2 The Hon. Sir Charles Parsons [Jan. 23, 



ill.- temperature was raised to that estimated to exist at the 

 depth below the surface of the earth corresponding to this 



- ire. 



Winn do heat was applied the holes in the granite showed no 

 alteration under a pressure equivalent to 30 miles deep, and in the 

 of limestone the specimen supported one-half of this pressure 

 without alteration. He then raised the temperature of the container 

 and specimen. WTien granite was heated to 550° C, a temperature 

 corresponding to 11 miles below the surface, it stood a pressure 

 equivalenl to 15 miles, and might have stood more but that the con- 

 tain, r became weakened by the heat. Limestone begins to decompose 

 al ;i temperature of 450° C, but even at this temperature it withstood 

 a pressure corresponding to 10 miles. 



Adams concludes that small cavities in granite will not close in 

 under the conditions of pressure and temperature at 11 miles below 

 the Burface, however long a time is allowed to lapse, and that the 

 cavil ies may persist to much greater depths ; but the softening of the 

 steel of the container precluded the carrying of his experiments to 

 still higher temperatures and pressures. 



As far as they go these experiments are reassuring as to the 

 permanence and safety of a pit shaft 12 miles deep sunk through 

 granite. But it would be more satisfactory to experiment on a larger 

 Bpecimen than one only J inch in diameter, as used by Adams, and 

 to electrically heat the specimen when submerged in graphite while 

 keeping the container cold, the temperature being indicated by a 

 thermo-couple in the specimen ; this could be carried out in a nickel 

 steel container like Fig. 5. 



In this connection P. AY. Bridgeman in 1911 submerged a sealed 

 glass tube containing a cavity under an external hydrostatic pressure 

 of 24,000 atmospheres (corresponding to a depth in the earth of 

 5G miles) for three hours, and the cavity showed no change in size 

 or form. It, however, appears that temperature will probably place 

 a limit to the depth that would be reached before closiDg in of the 

 ehafi commences to occur, for Judd, Milne and Mallet agree in the 

 view that the deepest origin of earthquakes is between 30 and 50 

 miles. This would seem to indicate that at greater depths than 

 '-'>" miles the temperature and pressure are such that changes of form 

 take place by plastic deformation, and not by sudden slips or the 

 formation of faults, which are the chief cause of earthquakes. Again, 

 Oldham states thai beyond 20 miles deep seismic waves which are 

 transmitted by compression and distortional vibrations change in 

 character in this respect: thai though the compressional waves are 

 only slightly affected in velocity, on the other hand the distortional 

 waves are reduced to one-half their velocity. This would seem to 



imply thai the I lulus of elasticity in shear has, at 20 miles depth, 



fallen to one-half owing to the rise of temperature, and it seems 

 probable that the rock also is weakening in its resistance to shear : in 



