GRAVITY ON SATURN AND URANUS. 



In like manner, while the bulk of Uranus is 82 times greater, 

 its weight is only 14 times greater than that of the earth. It 

 is therefore lighter, bulk for bulk, than the earth, in the propor- 

 tion of 14^ to 82, or 1 to 6 very nearly. 



In fine, while the bulk of Neptune is 107 times greater, 

 its weight is only 19 times greater than the earth, and it is 

 therefore lighter, bulk for bulk, than the earth, iu the proportion 

 of 19 to 107, or nearly 1 to 6, the same as Uranus. 



It has been proved that the earth is, bulk for bulk, 5^ times 

 heavier than water. It follows therefore that Saturn being 8^ 

 times lighter, bulk for bulk, than the earth, is lighter, bulk for 

 bulk, thau water in the proportion of 5^ to 8, or 1 to 1^. 



In like manner it appears that Uranus and Neptune, being- 

 nearly six times lighter, bulk for bulk, than the earth, must be 

 composed of materials equal in weight, bulk for bulk, with water. 



The weight of Jupiter is equal to that of some of the denser 

 sorts of wood, such as lignum vitee or ebony, and that of Saturn 

 is equal to the weight of the lighter sorts, such as deal. 



26. The weight of bodies placed upon the surfaces of Saturn, 

 Uranus, and Neptune, are ascertained by comparing their masses 

 with their magnitude, as in the case of Jupiter, and it is thus 

 found that it does not differ much from their weights on the 

 earth. On Saturn it is a very little more, and on Uranus and 

 Neptune a little less. 



27. It appears, therefore, that if these planets be inhabited, 

 the same organisation which prevails on the earth would be 

 sufficient to impart the same, or nearly the samp, degree of 

 stability and freedom of locomotion. A man, in fine, transferred 

 from the earth to Saturn, Uranus, or Neptune, would not be 

 sensible of much difference in his power of action and motion. 

 Trees and other vegetables, with their present strength, would be 

 equally stable, and artificial structures equally solid and durable. 



28. The importance of the atmosphere to all the functions of 

 animal and vegetable life, and its uses in the diffusion of light 

 and the retention and diffusion of heat, have been fully explained. 

 The existence of any atmosphere on a planet is, therefore, an 

 essential condition necessary to bring it into analogy with the 

 earth as an inhabited globe. 



The atmospheres of Jupiter and Saturn are rendered con- 

 spicuously apparent by the telescope. We see the clouds floating 

 in dense masses upon them ; so dense indeed and so unbroken 

 as to conceal from our view the characters of the surfaces of the 

 planets themselves. 



Uranus and Neptune are too remote for like observations in 

 the present state of the telescope, but it is in the highest degree 



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