1875 ] t)dl [Uhase. 



2. Urauus is to Saturn, as the time of describing radius in a circular 

 orbit is to the time of orbital revolution. 



3. Neptune is to Saturn, as the time of describing radius in direct fall 

 to the centre is to tlie time of orbital revolution. 



While thus using the convenient language of the nebular hypothesis, I 

 have looked merely to the known laws of centripetal and centrifugal 

 forces which are now operative, withovxt feeling bound by any special 

 theory. Whether planetary aggregation has sprung from gaseous or 

 vaporous clouds, or from meteoric fall, or from explosive nucleal action, 

 or from all combined, is immaterial ; in any case the equilibrating forces 

 would be called into play, and, if they act through the intervention of au 

 elastic medium, the law of harmonic differences should be traceable in 

 any resulting arrangement. "Subsidence, and the central aggregation 

 consequent on subsidence, may go on quite as well among a multitude of 

 discrete bodies under the influence of mutual attraction, and feeble or 

 partially opposing projectile motions, as among the particles of a gaseous 

 fluid."* 



Among the most important consequences of such conservation of force 

 as is indicated by the gravity-potential and its relation to light-velocity, 

 may jjerhaps be reckoned the provision which they seem to involve for the 

 perpetuation of physical activity. In the common interpretations of the 

 nebular hypothesis and of most of the modern thermodynamic theories, 

 continual contraction and heat-radiation have been supposed to tend 

 towards ultimate stagnation and universal death. In the almost ex- 

 clusive regard which has been paid to centripetal influences, the increasing 

 energy of the centi-ifugal force and its final preponderance have both been 

 overlooked. To this general bias of speculative thought Prof. Alexander 

 furnishes a weighty exception. In his Note on the origin of clusters and 

 nebuliB, he refers to appearances "as if, when they were released from 

 superincumbent pressure, by the rupture of the outer portions of the 

 spheroid, or other primitive form, their feeble central attraction could no 

 longer preserve them in form; and so their centres are always broken up."t 

 In illustration of the alternating destructive and conservative changes, 

 he closes the Note with the following words : 



" For the growing leaf is fed by the exhalations which it finds in the 

 atmosphere ; and the leaf, in its decay, nourishes the vegetating tree ; 

 the roots of that tree are embedded in the debris of a comparatively an- 

 cient earth ; the earth itself, in view of the nebular hypothesis (of La- 

 place), has beeU detached from the sun; and the sun and other stars 

 would now seem to be but the comparatively small fragments or drops of 

 greater masses : the one great plan pervading the whole, being, by means 



OF A PERMITTED DESTRUCTION, TO PROVIDE FOR A MORE PERFECT ADAP- 

 TATION AND DEVELOPMENT." 



*Hersehel, Outlines of Astronomy, §871. 



t Op. cit. p. 92. 



