348 SPECIAL RESULTS IN THE UltANOLOGlCAL 



important element. Encke has determined the mass of 

 Mercury at 4 8( \ a 7 5 1 of the Sun's mass, and about - t 7 '. f of 

 that of the Earth. Laplace, in accordance with Lagrange, 

 had made aoaaaTg-j but the true mass is only about ^tha 

 of that quantity. This correction refutes at the feame 

 time the previous hypothetical statement of the rapid in- 

 crease of planetary density with increasing proximity to the 

 Sun. If, with Hansen, we take the volume of Mercury 

 at -rf-o-ths of that of the Earth, the resulting density of Mer- 

 cury, as compared to that of the Earth, is only as 1'22 .* 1. 

 " These determinations," adds my friend, their author, " are 

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

 approximate much more nearly to the truth than Laplace's 

 assumption." Ten years ago, the density of Mercury was 

 still assumed to be almost three times greater than that of 

 the Earth (2'56 or 2'94), that of the Earth being =] -00. 



Venus. 



The mean distance of Venus from the Sun is '1 23331 7 

 in parts of the Earth's solar distance, or 15 German or 60 

 English millions of geographical miles. The sidereal or 

 true period of revolution of Venus is 224 days, 16 hours, 

 49 minutes, 7 seconds. No other planet approaches so 

 near to the Earth as does Venus : it may approach us within 

 5J German, or 21 English, millions of geographical miles ; 

 but it may also be as remote as 36 German, or 144 English, 

 millions of geographical miles : and hence the great varia- 

 bility of its apparent diameter, but which by no means de- 

 termines solely the intensity of its brightness ( 557 ). The 

 excentricity of Venus's orbit is only 0*00686182, expressed 



