' 324. Mr. 'G old ingham's Account of the 
of the others, it will be found, how much the latter are af- 
fected, by the eclipses having been observed when the atmos- 
phere was hazy, or the planet very low, during twilight, or 
when the moon was near the planet, or the satellite not far 
from the body of Jupiter ; and that even if correspondent ob- 
servations had been taken, under favourable circumstances, at 
a known meridian, the difference of longitude given by the 
comparison would have been far from correct : the same 
eclipse observed at two places, under similar unfavourable 
circumstances, would possibly give a result near the truth ; 
as the observations at both places would be affected in the 
same w r ay, but probably not in an equal degree, as it is not 
likely there would be exactly the same degree of haze, the 
same strength of twilight, &c. &c. at both places ; and there- 
fore those observations taken under the same favourable cir- 
cumstances can only be relied upon with certainty. 
It may not be an easy matter to have telescopes at both 
places of precisely the same powers for these observations : 
at Madras we had two telescopes in use, constructed at the 
same time, in appearance precisely alike, and intended by 
Dollond to have been so in all respects ; yet on repeated 
trials, one was found to have a decided advantage of several 
seconds over the other, shewing the emersions sooner, and 
the immersions later by that quantity. In order to do away 
the error arising from a difference in the powers of telescopes, 
immersions and emersions should be observed at both places ; 
the difference of longitude will be as much greater than it 
ought to be by one series as less by the other, but the medium 
will be the corredt difference of longitude of the places : it is 
possible also there may be some difference in the eyes of 
