SECT, xxvi.] AND OF THE PLANETS. 275 



grade degree, at least in this climate. The intensity of the 

 sun's light diminishes from the centre to the circumference 

 of the solar disc. 



In Uranus, the sun must be seen like a small but brilliant 

 star, not above the hundred and fiftieth part so bright as he 

 appears to us; but that is 2000 times brighter than our 

 moon ; so that he is really a sun to Uranus, and may impart 

 some degree of warmth. But, if we consider that water 

 would not remain fluid in any part of Mars, even at his 

 equator, and that, in the temperate zones of the same planet, 

 even alcohol and quicksilver would freeze, we may form some 

 idea of the cold that must reign in Uranus. 



The climate of Venus more nearly resembles that of the 

 earth, though, excepting perhaps at her poles, much too hot 

 for animal and vegetable life as they exist here ; but, in 

 Mercury, the mean heat arising only from the intensity of 

 the sun's rays must be above that of boiling quicksilver, and 

 water would boil even at his poles. Thus the planets, though 

 kindred with the earth in motion and structure, are totally 

 unfit for the habitation of such a being as man, unless, in- 

 deed, their temperature should be modified by circumstances 

 of which we are not aware, and which may increase or 

 diminish the sensible heat so as to render them habitable. 



It is found, by experience, that heat is developed in opaque 

 and translucent substances by their absorption of solar light, 

 but that the sun's rays do not sensibly alter the temperature 

 of perfectly transparent bodies through which they pass. 

 As the temperature of the pellucid planetary space can be 

 but little affected by the passage of the sun's light and 

 heat, neither can it be sensibly raised by the heat now 

 radiated from the earth ; consequently its temperature must 

 be invariable, at least throughout the extent of the solar 

 system. The atmosphere, on the contrary, gradually in- 

 creasing in density towards the surface of the earth, becomes 

 less pellucid, and therefore gradually increases in tem- 

 perature, both from the direct action of the sun, and from 



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