630 PHILOSOPHICAL TRANSACTIONS. [aNNO 176g. 



ever chiefly by recollection; for having had no experience of this observation 

 before, not having observed the transit of 1761, he had conceived a prqudice 

 that it would not be possible to observe the external contact with any accuracy, 

 and therefore he neglected to make any other minute of what he saw of it, but 

 that he was certain that the planet was on the sun by 5™ after 7, by his regula- 

 tor. Mr. Cyril Jackson, a student of Christ Church, who observed in the same 

 room with Mr. H., said, when all was over, that he thought he had notice of 

 the planet's approach, by a more vehement undulation in that part of the sun's 

 limb where the planet entered, than in any other, which he perceived a very 

 short time before he saw the planet. Mr. H. confessed that he was not sensible 

 of this circumstance. He observed with an 18 inch reflector; Mr. Jackson used 

 a refractor of Mr. Dollond's of 9 feet. The wind was high, and very trouble- 

 some to both of them, by the motion it gave to the instruments. 



When the planet had been so long on the sun's limb, and so large a part of 

 its circle was plainly entered, that he thought the internal contact was near at 

 hand, he was much astonished to find the shape of the black spot suddenly al- 

 tered from a large segment of a circle, to what appeared drawn out like a neck, 

 where the lower part, which still seemed the segment of a circle, was connected 

 with the sun's limb, by a kind of ligament of darkness terminated on each side 

 by right lines. The ligament detached itself from the sun's limb; and the light, 

 as bethought, was visible, all round the planet, at 7^ 21*" 52% by his regulator, 

 and not earlier to his eye. And this he set down as the internal contact. The 

 moment that he perceived the ligament detached from the sun's limb, he turned 

 his eye to the clock, to catch the minute, and to be satisfied that he was right in 

 counting of the seconds. And when he returned his eye to the telescope, which 

 was before or not later than the 55th second, he found that the thread of light 

 between the limb of Venus and the limb of the sun had sensible breadth, and the 

 shape of the planet was perfectly circular. Mr. Jackson reckoned the internal 

 contact at 7*'21'"51% by the regulator. HejudgedofitasMr.il. did, by thede- 

 tachment of the ligament, which hesaw, as well as Mr. H. from the sun's limb. The 

 regulator, at the time of the internal contact was 25^-J- or 26^ too slow for Mr. 

 Hornsby's observatory clock. Mr. H. was much surprized, on comparing notes 

 with Mr. Hornsby, to find that he had judged the internal contact 14'-^. or 15' 

 earlier than he did. 



After writing the above, Mr. H. received from Mr. Hornsby a minute of the 

 difference of his clock from mean time, at the time of observation. He now 

 subjoins his observations reduced to mean time at Oxford, reckoning Mr. 

 Hornsby's clock too fast for mean time by 5*4- at the hour of observation. 



External contact 7^ 3"* 23'-^ 



Detachment of the ligament ... . 7 22 12-|- 



