472 COSMOS. 



The mass of Mercury was determined by Ln grange upon 

 very bold assumptions as to the reciprocity of the relations of 

 distances and densities. A means of improving this element was 

 first afforded by Encke's Comet of short period of revolution, 

 The mass of this planet was fixed by Eucke at ?86 ^ 5T of the 

 Sun's mass, or about yi-.y of the Earth's. Laplace 18 gave the 

 mass of Mercury as ^- f ^ 810 according to Lagrange ; but the 

 true mass is only T 5 T of that assigned by Lagrange. By this 

 correction also, the previous hypothesis of the rapid increase 

 of density in the planets in proportion as they were nearer to 

 the Sun, was disproved. When, with Hansen, the material 

 contents of Mercury are assumed to be y^ those of the 

 Earth, the resulting density of Mercury is I '22. "These 

 determinations," adds my friend, the author of them, "are 

 to be considered only as first attempts, which, nevertheless, 

 come much nearer the truth than the numbers assumed by 



18 Laplace, Exposition du Syst. du Monde, 1824, p. 209. 

 The celebrated author admits, however, that in the determi- 

 nation of the mass of Mercury, he founded his opinion upon 

 the " hypothese tres precaire qui suppose les den sites da 

 Mercure et de la Terre reciproques a leur moyenne distance 

 du Soleil." " The very precarious hypothesis which supposes 

 the densities of Mercury and the Earth reciprocal to their 

 mean distance from the Sun." I have not considered it 

 necessary to mention either the chain of mountains, 61,826 

 feet in height, which Schroter states that he saw upon the 

 disc of Mercury and measured, and which Kaiser (Sternen- 



ly 1850, 57) doubts the existence of; or the visibi- 

 lity of an atmosphere round Mercury during his transit over 

 the Sun, asserted by Lemonnier and Messier (Delambre, Hist. 

 de lAstronomie au dixhuitieme siecle, p. 222), or the tempo- 

 rary darkening of the surface of the planet. On the occasion 

 of the transit which I observed in Peru on the 8th of 

 November, 1802, I very closely examined the outline of the 

 planet during the egress, but observed no indications of an 

 envelope. 



