1907.] AND CONTRACTION OF THE EARTH. 193 



there is for the premises which have been so long adopted in geology 

 and the related sciences. And to make the results of this criterion 

 as convincing as possible, it becomes necessary to adopt mathemat- 

 ical methods, which inspire the greatest confidence in the final con- 

 clusions. 



So long as contraction was the only available explanation of 

 earthquakes and mountain formation, and no serious doubt had 

 arisen as to the reality of the assigned cause, it seemed legitimate 

 to adhere to the traditional theories of the physics of the globe. 

 But since a different theory has been developed in the paper on 

 ** The Cause of Earthquakes, Mountain Formation and kindred 

 phenomena connected with the Physics of the Earth," which explains 

 the observed phenomena by the effects of water vapor accumulating 

 beneath the earth's crust, and seems to be justified by the harmonious 

 concordance of many facts not otherwise intelligible, it appears 

 necessary to re-examine the foundations of the earlier reasoning. 



It is true that the traditional theories were rejected in the paper 

 on the cause of earthquakes, for reasons there assigned; but this 

 step is a highly important one, and as the evidence may not be 

 entirely convincing to others, it seems advisable to give a more con- 

 nected view of the physical grounds on which the step was taken. 

 The general grounds assigned in the former paper need not be re- 

 peated here ; it is sufficient to recall that they were based partly on 

 the principles of probability and partly on observed phenomena 

 which appeared to admit of but one interpretation. 



The paper on the cause of earthquakes grew out of certain phe- 

 nomena observed during the great earthquake of San Francisco, and 

 was a natural continuation of the previous work on the " Physical 

 Constitution of the Heavenly Bodies " (cf. A. N., 3992, 4053, 4104, 

 4152). As a result of these researches on the internal state of the 

 heavenly bodies, the wTiter reached the conclusion that no convec- 

 tion currents exist within the earth, nor have such currents been 

 possible at any time since the globe developed a density comparable 

 to that which it has at present. It was therefore inferred that the 

 cooling extends only through a very thin crust, and consequently 

 the earth is not now appreciably contracting ; nor has the contraction 

 been sensible at any time since the terrestrial spheroid attained 



