CAMBRIDGE ESSAYS : HOPKTNs' '^GEOLOGY." 
519 
80 little depends on the latter cause, that although 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 cease, 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. No 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, WiU the external conditions of our 
planet be such as now exist ? will the sun give the same light and heal ? 
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. 
Until 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 
