552 PHILOSOPHICAL TRANSACTIONS. [aNNO I76I. 



corresponds with 34 below the cypher of Fahrenheit's. The frigorific mass, in a 

 degree of cold far below this, remained soft like a poultice. The cause of this 

 extraordinary phenomenon seems to be no other than a continuation of the solu- 

 tion of the snow, and its mixing with the nitrous acid. For as the production of 

 cold depends solely on the solution and mixture, it cannot happen that this mass, 

 which constitutes a fluid of a hard kind, should run into a solid consistence, so 

 long as the solution and mixture continue. 



XXXI I. Observations on the Transit of Venus over the Sun, June 6th 1761. Bt/ 

 the Rev. Nathaniel Bliss, M.A., Savilian Prof, of Geometry^ Oafordy ajid 

 F.R.S. p. 173. 



The present bad state of health of Dr. Bradley * his Majesty's Astronomer, 

 preventing him from making the proper observations of the transit of Venus, he 

 desired Mr. B. to attend at the Royal Observatory to supply his place in making 

 the necessary observations. The instruments they proposed to use, were a re- 

 flecting telescope, of 2 feet focal length, to which was fitted Mr. Dollond's mi- 

 crometer, both executed by Mr. Short. There were some additions necessary to 

 be made to this instrument, which could not be completed before the 2d inst. 

 But it is absolutely necessary that the telescope should be nicely adjusted to 

 distinct vision for the observer's eye, otherwise the apparent angle, measured by 

 the micrometer, will not be exactly true ; and as the eyes of dift^erent observers 

 may vary very much ; the weather was so very unfavourable, that Mr. B. had 

 not so much as one opportunity of seeing any celestial object, by which he might 

 fit it to the proper focus of parallel rays for his eye. Mr. Green therefore. Dr. 

 Bradley's assistant, was the only person who could use that instrument, having 

 adjusted it to his eye some time before. The instrument Mr. B. made use of, 

 was an exquisite micrometer, of the old form, made by the late Mr. Graham, 

 adapted to an excellent refracting telescope of 1 5 feet focal length. The sky was 

 so cloudy the morning of the transit, and the apparent probability of its clearing 

 up so small, that they almost despaired of being able to make any observation ; 

 for they had but one glimpse of the sun, and that only for about half a minute, 

 till half an hour after 7 o'clock. They then prepared to observe the distance of 

 Venus from each limb of the sun, on the chords parallel to the equator, by Mr. 

 Green, with the reflecting telescope, and its micrometer ; and Mr. B., with the 

 refracting telescope, and the old micrometer, observed differences of right ascen- 

 sion and declination from the consequent and southernmost limb of the sun. 



They observed the internal contact of Venus with the sun's limb, Mr. Green 



* Dr. Bradley died the year following (1762), and was succeeded by Mr. Bliss, as Astronomer 

 Royal. 



