VOL, LIX.3 PHILOSOPHICAL TRANSACTIONS. 63Q 



subject; and very possibly be able to imitate nature, in the formation of medi- 

 cated waters. 



XXXI. Of Several Phenomena observed during the Ingress of F^enus into the 



Solar Disk. By the Rev. fV. Hirst, F.R.S, p. 228. 



.■••'■;■*; sffv 



The telescope Mr. H. used was a reflector 2 feet in length, and magnified 55 

 times. Expecting the planet to enter the solar disk at or near the zenith, he 

 kept his eye constantly fixed at that part of the sun a considerable time before 

 the beginning of the transit. The first intimation which he had of the near ap- 

 proach of the planet, was by the sudden appearance of a violent corruscation, 

 ebullition, or agitation of the upper edge of the sun, as in pi. 17, fig. 3, when 

 he called out to governor Vansittart, who was taking the time, and desired him 

 to take care. Mr. H. had not taken this precaution above 5 or 6 seconds, when 

 he plainly saw a black notch breaking in upon the sun's limb, and which seemed 

 a portion of a much less sphere than that of Venus, as in fig. 4. Instantly he 

 desired Mr. Vansittart, by the word now ! to mark the time, which was 1 1*^ 57"* 

 35' sidereal time, by Dr. Halley's little clock, belonging to the royal observatory. 

 The last-mentioned time, allowing for the necessary corrections, and reduced to 

 apparent time, is 7^ 11™ 1 1^ 



As Mr. H. imagined, from the instructions of Dr. Halley, that the precise 

 and accurate time of observing the internal contact, is when the thread of light 

 should break in between the concave edge of the sun and the convex edge of 

 Venus, as in fig. 8 ; he waited till that particular period, which was when Dr. 

 Halley's clock marked IS"^ 15'" 45% sidereal time, or 7^ 29"" 18% apparent time, 

 the difference being 1 S"* 7' of apparent time. 



The same phenomenon of a protuberance, which Mr. H. observed at Madras, 

 in 1761,* at both internal contacts, he observed again at this last transit: at 

 both times, the protuberance of the upper edge of Venus diminished nearly to a 

 point before the thread of light between the concave edge of the sun, and the 

 convex edge was perfected, when the protuberance instantaneously broke off 

 from the upper edge of the sun, but Venus did not assume its circular form till 

 it had descended into the solar disk, at least to the distance, by estimation, from 

 the upper edge of the sun, as described in fig. 9. 



Though at the time of the contacts the atmosphere was remarkably clear, yet 

 as the sun descended towards the horizon, the atmosphere became more and 

 more hazy, so that the edge of the sun, as well as the edge of the planet, be- 

 came more and more tremulous, and caused the planet to assume, in appearance, 

 different configurations, resembling sometimes a prolate and sometimes an oblate 



* Phil. Trans, vol. lii. p. 396.— Orig. 



