l6o HISTORY OF GEOLOGY AND PALAEONTOLOGY. 



In comparison with the Earth, the relative density of the 

 planets is as follows : 



Sun . . .0.25 

 Mercury . . 1.12 



Venus 



1.03 



Earth . . . i.oo 

 Mars . . . o 70 



Jupiter . . 0.24 



Saturn . . o. 1 3 



Uranus . * o. 1 7 



Neptune . . o. 16 



The inner planets are therefore considerably heavier and more 

 firmly consolidated than the outer. 



Great advances have been made in our knowledge of the 

 physical constitution of the planets by means of improved 

 telescopic methods and the construction of the modern large 

 telescope. Mars has always been an interesting object of 

 astronomical observation. As early as 1659, Huygens 

 observed white spots at both poles, and the elder Herschel 

 in 1781 was able to draw a sketch of the surface of Mars, 

 which was afterwards improved by Hieronymus Schroter on 

 the basis of researches conducted between 1786 and 1803. 

 Beer and Madler distinguished pale, white, and yellowish-red 

 spots from dark greenish-blue spots, and regarded the former 

 as land masses, the latter as seas. Maps of Mars were pub- 

 lished by several other astronomers. The Milan astronomer, 

 Schiaparelli, published in 1878 a work which added much to 

 our knowledge of Mars. The dark streaks crossing the light 

 spots in straight or in bent lines, opening into the dark, iron- 

 grey seas, are regarded by Schiaparelli as canals, and are 

 mapped with hitherto unsurpassed precision, while he confirms 

 the observation that mountain-chains and solitary mountains 

 are quite absent. 



The telescopic examination of the rest of the planets has so 

 far brought less satisfactory results. The small planet Venus, 

 next in position to the Earth, seems to be surrounded by a 

 dense, cloudy atmosphere, which obscures the view of the 

 actual surface of the planet ; at the same time recent observa- 

 tions have demonstrated round or elliptical spots of light 

 colour (perhaps continents dimly visible through the atmo- 

 sphere), and these are separated from one another by dark 

 ribbon-like streaks. 



Keeler in 1889, by the use of the famous refractor of the 

 Lick Observatory, obtained the first information about the 

 constitution of Jupiter. With this instrument two reddish 



