VOL. LIX.] PHILOSOPHICAL TRANSACTIONS. 627 



lived, and which he had altered from sidereal to mean solar time, for the easier 

 comparison of those clocks, which several gentlemen had procured, in order to 

 observe this rare and curious phenomenon. The time was determined by meri- 

 dional transits of the sun, taken with a transit instrument made by Mr. Bird, 

 and placed very exactly in the plane of the meridian, the focal length of the 

 object-glass being 43 inches. The motion of both clocks was perfectly even 

 and regular. 



The atmosphere was so loaded with vapour, and the limb of the sun was in 

 such a constant state of undulation, that Mr. H. determined to observe the 

 external contact with a refractor of 12 feet, furnished with a system of eye- 

 glasses, and magnifying ()S times. He had found, by a previous computation, 

 that the planet would make the first impression on the sun's upper limb, about 

 19' to the right hand of a vertical circle passing through the sun's centre. He 

 therefore kept his eye constantly fixed on that part, and at 7^ 5"" 58' apparent 

 time, he perceived that a small part of the planet's diameter had certainly en- 

 tered on the sun's disk ; the impression, which he had observed for a few 

 seconds before, having continued on that part. While the planet was passing 

 over the sun's edge, Mr. H. determined, with the old micrometer applied to 

 the 1'2 feet glass, the annexed differences of 

 declination between the northern limb of the 

 sun, and the southern limb of Venus, with 

 as much accuracy as the unsteadiness of the 

 floor would permit. 



But as the time of the internal contact began to draw nigh, he directed a re- 

 fractor of 7t f'Set, with a double object-glass, to the sun, made by Mr. DoUond, 

 and magnifying QO times; and soon after 7'' 21™ mean time, perceived that the 

 planet appeared to be wholly entered on the sun, though the limbs of the sun 

 and Venus were not actually separated ; that part of the sun's edge, where the 

 ingress happened, being very sensibly obscured by a penumbra, and the limbs 

 appearing to be united, by a kind of ligament of a considerable breadth. This 

 ligament became narrower and narrower, and was at length reduced to a point, 

 and actually broken at 7*^ 21™ 57% mean time, or /*'24"' 13'^ apparent time. 

 At 7^ 24™ 23' apparent time, the thread of light between the edges of the sun 

 and Venus, which was before completed, now appeared of a very sensible 

 breadth, and to equal -rV^h of the planet's diameter. If he had estimated this 

 breadth properly, the true internal contact must have happened considerably 

 more than a minute sooner. The Swedish astronomers have described this 

 appearance very nearly as Mr. H. saw it ; but according to the account given by 

 Mr. Mallet, the interval of time between the true and apjjarent ingress, when the 

 limbs appeared perfectly to coincide, and when the ligament was observed to be 

 broken, did not exceed 53% according to Mr. Melander's observation, and 



4 L 2 



