7 id PHILOSOPHICAL TftANSACTIONS. [anNO J 763. 



1743 is = 26 by the external contact at egress. 



1753 is = 25 by the internal contact at egress. 



1753 is = 33 by the external contact at egress. 

 Hence we may safely conclude, that the difference of longitude between any 

 two places may be determined by one single observation of the contact of Mer- 

 cury with the sun's limb, made at each place, so that the error in the determi- 

 nation will not exceed 30* of time from the truth ; whereas in the above 33 ob- 

 ser\^ations of the eclipses of the first satellite of Jupiter, we find the limit be- 

 tween the errors to amount to 3*" 44* of time. If we take a mean of the said 

 observations of the first satellite, the difference of longitude between Greenwich 

 and Paris is = Q"" 29% and if we reject those which differ the most from the rest, 

 the mean of the remaining 25 observations gives the said difference = 9™ 40*, 

 and the mean of those 8 observations which are rejected, gives the said difference 

 = 8"* 53* , both which last determinations can be proved to be very far from the 

 truth by the observations of the late transit of Venus ; for by the said observa- 

 tions of Venus it appears, that the difference of longitude between Greenwich 

 and Paris cannot exceed 9*" 33% as before said; and if the said difference is = 

 8"* 53% then the parallax of the sun, by the Savile-house observation compared 

 with that of M. De la Lande at Paris, would amount to 20*, which we are sure 

 it cannot be. ^ 



On the whole therefore we may conclude, that the difference of longitude be- 

 tween the Royal Observatories of Greenwich and Paris, as determined by 63 

 observations of the contact of Mercury with the sun's limb made at each place, 

 is = 9™ 16*. This determination would have been perhaps more decisive, if 

 Mr. S. could have had recourse to the books containing the observations of the 

 late astronomer royal. Dr. Bradley. Observations! made by one of the greatest 

 astronomers, and by the best and most accurate observer, assisted by the best 

 and most accurate instruments, which are in any observatory; but alas! the 

 public are hitherto deprived of the ust of these most excellent observations.* 



In a former paper which Mr. S. gave into the r. s., concerning the parallax of 

 the sun, he assumed the difference of longitude between Greenwich and Paris 

 to be = 9™ 10*; and as the determination of this difference is now more certain 

 by the transits of Mercury above-mentioned, being found = 9™ 16*; and as this 

 difference of longitude will make sojne small difference in the result of the said 



• On Thursday following, being the 9th of June, a motion was made, at the meeting of the jt. s. 

 by the Rev. Nevil Maskelyne, f. b.. 8. and unanimously agreed to, recommending it to their council, 

 as visitors of the Royal Observatory, to take proper measures for obtaining and securing the astrono- 

 mical observations that have been made there in times past, for the benefit of the public; it was 

 also agreed on to publish them, when obtained, at the expence of the Society; and for the future 

 to publish the observations made at the Royal Observatory annually, in the Philosophical Transac- 

 tions. — Orig. 



