246 Prof. Hughes, On the relation of the appearance [May 5, 



The axis of revolution is the mean axis or the least axis, accord- 

 ing as 





If we put G> + G)' = 0, the ellipsoid is stationary, and we must 

 have 



4a^P 

 (a'+by 



the axis of revolution being then the least axis. 



Again we may have a prolate spheroid, rotating in equilibrium 

 about an equatorial axis, if 



„.U_,4£4,„j = 6=ji;_^^«>j = c»C; 



/ , a' 



[ 0) — (0 — 



\ a 





or 





+ bV {a' + hj 



( . 2a \ 



Monday, May 5, 1879. 

 Professor G. D. Liveing, President, in the Chair. 



Mr William Hillhouse, of Trinity College, was ballotted for 

 and duly elected an Associate of the Society. 



The following communication was made to the Society : — 



Professor T. M'' K. Hughes, On the relation of the appearance 

 and duration of the various forras of life upon the earth to the 

 breaks in the continuity of the sedimentary strata. 



Few subjects are more interesting than that of the first appear- 

 ance of life upon the earth and the enquiry into the circumstances 

 which have affected the duration of its various forms. 



There are many difficulties in the way arising from the imper- 

 fect record we have in the rocks of any one district and the 

 small knowledge of the surface of the earth which has yet been 



