Roijal Astronomical Society. 233 



servation of the dark lines, or threads, which connect the borders 

 of the sun and moon, at the formation and dissolution of the annu- 

 lus. His account is inserted in the first volume of the Memoirs of 

 this Society (page 146), accompanied with drawings, which coincide 

 almost exactly with those given by Mr. Baily. In nearly all the 

 accounts by other observers, the description of the phaenomenon 

 is restricted to the very commencement of the annulus, or to the 

 formation of the string of luminous points which on a sudden are 

 seen to surround that portion of the moon's limb about to enter on 

 the sun's disc; and no notice whatever is taken of the continuation 

 of the phuenomenon, or of the stretching out of the dark spaces 

 into parallel lines, as above mentioned : nor of their sudden rupture 

 and disappearance, which is by Air the most remarkable part of the 

 phaenomenon. 



How far any of these appearances may favour the hypothesis of 

 a lunar atmosphere, or whether, indeed, they^could be accounted 

 for on such an assumption, the author does not stop to discuss ; 

 but, with a view to assist those who are disposed to enter on such 

 an inquiry, he has adduced various accounts of a similar phaeno- 

 menon to that of the dark lines, observed at the transits of Venus 

 over the sun in 1761 and 1769, For on each of those occasions, 

 many astronomers remarked, thtit, at the interior contact of Venus 

 with the sun (both on its ingress and egress), there was formed a 

 sort of dark ligament between the border of Venus and the border 

 of the sun, which appeared like a protuberance from the planet, 

 and which continued several seconds. This dark ligament is repre- 

 sented, in the drawings which accompany the several memoirs on 

 this subject, to be much thicker, and to continue longer, than the 

 dark lines in a solar eclipse j so that the planet, during the progress 

 of the ingress and egress, assumes a shape which has been variously 

 described as resembling a pear, a Florence flask, and a skittle. But 

 all the accounts agree in stating the sudden rupture of the ligament, 

 and that immediately thereon the planet assumes its usual circular 

 shape. Nothing of this kind, however, has been noticed at the 

 transits of Mercury over the sun : on the contrary, we have the di- 

 rect evidence of Sir William Herschel (who examined Mercury, with 

 that special object in view, at the transit of November 9, 1802), 

 that he could not discern anything out of the usual course. He ex- 

 pressly states, that the whole disc of Mercury was as sharply defined 

 as possible ; and that there was no kind of distortion of the limb, 

 either at its ingress or egress: the appearance of the planet re- 

 mained well defined from first to last. 



Mr. Baily considers, and adduces certain facts to show, that the 

 circular edge of the moon is always distorted at those points which 

 are in contact (or nearly so) with the sun's circumference j and 

 which have occasionally given rise to the supposition of lunar moun- 

 tains in high relief*. He thence infers, that all measures of the 

 moon's diameter, when passing over the sun's disc, must be taken 

 with great caution, and with due attention to the proximity of the 

 [* See Lond. and Edinb. Phil. Mag., vol. ix. p. 73. — Edit.] 



Third Series. Vol. 10. No. 60. March 1837. 2H 



