384 PHILOSOPHICAL TRANSACTIONS. [aNNO 1769. 



present venture to suggest, as that may be better done when all the observations 

 that shall have been made of the transit are collected together. The telescope 

 which he used was an excellent reflecting one of 2 feet focus, made by the late 

 ingenious Mr. Short, and is the same with which the last transit was observed 

 here by Mr. Charles Green. Mr. M. applied the magnifying power of 140 times, 

 and used smoked glasses to defend the sight, which are much preferable to black 

 or red glasses, as showing the objects more distinct, and being much more plea- 

 sant to the eye. 



It had been thought by some, that Venus's circumference might probably be 

 seen, in part at least, before she entered at all upon the sun, by means of the 

 illumination of her atmosphere by the sun ; Mr. M. therefore looked out dili- 

 gently for such an appearance, but could see no such thing. He was also atten- 

 tive to see if any penumbra or dusky shade preceded Venus's first impression on 

 the sun at the external contact, such a phenomenon having been observed by the 

 Rev. Mr. Hirst, f.r.s., at the former transit of Venus, in 1761, which he ob- 

 served with much care and diligence at Madras, in the East Indies; but Mr. 

 M. could not discern the least appearance of that kind. He would not however 

 be therefore thought to call in question either Mr. Hirst's discernment or fidelity; 

 as he is sensible that the tremors of the limbs of the sun and Venus, occasioned 

 by the vapours at the altitude of 7°, might easily obscure a faint object. 



When Venus was a little more than half immerged into the sun's disk, Mr. M. 

 saw her whole circumference completed, by means of a vivid, but narrow and ill- 

 defined border of light, which illuminated that part of her circumference which was 

 oft" the sun, and would otherwise have been invisible. This he might probably 

 have seen sooner, if he had attended to it. He continued to see it till within a 

 few minutes of the internal contact, and grew apprehensive that it would prevent 

 the appearance of the thread of light, when it came to be formed ; but it disap- 

 peared about 2 or 3 minutes before, as well as he can remember : after which 

 the regularity of Venus's circular figure was disturbed towards the place where 

 the internal contact should happen, by the addition of a protuberance, dark like 

 Venus, and projecting outwards, which occupied a space on the sun's circum- 

 ference, which bore a considerable proportion to the diameter of Venus. Fifty- 

 two seconds before the thread of light was formed, Venus's regular circumference, 

 supposed to be continued as it would have been without the protuberance, seemed 

 to be in contact with the sun's circumference supposed also completed. Accord- 

 ingly, from this time, Venus's regular circumference, supposed defined in the 

 manner just described, appeared wholly within the sun's circumference ; and it 

 seemed therefore wonderful that the thread of light should be so long before it 

 appeared, the protuberance appearing in its stead. 



At length, when a considerable part of the sun's circumference, equal to -J- or 



