42 



PRESIDENTIAL ADDRESS SECTION A. 



the earth. Oldham, in 1906, published the accompanying diagram 

 (Fig. 3) in explanation of the results he had collated up to that 

 date. He considers that there is clear evidence that the speed 

 of the transverse vibration attains a maximum at a depth of about 

 2,400 miles, and thereafter decreases somewhat sharply to a 

 steady value. This would argue the presence of an innermost 

 core somewhat less stiff than the parts immediately outside of 

 it, which would bend the waves along the paths shown, much as 

 a sphere of glass bends and condenses the waves of light. 

 Wiechert agrees with Oldham in the main, but believes himself 

 able to differentiate a considerable number of layers between 

 Oldham's core and the surface. He considers that the main 

 waves are confined to a crust of thickness of about 19 miles, and 

 that there are changes of stiffness at about 750 miles. 1.030 miles. 

 and 1,530 miles. 



Fig. -.i. Oldham's Eaitli-Wave Patlis. 



Longitudinal Vibrations. 



Transver^^e Vibrations. 



I may add that recent investigations by Barrel! on the 

 strength of the earth's crust to resist deformation, as evidenced 

 by the enormous deltas of the Nile and Congo, and the remark- 

 able experiments of Adams in Canada on the crushing stresses 

 of different kinds of rocks, both bear witness to the enormous 

 increase of the stiffness of rocks when supported laterally. 

 Adams found that granite which, if unsupported laterally, would 

 yield under a crushing stress of 23,000 lbs to the square inch, 

 gained immensely in power of resistance to such stresses if given 

 lateral support, till with a lateral support of 20,000 lbs to the 

 square inch it needed a crushing stress of some 80.000 lbs. per 



