492 Dr. P. Lowell on Photographs of Jupiter 



see that Jupiter with an ellipticity of -j^ fulfils ClairauVs 



law, and furthermore that the matter composing him comes 

 nearer to central condensation than to homogeneity. 



Going into this more carefully and comparing the cor- 

 responding data for the Earth, Mars, and Saturn, we get the 

 following table : — 



<p 5 a — b a — b Ratio 



2" 4^ e= ~b~ V= ~^T' eto0. 



1 1 1 1 n ^ 



Earth 577 231 296 296 = ' 9 ' 5 



1111 

 Mars 435 174 189 190 =1 ' 151 



Jupi ter .... .gjg ~ ± ^ =0-682 



1 1 1 1 



featurn . . . . ^ j ^ ^ =0-575 



These are the only planets for which we have the complete 

 data. Mercury and Venus rotate so slowly, in ^ and 225 

 days respectively, that their oblateness is too small to be 

 measured, so that nothing can be deduced of their laws of 

 density. Of Uranus and Neptune we lack as yet the rotation 

 periods, and in the case of the last even the flattening, so 

 that their distribution of matter remains equally hid. 



In considering the table we see that the Earth and Mars 

 lie nearer homogeneity than central condensation, since 

 the conditions for these states are e=0*5<£ and e= 1*250. 

 Jupiter and Saturn, on the other hand, approach central 

 condensation, Saturn indeed almost reaching it. 



Now, when we reflect upon the great pressures to which 

 the matter composing the major planets must be subjected 

 owing to their enormous masses, we perceive that such slight 

 densities as they present can only be maintained through 

 great internal heat. The force of attraction at Jupiter's 

 equator, for instance, would be 2*61 times the Earth's were 

 his substance homogeneous, and 2*51 times were all his 

 matter concentrated at his centre, being given by 



. ■ /-*¥(!+ J- : i'+*-> 



where b is the polar radius, in the one case, and by 



m 



f=k 



a- 



in the other. 



