VOL. LIII.l PHILOSOPHICAL TRANSACTIONS. 7i3 



XXIX, The Difference of Longitude between the Royal Observatories of Green- 

 wich and Paris, determined by the Observations of the Transits of Mercury 

 over the Sun in 1723, 1736, 1743, and 1753. By James Short, M.A., 

 F.R S. p. 158. 



It will doubtless appear surprizing, Mr. S. says, that he should attempt to de- 

 termine the difference of longitude between two of the most celebrated observa- 

 tories in Europe; and in which some of the greatest astronomers that ever lived, 

 have for above 80 years been constantly observing the motions of the heavenly 

 bodies: yet it is most certain that to this day we are ignorant of the said differ- 

 ence of longitude; the English astronomers reckoning it to be = Q™ 20% and 

 the French setting it down at Q"" 10% which they tell us was found by M. Cas- 

 sini, by observations of the eclipses of Jupiter's first satellite made by him while 

 iu London in the year 1 698 : we are no where told by what observations the 

 English astronomers have fixed this difference at Q*" 20^ 



In the Memoirs of the Royal Academy of Sciences at Paris for the year 1734, 

 there is an account given of 33 corresponding observations of the eclipses of the 

 first satellite of Jupiter, made at Greenwich and Paris, from the year 1677 to 

 the year 170I: the mean of these 33 observations gives the difference of longi- 

 tude between Paris and Greenwich = Q'" 29'. 



I have calculated, says Mr. S., and it may be demonstrated, that if we com- 

 pare the observations of the late transit of Venus made at Greenwich, and by 

 M. de la Lande at Paris, and suppose that the difference of longitude between 

 these 2 places is = 9"^ 25% it will follow that the sun and Venus are at an infi- 

 nite distance, which is absurd. Again, if we suppose the difiference to be greater, 

 it will follow that the sun and Venus are more than infinitely distant, which is 

 likewise absurd. We are therefore certain, if these observations are to be de- 

 pended on, that the difference of longitude between Greenwich and Paris is less 

 than 9™ 25^ If we compare the observations made at Savile-house, with the 

 same observation by M. de la Lande at Paris, and reason in the same manner, 

 we shall find that the difference of longitude between Greenwich and Paris must 

 be less than 9*" 33\ Thus far then a limit one way is fixed for the difference of 

 longitude between these 2 places. 



The late transit of Venus was the only one which had ever been observed at 

 Greenwich and Paris, and by comparing the observation at Greenwich with that 

 made by M. de la Lande at Paris, the difference of longitude comes out = 9"^ 8^ 

 And if we compare the observations at Savile-house (30' of time west of Green- 

 wich) with that of M. de la Lande,* the said difference of longitude comes out 



* M. de la Lande saw the internal contact of Venus with the sun's limb at 8** SS*" 25' 



Pere Clouet 8 28 25 



VOL. XI. 4 Y 



