AGE OF THE EARTH AS AN ABODE FITTED FOR LIFE. 353 



and, judging alone from what we know of properties of matter, we 

 should expect gigantic cracks to occur from time to time in the upper 

 crust, tending to shrink as it cools and prevented from lateral shrink- 

 age by the nonshrinking uncooled solid below it. When any such 

 crack extends downward as far as a pocket of mother liquor under- 

 lying the wholly solidified crust, we should have an outburst of traj) 

 rock or of volcanic lava just such as have been discovered by geologists 

 in great abundance in many parts of the world. We might even have 

 comparatively small portions of high plateaus of the primitive solid 

 earth raised still higher by outbursts of the mother liquor squeezed out 

 from below them in virtue of the pressure of large surrounding por- 

 tions of the superincumbent crust. In any such action, due to purely 

 gravitational energy, the center of gravity of all the material concerned 

 must sink, although portions of the matter may be raised to greater 

 heights; but we must leave these large questions of geological dynam- 

 ics, having been only brought to think of them at all just now by our 

 consideration of the earth antecedent to life upon it. 



38. The temperature to which the earth's surface cooled within a few 

 years after the solidification reached it must have been, as it is now, 

 such that the temperature at which heat radiated into space during 

 the night exceeds that received from the sun during the day, by the 

 small difference due to heat conducted outward from within.^ One 

 year after the freezing of the granitic interstitial mother liquor at the 

 earth's surface in any locality the average temperature at the surface 

 might be warmer by 60° or 80° G. than if the whole interior had the 

 same average temjDerature as the surface. To fix our ideas, let us sup- 

 pose at the end of one year the surface to be 80° warmer than it 

 would be with no underground heat; then at the end of one hundred 

 years it would be 8° warmer, and at the end of ten thousand years it 



1 Suj)pose, for example, the cooliug aud thickening of the upper crust has proceeded 

 so far that at the surface, and therefore approximately for a few decimeters below 

 the surface, the rate of augmentation of temperature downward is one degree per 

 centimeter. Taking as a rough average 0.005 c. g. s. as the thermal conductivity of 

 the surface rock, we should have for the heat conducted outward 0.005 of a gram 

 water thermal unit centigrade per square centimeter per second (Kelvin Math, and 

 Phys. Papers, Vol. Ill, p. 226). Hence if (ihicl., p. 223) we take soW as the radiational 

 emissivity of rock and atmosphere of gases and watery vapor above it radiating 

 heat into the surrounding vacuous space (ether), we find 8000 X .005 or 40° C. as 

 the excess of the mean surface temperature above what it would be if no heat were 

 conducted from within outward. The present augmentation of temperature down- 

 ward may be taken as 1° C. per 27 meters as a rough average derived from observa- 

 tions in all parts of the earth where underground temperature has been observed. 

 (See British Association Reports from 1868 to 1895. The very valuable work of this 

 committee has been carried on for these twenty-seven years with great skill, perse- 

 verance, aud success by Professor Everett, and he promises a continuation of his 

 reports from time to time.) This with the same data for conductivity and radia- 

 tional emissivity as in the preceding calculation makes 40°/2700 or 0.0148° C. per 

 centimeter as the amount by which the average temperature of the earth's surface 

 is at present kept up by underground heat. 

 SM 97 23 



