32 PRESIDENTIAL ADDRESS — SECTION A. 



key to the mystery of those incessant rises and falls, those thrusts 

 and bucklings and occasional tearingfs of the apparently rigid 

 crust, of which the surface rocks are so plain a record. For 

 the evolutionist student of the biological sciences and of the 

 history of man, the question is wrapped up with such important 

 matters as the area and extent and connections of old land 

 surfaces, their possible climates as affected by changes in alti- 

 tude and other terrestrial and astronomical conditions, the per- 

 manence and physical state of the oceans, and the like. Finally, 

 the subject has presented tO' the mathematician problems at once 

 of the greatest interest and of the greatest difficulty, problems 

 that test in the 'highest degree his resources of imagination and 

 his clarity of thinking. 



I shall discuss the question chiefly from the side of geo- 

 physics, partly because it is on this side that my own ignorance 

 of the subject is perhaps less complete than on others, and partly 

 because the methods of geophysics, because of their quantitative 

 character and comparative simplicity, are those which have of 

 late years been the most fruitful in trustworthy results. I say 

 this with no want of respect for the large body of exact know- 

 ledge accumulated by the researches of geologists. But it appears 

 to me that to a considerable extent the work of the geophysicist 

 begins where that of the geologist ends, and that it lies with 

 the former__to discover, as far as may be, the exact processes by 

 which those incessant movements have been maintained to which 

 the geological record is so indubitable though perplexing a 

 witness. 



The methods that have been of greatest service in geo- 

 physical research are the following: — 



(i) Exact geodetic surveys, accompanied by astronomical 

 observations of latitude, longitude, and meridian. 

 Such surveys have not only given the size and shape 

 of the earth so far as these can be determined from 

 land observations, but have also in recent years thrown 

 a considerable amount of light on the probable density 

 and strength of the earth's crust and the layers lying 

 immediately thereunder. 



(2) Determinations of the total mass of the earth and of 



the densities of the chief rocks. 



(3) Exact determinations of the intensity of gravity at 



points kno'wn in geodetic height and position. 



(4) Determinations of temperature, heat conduction and 



radium content in surface rocks. 



(5) Laboratory determinations of the elastic constants of 



rocks, and of their greatest stress-bearing power at 

 the ordinary and higher temperatures. 



(6) Investigations into the small deformations of the earth 



that occur under the tidal forces of the moon and sun. 



(7) And perhaps the most fruitful of all, investigation by 



seismometers of the rates of propagation of earth- 



