very readily furnished us with 
his own observations of the same phenomena. A. second list was after- 
wards sent, but his answer has not yet been received, and as in the first the 
number of observations is in. no degree comparable to the total number 
made, it has been thought most adviseable for the present, while waiting 
a more correct determination, to present here the results obtained from a 
‘comparison with the Ephemeris. It is to be noted, that whatever error 
may be occasioned in the longitude, as deduced from emersions, owing to 
want of power in the telescope, will be counteracted by an equal error 
inv a contrary sense affecting the immersions—so that supposing the tables 
tolerably correct—a mean of the results of emersions and immersions,, will 
qwe think be found not far removed from the truth.. 
Tus differences of longitude are im most cases found either wholly, or 
the chief part, trigonometrically. In avery few mstances, and for very 
small distances, the route survey checked and corrected, is necessarily 
taken.. The error in this part of the calculation can in no single case 
bs » e 
amount even to 4, and on the mean must be insensible. 
VOL. XIV. 3% 
