vol" XLII.] PHILOSOPHICAL TRANSACTIONS. 6l5 



the known difference of meridians of these places, the egress must have hap- 

 pened at Greenwich at S** 1"" mane. 1 



The observation of Mr. Wurtzelbaur will greatly avail at coming at the dura- 

 tion of the transit. It is mentioned, that Mercury left the sun's limb 73° 30' 

 from his vertex to the right. Now at that time at Nuremberg, the angle of 

 the ecliptic with the vertical passing through the sun's centre, was 42° S' 5"', 

 therefore the last point of contact on the sun's limb was observed 31° 26' 55* 

 from the ecliptic to the south, and consequently his latitude was 8' "28" south 

 at that time. 



To find the point on the sun's limb of the ingress, in order to come at the 

 duration of the transit, we must be beholden to computation and the theory of 

 Mercury's motion: I have therefore, from the tables from which the above 

 times of the transit of 1743 are drawn, carefully computed his motion along 

 his path crossing the sun's disk, and find that he moved along it after the rate 

 of 5' 53y in an hour, and the difference of latitude in 5 hours 4' 2i", and his 

 elongation 29' 7": therefore the angle of his visible way was 8° 29' 50", which 

 doubled, and added to 31° 26' 55", gives 48° 26' 35", his distance, on the limb 

 of the sun, from the ecliptic, also to the southward at his ingress on it: there- 

 fore the nearest approach of his centre to that of the sun was lO' iQ", and the 

 length of the path run during the transit 25' 14", and consequently the time 

 of running it 4*' 17'", the half of which 2'' S-i-'", subtracted from 20*" 1", the 

 end of the transit at Greenwich, gives the middle there at 17'' 52"" SO*, earlier 

 by 1 84^" than the series of moments, &c. give it. 



Now as the said series makes the middle of the transit of 1743, at ll** 2™ 

 mane, and as it corresponds with that of 1697; and the computation of that is 

 18-i^" too late by the series of moments, &c. it may be reasonably expected, 

 that the same computation for this of 1743 will be so much too late also; and 

 if so, the middle may be set down at 43.^*" past 10, or 44"* at farthest, Oct. 25 

 in the forenoon. 



By computation from the tables abovementioned, with the correction of the 

 node, I make the distance of the centres at the nearest approach in 1697, to 

 be 10' 33", but by the observations of Mr. Wurtzelbaur it turns out only )o' 

 19", less by 14". Should therefore their distance in 1743, computed in the 

 same manner at 9™ 10*, be as much diminished, the duration of the transit will 

 be protracted no less than 5"" 24', and the first contact will be 2™ 42' earlier, 

 and the last so much later, than the times abovementioned for them. 



In the computation of the transit of 1743, the semidiaineter of the sun is 

 supposed 16' 14V', and that of Mercury 4-i-"; but in that of 1697, have taken 

 Mercury's only 3-^^", imagining the precise moments of the first and last exte- 



