THE AGE OF THE EARTH. 31 



rafter the freezing of the granitic interstitial mother hquor at 

 the earth's surface in any locahty, tlie average temperature 

 at the surface might be warmer, by 60° or 80° Cent., than if 

 the whole interior had the same average temperature as the 

 surface. To fix our ideas, let us suppose, 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 100 years it would be 

 8° warmer, and at the end of 10,000 years it would be '8 of 

 a degree warmer, and at the end of 25 million years it would 

 be '016 of a degree warmer, than if there were no under- 

 ground heat. 



§ 39. When the surface of the earth was still white-hot 

 liquid all round, at a temperatiu'e fallen to about 1200° Cent., 

 there must have been hot gases and vapour of water above 

 it in all parts, and possibly vapours of some of the more 

 volatile of the present known terrestrial solids and liquids, 

 such as zinc, mercury, sulphur, phosphorus. The very 

 rapid cooling which followed instantly on the solidification 

 at the surface must have caused a rapid downpour of all the 

 vapours other than water, if any there were ; and a little 

 later, rain of Avater out of the air, as the temperature of the 

 surface cooled from red heat to such moderate temperatures 

 as 40° and 20° and 10° Cent., above the average due to sun. 

 heat and radiation into the ether around the earth. What 

 that primitive atmosphere was, and how much rain of water 

 fell on the earth in the course of the first century after 

 consolidation, we cannot tell for certain; but Natural History 

 and Natural Pliilosophy give us some foundation for endea- 

 vours to disco vei- much towards answering the great questions, 



thermal unit centigrade per sq. cm. per sec. (Kelvin, Moth, and Ph/s. 

 Papers, vol. Ill, p. 226). Hence if {ibid. p. 223) we take ^^Vo ''^^ t'le 

 radiational emissivity of rock and atmosphere of gases and watery vapour 

 above it radiating heat into the surrounding vacuous space (ether), we 

 iind 800U X "OOS, or 40 degrees Cent, as the excess of the mean surface 

 temperature above Avhat it would be if no heat were conducted from 

 within outwards. The present augmentation of tem))erature downwards 

 may be taken as 1 degree Cent, per 27 metres as a rough average derived 

 from observations in all parts of the earth where underground temperature 

 has been observed. (See British Association Eeports, from 1868 to 189o. 

 The very valuable work of this Committee has been carried on for these 

 twenty-seven years with great skill, perseverance, and success, by 

 Professor Everett, and he pi'omises a continuation of his reports from 

 time to time.) This with the same data for conductivity and radiational 

 emissivity as in the preceding calculation makes 40°/2700 or 0-0148° Cent, 

 per centimetre as the amount by which the average temperature of tho 

 earth's surface is at present kept up by underground heat. 



