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HISTORY OF PHYSICAL ASTRONOMY.

tuberance, dark like Venus , and projecting outwards, which occupied aspace upon the sun’s circumference which bore a considerable proportionto the diameter of Venus . Fifty-two seconds before the thread of lightwas formed, Venus ’s regular circumference (supposed to be continued asit would have been without the protuberance) seemed to be in contact withthe sun’s circumference, supposed also completed. Accordingly, fromthis time, Venus ’s regular circumference (supposed defined in the mannerjust described) appeared wholly within the sun’s circumference, and itseemed, therefore, wonderful that the thread of light should be so longbefore it appeared, the protuberance appearing in its stead. At length,when a considerable part of the sun’s circumference (equal to one-tbirdor one-fourth of the diameter of Venus ) remained still obscured by theprotuberance, a fine stream of light flowed gently round it from each side,and completed the same in the space of three seconds of time ; and Venus appeared wholly within the sun’s lucid circumference. But the protube-rance, though diminished, was not taken away till about twenty seconds more ;when, after being gradually reduced, it disappeared, and Venus 's circularfigure was restored." *

Dr. Bevis states in the account of his observations that “ the planetseemed quite entered upon the disk, her upper limb being tangential tothat of the sun; but instead of a thread of light, which he expected im-mediately to appear between them, be perceived Venus to be still con-joined to the sun’s limb by a slender kind of tail, nothing near so blackas lier disk, and shaped like the neck of a Florence flask. The said tailvanished at once; and/or a, few seconds after, the limb of Venus , to whichit had been joined, appeared more prominent than her lower part, somewhatlike the lesser end of an egg, but soon resumed its rotundity.” f

The Rev. Mr. Hirst thus describes the appearance presented duringthe transit:—“ The same phenomena of a protuberance which I observedat Madras in 1761, at both internal contacts, I observed again at this lasttransit. At both times the protuberance of the upper edge of Venus diminished nearly to a point before tire thread of light between the con-cave edge of the sun and the concave edge of the planet was perfected,when the protuberance broke off from the upper edge of the sun: butVenus did not assume its circular form till it had descended into the solardisk some distance (.

Mr. Dunn, who observed the transit at Greenwich, remarks that “ hesaw the planet held as it were to the sun’s limb by a ligament formed ofmany black cones whose bases stood on the limb of Venus , their verticespointing to the limb of the sun.”§

Mr. Pigott states that Venus , before, she separated from the sun, wasconsiderably stretched out towards his limb, which gave the planet nearlythe form of a pear; and even after the separation of the limbs, Venus wastwelve or more seconds before she resumed her rotundity ,”j|

With respect to the physical cause of the phenomena above referred to,the most probable hypothesis of their origin is that which attributes themto the influence of irradiation. Lalande first shewed that the dark liga-ment connecting the limbs of the planet and tlie sun might he satisfacto-rily accounted for upon this principled. The most consistent explanationthat can he given of the protuberance on the limb of the planet, when itappeared wholly separate from the sun’s limb, is that proposed by Prof.

* Phil. Trans., 1768, p. 358. f Ibid., 1769, p. 190. J Ibid., p. 229.

§ Ibid., 1770, p. 70. || Ibid., p. 202. d Mem. Acad, des Sciences, 1770, p. 409.