464 Astronomical Society. 



but the formation of tables even more perfect than observation 

 itself*: and in addition, a mass of most valuable and instructive 

 information on the general nature of planetary perturbations, am- 

 ply repaying all the labour of the inquiry, and adding fresh lustre 

 to the already imperishable glory of his name. 



This slight sketch of the history of the satellites of Jupiter may 

 serve to show how intimate is the connection of distant parts of 

 science with each other, and that in it we are to regard nothing as 

 trivial and nothing as great in itself, but in respect of the instruc- 

 tion we may draw from it; — to show, in fine, how deep are the 

 foundations and how wide spread the ramifications of that tree 

 of knowledge which, in the poet's words, 



.... quantum caput ardua ad astra 

 Attollit — tantum radice in Tartara tendit. 



which draws its increments from small beginnings and matters of 

 speculative curiosity, and ends in becoming the ornament, the 

 shelter, and the support of society. 



It is by observations of the eclipses of the satellites alone that their 

 theory can be compared with nature, their apparent distances from 

 the planet being too small and its change too slow to admit of mi- 

 crometrical measurements precise enough for the purpose, though 

 perhaps the modern improvements both in the telescope and mi- 

 crometer may authorize a hope that this may not long be an insu- 

 perable difficulty. Accordingly, from the time of Roemer down- 

 wards, a series of eminent astronomers have occupied themselves 

 with observations of these phaenomena, and it is on no less than 

 two thousand of such observations that Delambre, improving on 

 the tables of Wargentin by the aid of the profound theory just al- 

 luded to, succeeded in calculating the first series of tables laying 

 claim to precision. 



The longitude is so much better ascertained now by lunar di- 

 stances and occultations, that these observations are less resorted 

 to than heretofore for that purpose. Nevertheless they are occa- 

 sionally used, especially those of the first and second, whose 

 eclipses not only happen much more frequently, but are much more 

 definite, than those of the exterior ones. Indeed, the observations 

 of the latter have been declared by high authority, utterly useless. 

 It is not always good, however, to trust to authority ; and Mr. South 

 by a comparison of his own with Colonel Beaufoy's observations, 

 has arrived at a very different conclusion, at least for the cases 

 when both the beginning and end of the eclipse can be seen. Still, 

 however, it is highly desirable that they should continue to be assi- 

 duously observed, not merely to furnish corresponding observations, 

 but to afford the means of further perfecting the tables, so as ulti- 

 mately to enable us to dispense with corresponding observations 

 altogether. 



Colonel Beaufoy has for many years past been a most careful 

 and assiduous observer of these eclipses and indeed of all occasional 

 phaenomena ; such as occultations, eclipses both Solar and Lunar, 



* Than any single observation. — Delambre. 



and 



