10 
is an almost equal error on each side. Each theory is based 
on data wholly insufficient to establish its truth. The doctrine 
of uniformity, I believe, is untrue for many reasons, but not 
for the reason which Mr. Croll, following* Sir W. Thomson, 
has assigned. There is no proof that the sun was much 
hotter a hundred or fifty millions of years ago than at present. 
If there be a difference, which is probable, I think it much 
more likely that it would be of an opposite kind, and that its 
heat has increased by condensation, more than it has lost by 
dissipation. In the “ Theory of Helmholtz/'* which Sir William 
has latterly espoused, having abandoned Meyer’s meteoric hypo- 
thesis, the heat of the sun is now thought to be supplied by 
condensation, which replaces the ceaselesswaste from dissipation 
or radiation into space. Now if the sun has reached its present 
high state of heat and light from an earlier stage, when it was 
neither hot nor luminous, what proof can there be that the 
process has been reversed for the last million of years, and 
the waste exceeded the supply for so long ? But this very 
idea, that all the heat radiated into space is dissipated and 
lost, is an assumption without solid reason. If it arose at first 
from a transformation of potential into kinetic energy, or 
attractive force into motion, by the condensation of the solar 
mass, it can only cease or be lost by a reconversion of this 
kinetic energy into potential energy of another kind ; namely, 
the condensation of repulsive ether. Thus the energy which 
flows out from the sun as sensible heat and light, in the sector 
of space bordering on the sun’s equator, will return to it in- 
visibly and insensibly, in the neighboui’hood of the poles, 
and the sun would thus be an immense magnet by virtue of 
its revolution. 
16. The general climate of the earth, Sir W. Thomson 
further remarks, “ cannot have been sensibly affected by con- 
ducted heat, at any time more than ten thousand years after 
the solidification of the surface.” This may be true, if wo 
take the phrase “conducted heat” in a rigorous sense, and 
exclude all liquefaction, convection, regelation, or fresh gene- 
ration of heat by condensation from pressure or chemical 
change. But those omitted or excluded elements are those of 
chief importance in the actual problem. A solution which 
omits them may be true as an abstract dynamical theorem, 
but can have little bearing on the actual course of geological 
change. 
17. The first volume of Sir W. Thomson’s and Professor 
Tait’s comprehensive “Treatise on Natural Philosophy” 
closes with theso remarks on the once current hypothesis of 
the earth’s fluidity below a thin superficial crust. 
