On the supposed Intra- Mercurial Planet. 35 



Art. XI. — On the supposed Intra-Mercurial Planet 

 By R. L. J. Ellert, F.RS. 



[Read 14th November, 1878.] 



The announcement that during the total eclipse of the 

 29th July last, visible in the United States of America, 

 Professor Watson had discovered an unknown body near 

 the sun, supposed to be an intra-mercurial planet, has 

 revived the almost dormant question of the existence of 

 such a body, and awakened fresh interest in the earlier 

 observations of the supposed planet Vulcan. It will be 

 known to some of you, no doubt, that long since, the 

 celebrated Leverrier demonstrated that Mercury's perihelion 

 moved 40 seconds per century faster than it should do, 

 taking into account the gravitating action of only the known 

 planets of the system. This he most easily accounted for by 

 supposing that there were between Mercury and the sun a 

 group of small planets. Adopting this theory, various re- 

 corded observations of the passage across the sun's disc of 

 dark round bodies, at a more rapid rate than ordinary sun 

 spots, were adduced as evidence of the existence of such 

 planets; but the untfustworthiness of some of these ob- 

 servations, and the failure of experienced observers to detect 

 the phenomena while scrutinising the sun's surface at the 

 very times the reputed passages occurred, has hitherto so 

 weakened the only proofs adduced — except the theoretical 

 one of Leverrier 's — that he alone, I believe, out of all expe- 

 rienced astronomers, still had strong faith that intra- 

 mercurial planets or a planet would yet be discovered. 

 On March 21st, 1877, a transit of the supposed body 

 across the sun's disc was announced as probable by Lever- 

 rier, and a systematic search was kept up by aU the 

 principal observatories of the world during the days indi- 

 cated, but nothing was discovered. The American astrono- 

 mers, probably made more sanguine by the recent discovery 

 by one of them of the satellites of Mars, seized the oppor- 

 tunity of the late eclipse for examining systematically the 

 immediate vicinity of the sun during the moments of totality, 

 at which times it is possible to detect comparatively small 

 stars very close to him, except in the rays of the corona. 



