io6 POPULAR SCIENCE MONTHLY 



observation to be now conducted out of the earth yearly is so great 

 that if this action had been going on with any apparent uniformity, 

 the history of life on the earth could not exceed a few thousand million 

 years." Another consideration leading to similar conclusions was 

 based on the shape and rigidity of the earth. With Sir William 

 Thomson, the age of the earth continued to be a question studied with 

 great predilection. His aim was not so much to determine the exact 

 age as to fix an upper age limit. As the years passed by, investiga- 

 tion supplied much of the knowledge which was at first wanting regard- 

 ing the thermal properties of rocks, and Sir William Thomson was able 

 greatly to reduce this upper limit. 



" The Physical Condition of the Earth " was the topic of Sir 

 William Thomson's presidential address in 1876, before Section A of 

 the British Association. He took the gradual increase of temperature 

 downwards to be on an average 1° C. for 30 meters of descent and 

 gave reasons for his belief that for great depths the rate of increase 

 does not diminish. He concludes that if at great depths the tempera- 

 ture does not exceed 4,000° C, then the geological age of the earth 

 does not exceed 90 million years. This argument involves some very 

 uncertain factors. Sir William Thomson has shown quite conclusively 

 that the earth's interior is solid, but at what temperature the substance 

 of the earth would begin to melt under the high internal pressures was 

 a matter of pure conjecture. 



About 1885 Carl Barus, of the United States Geological Survey, 

 made a series of very important experimental researches on the phys- 

 ical properties of rocks at high temperatures, for the purpose of sup- 

 plying trustworthy data for geological theory. 35 Mr. Clarence King, 

 in an article published in the American Journal of Science/ 5 used the 

 data on specific heats, thermal conductivities and temperatures of 

 fusion of rocks, which had been supplied by Barus, for a more accurate 

 determination of the age of the earth. King concludes from these 

 experimental data on diabase, " that we have no warrant for extending 

 the earth's age beyond 24,000,000 years." A computation made by 

 Lord Kelvin led to about the same figure. 37 These results were em- 

 bodied by him in his address of 1897 before the Victoria Institute. 



What was the attitude of geologists toward these researches? In 

 England, geologists did not pretend to be able to find any flaw in the 

 argument of Lord Kelvin, but they were in a position described in the 

 well-known couplet, 



" A man convinced against his will 

 Is of the same opinion still." 



35 Am. Jour, of Science, 3d S., Vol. 42, p. 498; Vol. 43, p. 56. 

 38 "On the Age of the Earth," 3d S., Vol. 45, 1893, pp. 1-20. See also 

 Smithsonian Report, 1897, p. 345. 



37 Smithsonian Report, 1897, p. 346. 



