CAMBRIDGE ESSAYS : HOPKINS' '^GEOLOGY." 



519 



SO little depends on tlie latter cause, that althougli the centre of this 

 globe may consist of a mass of molten matter, the temperature of its 

 surface would not thereby be increased by more than the small fraction 

 of a degree; and Poisson has calculated that ''it would require a 

 thousand millions of centuries to reduce this fraction of a degree by half 

 its present amount." 



The continued reduction of the earth's temperature, and the conse- 

 quent diminished intensity of all subterranean action, would be fol- 

 lowed, in the process of time, by a somewhat startling result. The 

 processes of denudation and of elevation have hitherto preserved the 

 balance of sea and dry land ; but if the elevatory movement should 

 thus ceascj while the denuding forces continue to act with unabated 

 energy, '' the inevitable result would be, that every mountain- top 

 would in time he brought low. ISo earthly barrier could declare to the 

 ocean that there its proud waves should be stayed. Nothing would 

 stop its ravages till all dry land should be laid prostrate, to form the 

 bed over which it would continue to roll an uninterrupted sea. 



But when we contemplate the change produced by such immeasurable 

 periods of time, we are led to ask. Will the external conditions of our 

 planet be such as now exist ? will the sun give the same light and heat ? 

 will the earth move in the same orbit ? will the whole solar system 

 maintain the same relative position to the stars which now surround it ? 



'* The heat and light which the earth derived from the sun in the 

 most ancient geological periods cannot have been very different from 

 that we derive from him at present. "We are too ignorant, however, of 

 the sun's mass and nature to have sufficient foundation for definite 

 speculation." 



" According to the present order of nature, heat is largely dissipated 

 from the sun and stars into surrounding space, and unless there be 

 some means by which its reconcentration may be hereafter effected, or 

 some inconceivable cause for the generation of heat ad infinitum in the 

 sun's mass, it is certain that he cannot continue to radiate the same 

 amount of heat as at present." 



It is a noble conclusion of the mathematical philosopher that the 

 solar system is framed to last for ever, provided the space in which it 

 exists is an actual vacuum ; if, however, the planetary space is not a 

 perfect vacuum, but filled with matter — however rare and ethereal that 

 matter may be — offering the slightest possible resistance to the motions 

 of the planets, the doom of the system is sealed. For evidence of the 

 existence of this ethereal matter forming what is termed a resisting 

 medium, we look especially to the comets as the lightest bodies of the 

 system, and therefore the most sensitive to such a medium. But the 

 only testimony as yet is the periodic retardation of Encke's comet. 



"tntil within the last few years, the motion of the whole plane- 

 tary system through space was only a matter of speculation, but 

 astronomers have estimated both the direction and velocity of the 

 motion. 



Such determination is at present necessarily somewhat vague ; but, 

 if only approximately correct, it might require nearly a million of years 

 for the system to traverse a space equal to that which separates us from 



