74 PHILOSOPHICAL TRANSACTIONS. [aNNO 1725. 



centre 5' 56". Now the horary motion, in this chord, being b' 56", the whole 

 duration of this mercurial eclipse becomes 5'^ 6"^ in respect of the planet's centre ; 

 and therefore the nearest approach of their centres was at 5'^ 14"" 30* at Greenwich, 

 and the exit at 7'^ 47^"", both visible in our American plantations, had there 

 been any curious person there qualified to observe them. 



It follows likewise, by the observed diameter of Mercury, ]0" 45'", that he 

 was very little less than '2 minutes of time in passing the limb ; and, by the 

 given nearest distance to the sun's centre, it is concluded that he was in con- 

 junction, in point of longitude, at 5'' 23'" 15% having then precisely 6' OO" 

 north latitude. Nor can it be doubted, but that all this would have been found 

 exceedingly near to truth, had not the too early setting of the sun deprived all 

 Europe of the desirable sight. 



There being a very remarkable period of the motion of Mercury in 46 years, 

 in which time he makes I91 revolutions about the sun ; this transit is found to 

 have been preceded by two others at that interval : the first, in the year l631, 

 when Gassendus at Paris, on the 28th day of October, old style, was the first 

 that ever observed this appearance of Mercury within the sun's disk, and found 

 him to pass off at 10^ 28™ mane. The second was, Oct. 28, 1677, when Dr. 

 H. had the good fortune to observe both the ingress and egress of the planet, 

 in the island of St. Helena ; the middle time, when he was nearest to the sun's 

 centre, being there but 3"" 50^ past noon, and the visible duration of the transit 

 of the centre of the planet, 5*^ 14'" 20''; which was some small matter con- 

 tracted by parallax, and most likely might have been 0^ 15"' 00* without it. 

 Now in 5'' 15"' Mercury described the chord of 146° 52' in the sun's limb, be- 

 ing 31' 9", and consequently the nearest distance to the centre was 4' 38", or 

 the sine of 16° 34', the sun's semidiameter being radius; that is, l' 18* less 

 than we found it in 1723. Hence also it follows, that the true conjunction in 

 longitude was 7 min. of time later than the nearest approach of the centres, 

 viz. at O'' 10"' 50' at St. Helena, or at o'' 35™ past noon at Greenwich : and 

 that the north latitude of the planet, at that time, was 4' 41''. 



Supposing therefore the nearest distance of the centres, in the transit of 

 1631, to have been 3' 20", that is, l' 18" less than in 1677, we shall find that 

 Mercury then described a chord of 156° 20', traversing the sun's disk in 

 5'' 21"' oO*; so that supposing his exit at 10*' 28'" at Paris, that is lO'' 18"' 40* 

 at Greenwich, he entered on the sun at 4'' 57"' 10* in the morning; and was 

 nearest his centre at 7*' 38"' app. time, but in the same longitude with him at 

 7^ 43"", or Oct. 27<* 19'' 43'" app. time, having then 3' 22" north latitude. 



And here the Dr. observes, that above 30 years before, viz. in Philos. Trans. 

 N° 193, for the month of March 1O9O-I, he predicted, by help of the two former. 



