VOL. LII.] PHILOSOPHICAL TRANSACTIONS. 357 



At 8^ 35"^ per clock, the external contact was near, and not incumbered with 

 such a penumbra or partial light as the internal contact had been. At 8^ 35™ 

 4^ the least dent possible, quite black, appeared in the sun's limb. And at 8^ 

 35™ 6% the limb was restored to its perfect form, there having been a small 

 trembling light between the narrow watery border of Venus, and the vanishing 

 point of contact in the sun's limb, foi these 1 seconds of time. From which the 

 external contact at Chelsea was 8^ 34™ 30* equal time, and 8^ 36™ 2V apparent 

 time; which makes 8^ 37"^ '2* apparent time at Greenwich. 



From the foregoing circumstances, it appeared that the external contact was 

 more easily to be determined than the internal one, which was contrary to what 

 he had before expected. And considering the aforesaid penumbra, or border of 

 partial light, surrounding Venus as an atmosphere of that planet, with the time 

 of its vanishing, 2± seconds of time; and reducing this to the diameter of Venus, 

 with due allowance for the oblique direction over the sun's limb, the atmosphere 

 of Venus comes out 84- thirds of a degree, which is nearly about -^^ part of 

 Venus's diameter; which diameter being nearly equal to the earth's, the atmo- 

 sphere of Venus comes out nearly 50 geographical miles. 



XXXVI. Account of the Observations made on the Transit of Venus, June 6, 



1761, in the Island of St. Helena. By the Rev. Nevil Mashelyne^ M.A.y 



F.R.S. p. 196. 



From the very cloudy weather which prevailed here for the whole month pre- 

 ceding the transit, Mr. M. almost despaired of obtaining any sight of it at all. 

 He was however fortunate enough to obtain two fair views, though but of short 

 continuance, of this curious celestial phenomenon. The first was a few minutes 

 after sun-rise, when he was surprized not only at seeing Venus so very large, 

 but also so much nearer the sun's limb than he had reason to expect from the 

 best-grounded calculations; which last circumstance foreboded that she would 

 make a more speedy exit from the sun's body than the same calculations allowed; 

 which accordingly happened. At this time her limb, as well as the sun's, ap- 

 peared exceedingly ill defined, which was no more than what one might naturally 

 expect, from their great proximity to the horizon. 



This alone was sufficient to prevent making any observations at that time, 

 which could admit of any exactness, if the clouds had not presently come up, 

 and totally deprived him ofthe sight both of Venus and the sun. In this man- 

 ner the skies continued unfavourable for about an hour, when they became again 

 extremely clear, and he had the pleasure of seeing Venus appear as an intensely 

 black spot on the sun's body, and perfectly well defined. At this time he mea- 

 sured the distance of the nearest limbs of Venus and the sun from each other, 

 with the curious object-glass micrometer adapted to the reflecting telescope, ac 



