VOL. XXXIII.] PHILOSOPHICAL TRANSACTIONS. 73 



In order to deduce from this phenomenon, so accurately observed, what may 

 contribute to perfecting the theory of Mercury's motion, which seems to need 

 but very Httle correction; Dr. H. carefully computed, from his tables, the mo- 

 tion of the planet in 5 iiours, and found his apparent motion on the sun, to be 

 in longitude 29' 21" retrograde, and that his latitude increased northerly 4' 17-^" 

 in the same time ; whence the horary motion in longitude was 5' 52", and in 

 latitude o' 5]-^"; and thence the angle of the visible way with the ecliptic 8° I9', 

 and the horary motion in that way 5' 56". Again, the angle of the ecliptic 

 with the meridian, being in this place 73'^ 24', the visible way of Mercury made 

 an angle of 65° 5', with the meridian passing through the sun's centre; whence 

 the horary change of declination becomes exactly 2' 30". 



These data Dr. H. chooses rather to take from the theory, than from im- 

 mediate observation ; because there is always an unavoidable, though small 

 uncertainty, in what we observe, yet greater than there can be in the computa- 

 tion for so small a space of time, especially now the theory is so very near the 

 truth. 



This premised, let us now inquire the true time of the central ingress, and 

 the latitude of the planet at that time. And first, by Dr. H.'s account, Mer- 

 cury was in the parallel of the sun's centre at 21-l minutes after the central in- 

 gress, in which time he ascended to the northward 54"; and so much therefore 

 was he more southerly than the sun's centre at his ingress. Mr. Bradley, 7 ,; 

 minutes after the said ingress, in which the planet ascended 19', found his de- 

 clination 45" south, and therefore at the ingress his declination was l' 4" south. 

 And by Mr. Graham's observation. Mercury was more northerly than the sun's 

 centre l' 11", 53"" 20* after the central ingress; but in that time Mercury 

 ascended 2' 13"; therefore, according to him, at the ingress the planet had 

 1' 2" south declination. We shall not therefore err above a semidiameter of 

 Mercury, if we assume his declination, at that time, to have been precisely one 

 minute. 



Now the sun's semidiameter being then 16' 15", one minute is the sine of 

 3° 32' in the arch of the sun's limb; and consequently, the point of this ingress 

 was 13° 4' more northerly than the ecliptic; whence the latitude of Mercury 

 was then 3' 40" north, and difference of longitude 15' 50", by how much he af 

 that time followed the sun's centre. 



If therefore, to the arch of 13° 4', we add the double of 8° 19', or of the 

 angle which the visible way made with the ecliptic, we shall have 29° 42' for the 

 point on the sun's western limb, at which the planet made his exit, likewise to 

 the north of the ecliptic. Hence the chord described in the whole transit, was 

 of 137" 14', and the chord itself 30' 16"; and the nearest distance to the sun's 



VOL. VII. L 



