Eclipses of the Satellites of Jupiter. 325 
observers, any error which may arise from this source will 
also be done away by this method. 
Hence it would appear, that in order to obtain a correct dif- 
ference of longitude of two places from correspondent eclipses 
of the satellites of Jupiter, the circumstances at both places 
should be similar and favourable ; and that the telescopes 
should have equal powers, or that both immersions and emer- 
sions should be observed, which indeed ought always to be 
done, where time will admit: also, that the circumstances 
being favourable at one place and not so at the other, a result 
very different from the truth will be obtained. 
