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Ihus far all the facts which I have mentioned depend on 
observations of unquestionable accuracy and admit of no dis- 
pute, while, at the same time, they offer no materials for 
speculation on the origin or the probable duration of the 
solar system. But, in connection with the doctrine of thb 
conservation and dissipation of energy, speculations of a very 
bold and interesting character have been made by Sir William 
lhomson, which may profitably detain us for a moment. 
Several years ago his attention was called to the fact that 
the sun is constantly radiating heat into space in enormous 
quantities ; and, to avoid the self-evident conclusion, that this 
vast globe must inevitably be cooling down, and that thus, 
at some time or other, however distant, the heat-energy of the 
solar system would be expended, he proposed the theory that 
a constant amount of heat was probably kept up by the falling 
.on his surface of nebulous masses, comets, &c., either drawn 
within the sphere of his attraction from remote regions of 
space, or gradually brought to that condition by the resistance 
to motion in the densely nebulous neighbourhood of his body. 
This theory, however, was shortlv given up, and the con- 
clusion at present held by himself and many other physicists, 
is that the cooling process is really going on, though we are 
not sme t-nat any effect whatever has been observed during 
the term of man's occupation of the earth. If this be so, it is 
quite certain that a time will come, measured perhaps by a 
large multiple of millions of years, when the solar system will 
be a complete wreck, the sun himself a dark inert mass, and the 
attendant planets, like the moon, unfit habitations for organized 
and sentient beings. 
The earth too, even if the sun were to retain its heat, gives 
evidence that it was not intended for an eternal existence in 
its present state. It has been surmised, and the guess 
assumes something like verification from the accurate mathe- 
matical calculations of Delaunay, Airy, and others, that the 
faction of the tides contrary to the direction of diurnal motion 
is sufficient to produce a small but calculable increase in the 
time of the diurnal rotation. No one doubted that the tides 
would produce some effect of this kind, and calculation seems 
to prove, on certain assumptions, that the effect is sensible, 
and that it will some time or other bring the earth to rest. 
These are grand speculations, and they appear to be based 
on data which are unquestionable. By analogies drawn from 
the fixed stars we are also brought to nearly the same con- 
clusion. Many of these are variable, aud some, from a high 
degree of brightness or magnitude, fade away at regular 
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