PORTION OF THE COSMOS. THE PLANETS. 337 



and Poisson ( 538 ), the probable alterations of the ellipse 

 formed by the Earth's path are comprised within such narrow 

 limits, that they can only modify the climates of the different 

 terrestrial zones to a very moderate extent, and, moreover, 

 very gradually, and in very long periods. Although the 

 analysis by which these limits are exactly determined is not 

 yet quite completed, yet it has at least shown that the ex- 

 centricity of the Earth will never be transformed into that 

 of Juno, Pallas, or Victoria. 



10. Strength of the Sun's light on the different 

 planets. If we make the strength of the Sun's light on 

 the surface of the Earth = 1, we find for 



Mercury 6-674 



Yenus 1-911 



Mars 0-431 



Pallas . . . . 0-130 



Jupiter 0-036 



Saturn 0-011 



Uranus 0-003 



Neptune 0-001 



Owing to the great excentricities of the orbits of some 

 of the planets, the intensity of light on their surface differs 

 much at their greatest and least distance from the Sun. 

 Thus it is in 



Mercury, when in perihelion, 10*58; in aphelion, 4-59 

 Mars 0-52; 0'36 



Juno 0-25; 0'09 : 



while the Earth, from the small excentricity of its ellipse, 

 has in perihelion TO 34, and in aphelion 0*967. If the 



