206 Astronomical and Nautical Collections. 



should always be on the same side, with respect to the third 

 satellite ; but it is still more singular that it should not have 

 occurred to him, that this last circumstance sufficiently proves 

 that the error could not have originated with the computers. 

 Whatever may have been the source of this discordance, it was 

 known to the editor of the Nautical Abnanac long ago. But 

 the error is comprehended within the limits of probable accu- 

 racy, assigned in the explanation, p. 168, and even if this 

 remark of Dr. Maskelyne be thought somewhat obsolete, it 

 will not be denied that Delambre's own authority is of some 

 weight ; and he says expressly, in the Introduction to his new 

 tables, p. li., " Onverra Messier et Mechain, dans la mime ville, 

 et presqiie dans le meme guartier, tons deux munis d'excellens 

 instrumens et d'une vue excellente, ne s'accorder cependant gu'd 

 quelqxies viinutes pres sur la meme eclipse. II est evident, de 

 meme, que pour les differences des meridiem, on ne peut se 

 FiER Qu' AU PREMIER SATELLITE ; les ttutrcs 716 sout gutre 

 bons qu' li eclaircir quelque point de physique celeste; le premier 



SATELLITE EST LE SEUL QUI PUISSE ETRE VRAIMENT UTILE 



aiix astronomes et aux gcographes." It was then amply suffi- 

 cient to have lost no time in procuring copies of the new 

 tables for the future use of the computers, who are always ex- 

 pected to be five or six years in advance with their computa- 

 tions ; nor could it be necessary to reject the computations al- 

 ready completed, while it was well known that their results 

 could never be employed for nautical purposes, at least in 

 the present state of the art of observing at sea. 



6. P. xxiii. " * The telescopes proper for observing the 

 eclipses of Jupiter's satellites, are common refracting telescopes, 

 ivom fifteen to twenty feet." So says the Nautical Almanac, 

 but I much doubt whether any one of the Commissioners of the 

 Board of Longitude ever saw a telescope of this kind ; nor do I 

 think there is such a thing in existence. How absurd, then, it 

 appears to recommend the use of them, and thus mislead (as I 

 know it has done,) those entering the career of science." 



Now, in the Nautical Almanac for 1823 and 1824, the expla- 

 nation is expressly attributed to " the late Dr. Maskelyne," p. 

 161, so that it becomes merely historical, with respect to the 

 minute description of the instrument to be employed. But in 

 order to avoid any possible misconception, the paragraph in 

 question is printed thus : 



" The telescopes proper for observing the eclipses of Jupi- 

 ter's satellites, are " common" refracting telescopes from 15 to 

 20 feet ; reflecting telescopes of 18 inches, or two feet focal 

 length, and telescopes of Mr. Dollond's construction, with two 

 object glasses, from 5 to 10 feet; or, which are still more con- 

 venient, those of 46 inches focal length, and 3f inches aper- 

 ture, constructed with three object glasses, which are as ma- 



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