138 cosmos. 



The mass of Mercury was determined by Lagrange 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 rev- 

 olution. The mass of this planet was fixed by Encke at 

 4"3"65 TsT °f the Sun's mass, or about T i. T of the Earth's. La- 

 place # gave the mass of Mercury as 2025 3T0 according to La- 

 grange ; but the true mass is only ^ of that assigned by La- 

 grange. 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 

 T |o those of the Earth, the resulting density of Mercury is 

 1-22. " 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 Laplace." Ten years ago the density of Mer- 

 cury was taken as nearly three times greater than the dens- 

 ity of the Earth— as 2*56 or 2-94, when the Earth =1-00. 



Venus. 



The mean distance of this planet from the Sun, expressed 

 in fractional parts of the Earth's distance from the Sun, i. e., 

 60 million geographical miles, is , 7233317. The period of 

 its sidereal, or true revolution, is 224 days, 16h. 49m. 7s. 

 No principal planet comes so near the Earth as Venus. She 

 can approach the Earth to within a distance of 21,000,000 

 miles, but can also recede from it to a distance of 144,000,000 

 miles. This is the reason of the great variability of her ap- 



but in the edition of the Astronomie of 1849, Madler has given the pref- 

 erence to Bessel's result. 



* Laplace, Exposition du Syst. du Monde, 1824, p. 209. The cele- 

 brated author admits, however, that in the determination of the mass 

 of Mercury, he founded his opinion upon the " hypothese tres precaire 

 qui suppose les densites de 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 disk of Mercury and measured, and which 

 Kaiser (Sternenhimmel, 1850, § 57) doubts the existence of, or the vis- 

 ibility of an atmosphere round Mercury during his transit over the Sun, 

 asserted by Lemonnier and Messier (Delambre, Hist, de V Astronomie 

 au dixhuitieme siecle, p. 222), or the temporary 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 



