VOL. LIII.] PHILOSOPHICAL TRANSACTIONS. 53 



happened later ; for whether we make use of the sun's mean R. ascension from 

 the best solar tables extant, or the sun's apparent R. ascension reduced to the 

 meridian of the place, as determined by actual observation on the day of the 

 transit, the true apparent time of the contact will be'found to have happened at 

 2ih 3Q' 54'' — or at SI*' SQ'" 54"-i- if the time by the star Antares be used, whose 

 situation was more favourable to an observer in 34° of south latitude. Mr. H. 

 therefore supposes the internal contact to have happened at 2l''39"' 52% by tak- 

 ing a mean of the two observations.* 



The Royal Observatory at Paris was supposed by Sir Isaac Newton, in his 

 Principia, to be Q'' 20^ to the east of Greenwich. And the editor of Dr. Hal- 

 ley's tables has followed that determination, which has also been generally used 

 by the English astronomers. The French astronomers have till very lately ima- 

 gined the difference of meridian to be p'" 10% as deduced from a single observa- 

 tion of an eclipse of Jupiter's first satellite made by M. Cassini when in London, 

 with a telescope of similar size and construction with that used at Paris when the 

 same eclipse was observed. In the year 1734 M. Maraldi published a compari- 

 son of 33 eclipses observed at Greenwich by Mr. Flamsteed, and at Paris by the 

 French astronomers, 1 Q of which are immersions, and the rest emersions. The 

 longitudes resulting from each correspondent observation differ widely from each 

 other, the two observatories being U'" 27* distant by an immersion of the 2d 

 satellite, and only 7'" 43* by an emersion of the first. But if we take a mean of 

 the whole, the difference of longitude will be Q'" 24* ; and if we exclude the ob- 

 servation of the 2d satellite above mentioned, which must be very faulty, the 

 difference of meridians will be 9'" 22% a result which in all probability is but a 

 very few seconds from the truth. It may be observed that the immersions all 

 give the difference of longitude too great, and almost all the emersions too 

 little; a circumstance owing either to the badness of the air here in England, or 

 to an inequality in the goodness of the telescopes, or perhaps to both ; for what- 

 ever was the advantage in observing the immersions, was balanced by the emer- 

 sions: for which reason, whenever the eclipses of Jupiter's satellites are used, 

 the longitude should, if possible, be deduced both from immersions and emersions. 



As the observations of transits of Mercury may be very useful in settling the 

 longitudes of places which are not far distant, Mr. H. examined the several ob- 

 servations made at Paris, and either immediately at Greenwich, or in such parts 

 of London whose longitude from Greenwich is known within one second of time. 

 And the result of such comparisons is as follows. On the 29th of October 1723, 

 Dr. Halley observed the first interior contact of the limbs of Mercury and the 



* Mr. Mason, before he left England, acknowledged, in a letter to Mr. H., that he had com- 

 mitted a mistake in his calculations, by foigetting to apply to the sun's place the equation of preces- 

 fioD, which on the day of the transit amounted to — 15". 6. — Orig. 



