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THE POPULAR SCIENCE MONTHLY. 



surface. In a somewhat wider zone, the planetary structure could be 

 only preserved under the form of an ellipsoid, which would deviate 

 widely from a sphere, not so much on account of its rotation as from 

 the effects of the unequal attraction which its different parts receive 

 from the primary. The accompanying diagram shows the equilibrium 

 form of a satellite moving in a circle around a large sphere, both bodies 







being equally dense and separated by small distances. In a somewhat 

 greater proximity to the primary, the satellite would be reduced to a 

 state of instability ; and its matter, if not kept together by a great co- 

 hesive power, would scatter into independent orbits, and ultimately form 

 a ring. There are, perhaps, in our solar domain too few cases to show 

 how excessive tidal action can produce its definite results in its various 

 degrees of power. If the nearest satellite of Mars bore to its primary 

 such relations of size and density as subsist between the earth and the 

 moon, it would, no doubt, surpass all known planets in deviating from 

 a true sphere, even though its materials were not of the most yielding 

 character. If similar relations existed between Saturn and his closest 

 secondary, the latter would show a greater deviation than the primary 

 does from a spherical form. 



