436 SCIENCE PROGRESS 



exact moment when he sees first contact ; by this time Venus 

 will have reached another position, V^ Thus the times when 

 Venus is at V and V* are both known, and hence their difference 

 is also known. This difference is the time that Venus has 

 taken to go from V to V\ and from it we can calculate the solar 

 parallax. 



One further word may not be inappropriate in this article. 

 The movements of Mercury have always given astronomers 

 much more trouble than have those of any other planet, and for 

 a long time seemed merely to throw discredit on the calcula- 

 tions. In 1869, however, Leverrier pointed out that the dis- 

 crepancies could be accounted for by the disturbing effects of 

 another planet between Mercury and the sun. Such a planet 

 would undoubtedly be too close to the sun to be visible as an 

 evening or a morning star, and the only way in which its exist- 

 ence could be demonstrated visually would be by seeing it make 

 a transit across the solar disc. In 1859 a physician named 

 Lescarbault thought he saw such a phenomenon, but he said 

 nothing about it until he heard of Leverrier 's announcement. 

 He communicated with the astronomer, and the latter, after 

 many questions and a personal inspection of the apparatus, was 

 convinced of the genuine nature of the observation and named 

 the planet Vulcan. It has, however, not been seen again, and 

 it is not improbable that what the doctor actually saw was a 

 sun-spot without penumbra. There is no doubt at all that if 

 such a planet really exists it must sometimes make these transits 

 across the sun, and, as the distance between the two bodies 

 must be comparatively small, these occurrences must be fairly 

 frequent. Close observations of the sun have been main- 

 tained for a great many years, but Vulcan has not yet been 

 found. It is most improbable that it exists. 



