218 Mr. South on the Eclipses of [March, 



other object. The phenomena offour moons were naturally interest- 

 ing, nor was the inquiry unattended with important consequences. 

 The velocity of light repaid the labours of Roemer, and the eclipses 

 of the satellites opened a new, easy, and at that time a compa- 

 rative accurate mode of determining the differences of longitude 

 of distant stations. Hence astronomers were taught to look out 

 for these phenomena, and their observations became recorded : 

 in proportion, therefore, as the opportunities of observing these 

 eclipses were more or less frequent, were they supplied with 

 means of improving their tables ; ,till at length something like 

 accuracy was arrived at, as far as relates to those of the first and 

 second satellites ; to this also we must add another cause which 

 will be found in the nature of the instruments at this time em- 

 ployed ; for the rapid motion of these two satellites is such, that 

 the intervening period between their first entrance into the 

 shadow, and their complete obscuration by it, is short; hence 

 telescopes were able to give something like uniformity to the 

 observations of various observers — in short, theory and practice 

 assisted each other. 



But not so with the outer satellites ; eclipses of them were 

 comparatively of very rare occurrence, and the time of their 

 entering the shadow till their complete obscuration being many 

 times greater than in the case of the two first satellites, the 

 observations became more difficult ; and the instruments were 

 inadequate to the purposes for which they were now wanted: 

 observations, therefore, of different observers differed considera- 

 bly with each other, and theory and practice were everlastingly 

 at variance. Hence observations of these satellites came into 

 disrepute, and almost into disuse. 



At length, however, in the preface to a work entitled " Tables 

 Ecliptiques des Satellites de Jupiter," the monstrous discordan- 

 cies between the existing observations of the eclipses of the 

 third and fourth satellites were dwelt upon, with considerable 

 energy, by the celebrated Delambre ; and, perhaps, to the senti- 

 ments expressed by this great man, may we trace the principal 

 cause why at the present moment, observations of the eclipses 

 of these two satellites are almost generally neglected. When, 

 however, prejudice seems distributed by one whose name, like 

 that of Delambre, is never mentioned but with respect, does it 

 become dangerous; and more and more imperative is it, upon the 

 humblest labourer in science, to point out the errors which it 

 leads to — this must plead my excuse for the present communi- 

 cation. 



As the work to which I allude, may not be in the library of 

 every practical astronomer, I shall quote from it some of the 

 passages calculated in my mind to prejudice observers. 



Page 51. — Having alluded to some trifling equation which 

 might be applied as a correction to his tables of the third satel- 

 lite. He says, " Je n'ai pousse l'examen plus loin ; mais il 

 parait en resulter que cette equation ne s'accorde pas plus avec 



