320 



THE GEOLOGIST. 



above the boiling-point, the water would exist only as a vapour-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 volcauos 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 vapour 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. Grlaisher'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 anot her, 

 and the influence of the advance of geology upon them. Agreeably and 

 logically framed, Mr. Ansted's discourse will afford half an hour's intel- 

 lectual gratification of an instructive and highly suggestive character. 



