628 PHILOSOPHICAL TRANSACTIONS. [anNO 176p. 



amounted to 56', according to Mr. Wargentin. This appearance, in all pro- 

 bability, is occasioned by the refraction which the rays of the sun suffered in 

 passing through the high and dense atmosphere of the planet, and was perhaps 

 rendered more sensible by the vapours near the horizon ; as a similar appearance 

 was observed at the 2d internal contact, in 1761, at very considerable altitude, 

 though in a smaller degree. But it will, Mr. H. fears, occasion a much greater 

 uncertainty in the quantity of the sun's parallax deducible from these observa- 

 tions, than was reasonably expected. 



By a mean of 6 observations, Mr. H. found the planet's diameter = 58".!; 

 being not greater than 5g".0 from 4 of the observations, all agreeing precisely 

 to the same part of a second ; nor less than 56".Q by the least of the other two. 

 About 1 5 minutes after the internal contact, a very thick and black cloud, 

 which moved towards the east, with a slow motion, along the skirts of the ho- 

 rizon, prevented any further observations. The next morning, the sky being 

 perfectly clear, and the limb of the sun undulating but in a small degree, Mr. 

 H. made the following observations of the sun's eclipse : 



Apparent time. At IS*" 33"' 45' beginning of the eclipse. — 20*' 18™ 36' nearly 

 ended. — 20** J S"" 42'i ends. 



Many irregularities were observable on the moon's limb ; though none of 

 them were so pointed as some which he observed in the eclipse of the sun, on 

 August 16, 1765. 



On the top of New College tower, the Rev. Mr. Lucas, fellow of New Col- 

 lege, with an excellent acromatic telescope of 6 feet, magnifying dO times, was 

 certain that the external contact of Venus with the sun was passed at 7^ 6"' 12' 

 apparent time, having perceived a small impression on the sun's edge several 

 seconds sooner; and the Rev. Mr. Clare, Fellow of St. John's College, with the 

 same instrument, judged the thread of light to be completed at 7^ 24"" 28', 

 having observed the limbs to be in contact several seconds sooner. 



The next morning Mr. Lucas observed the beginning of the sun's eclipse at 

 \S^ 33™ 47\ and the end at 20'' 18™ 37^ 



Mr. Sykes, of Brazen Nose College, with an acromatic refractor of 3-j- feet, 

 made by Mr. Dollond, first saw Venus on the sun at 7*^ 6™ O*, and observed the 

 thread of light to be completed at 7^ 24™ 22'. 



Mr. Shuckburgh, of Balliol College, observed there the external contact of 

 Venus with the sun at 7^ 6™ 8' apparent time, and the internal contact at 

 7^ 24™ 25'; though at 7*' 23™ 16', he judged that the centre of the planet was 

 removed more than its own semidiameter from the sun's limb, or that the true 

 internal contact was then actually passed. He is of opinion that the observation 

 of the completion of the thread of light could not be made nearer than to 8' or 

 10', on account of the undulation of the limbs : and he further adds, that when 

 Venus was wholly entered on the sun, he could no longer perceive the penumbra 



