320 
THE GEOLOGIST. 
above the boiling-point, the water would exist only as a vapoiir-atmo- 
spliere. In this case also its circulation would be arrested, and life would 
cease. In this way the subject might be extended to zoology, and on 
through chemistry, physical geography, and the whole circle of the 
sciences. At present geologists are apt to assume the internal igneous 
fluidity of the earth, and much useless speculation has been indulged in as to 
the thickness of its crust. Calculating from established observations. Pro- 
fessor W. Thomson considers that the solid substance of the earth must 
consist, on the whole, of solid material, more rigid than steel, because if it 
were not so, the actual height of the tides and the amounts of precession 
and nutation would be smaller than they are found experimentally to be. 
But the rigidity of the upper or surface-crust of the earth is less rigid than 
glass, and therefore the interior must be more rigid than steel, — a fact 
utterly at variance. Professor Ansted admits, with the hypothesis of the 
earth being a mass of molten matter enclosed in a hollow shell less than a 
hundred miles thick, open to volcanos and disturbed by earthquakes, 
whilst he considers it agrees with the calculation formerly made by Mr. 
Hopkins, from other data, that the crust cannot be less than 800 miles 
thick. Professor Thomson, however, concludes that no thickness of crust, 
less than half the earth's radius, could enable our planet to preserve its 
figure with sufficient rigidity to allow the tidal phenomena and the pheno- 
mena of nutation and precession to be as they are. 
Again, the observations recently made by Professor Tyndall on the 
effect of A'apour on solar heat-rays reflected back from the earth into the 
air, though not having apparently much to do with the rigidity of the 
earth, and strongly contrasting in the nature of the method employed, 
point to the same general conclusion, and afford another example of the 
correlation and of the intimate dependence of different departments of 
inquiry. It results from Professor Tyndall's experiments, that of the heat 
radiating from the earth in England, more than ten per cent, is stopped 
within a distance of ten feet from the surface. In proportion as the air 
contains more vapour it radiates less rapidly, and it would seem to follow 
that a uniform steam-temperature of the earth's surface must so completely 
intercept the solar heat-rays as to render the earth unfit for any kind of 
life, animal or vegetable, with which we are acquainted. In other words, 
the balance of heat received from the sun must probably have existed at 
all times nearly as it does now, to allow of such organic life as we know of. 
A very much higher temperature. Professor Ansted thinks, would, by dis- 
turbing this equilibrium, unfit the earth for the existence of races so 
nearly resembling those now living upon it as are indicated by even the 
oldest fossil remains. Mr. Glaisher's balloon experiments confirm Pro- 
fessor Tyndall's deductions as to the state of the atmosphere in the higher 
regions, and so unexpectedly these two seemingly unconnected meteorolo- 
gical experiments come in as evidence. It is in this manner that Mr. 
Ansted brings forward the most interesting facts in the modern progress 
of the sciences, to show their relationships to geology and to one another, 
and the influence of the advance of geology upon them. Agreeably and 
logically framed, Mr. Ansted's discourse will aflbrd half an hour's intel- 
lectual gratification of an instructive and highly suggestive character. 
