6/8 



NATURE 



[February 19, 1920 



was raised to that estimated to exist at ttie depth 

 below the surface of the earth corresponding to this 

 pressure. 



When no heat was applied the holes in the granite 

 showed no alteration under a pressure equivalent to 

 thirty miles deep, and in the case of limestone the 

 specimen supported one-half of this pressure without 

 alteration. Adams then raised the temperature of the 

 container and specimen. When granite was heated 

 to 550° C, a temperature corresponding to eleven 

 miles below the surface, it stood a pressure equivalent 

 to fifteen miles, and might have stood more but that 

 the container became weakened by the heat. Lime- 

 stone begins to decompose at a temperature of 450° C, 

 but even at this temperature it withstood a pressure 

 corresponding to ten miles. 



Adams concludes that small cavities in granite will 

 not close in under the conditions of pressure and 

 temperature at eleven miles below the surface, how- 

 ever long a time is allowed to lapse, and that the 

 cavities may persist to much greater depths, but the 

 softening of the steel of the container precluded the 

 carr>ing of his experiments to still higher tempera- 

 tures and pressures. 



So far as they go, these experiments are reassuring 

 as to the permanence and safety of a pit shaft twelve 

 miles deep sunk through granite, but it would be 

 more satisfactory to experiment on a larger specimen 

 than one only \ in. in diameter as used by Adams, 

 and to heat the specimen electrically 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 that shown in Fig. 2. 



In this connection P. W. Bridgeman in 19:1 sub- 

 merged a sealed glass tube containing a cavity under 

 an external hydrostatic pressure of 24,000 atmospheres 

 (corresponding to a depth in the earth of fiftv-six 

 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 could be reached before the closing in of the shaft 

 commences to occur, for Judd, Milne, and Mallet 

 agree in the view that the deepest origin of earth- 

 quakes is between thirty and fifty miles. This would 

 seem to indicate that at greater depths than thirty 

 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 that beyond twenty miles deep seismic 

 waves which are transmitted by compression and dis- 

 tortional vibrations change in character in this respect : 

 that though the compressional waves are only slightlv 

 affected in velocity, on the other hand the distortionnl 

 waves are reduced to one-half their velocity. This 

 would seem to imply that the modulus of elasticitv 

 in shear has, at twentv miles depth, owing to the 

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

 probable that the rock also is weakening in its resist- 

 ance to shear ; in fact, that the rock is becoming more 

 plastic, and that cavities would probably close up at 

 twenty miles below the surface. 



The greatest depth to which n shaft has as 

 vet been sunk is onlv about li miles. The deepest 

 single-stage shaft on the Rand is that of the 

 Hercules East Rand Proprietary Mine. It is 

 iiflo ft. verticallv, and rectangular in section. The 

 deepest sb.nft in the world is thnt at Morro Velho, 

 Brazil. The bottom is 6400 ft. verticSllv below 

 the surface, and it has been sunk, and is worked, in 

 stages, two of which are about T200 ft. vertical. The 

 deepest shaft designed on the Rand is bv the Citv 

 Deep Co. It is ycoo ft. vertically, is circular of 20 ft. 



NO. 2625, VOL. 104] 



diameter, and is to be worked in two stages of 

 3500 ft. each. The most rapid sinking record was 

 made at the Crown Mines No. 15 Shaft, where 310 ft. 

 were sunk in a month ; the shaft is circular, and of 

 20 ft. in diameter. 



There are several interesting departures from 

 ordinary mining practice necessary. The haulage 

 is arranged in stages of about half a mile, prin- 

 cipally in order to economise the weight of rope 

 and also the power for winding. In countries 

 where the atmosphere is dry the sides of the shaft 

 are cooled by sprinkling water upon them, which by 

 evaporation cools the rock. It is, however, possible 

 to augment this effect by artificially drying and 

 cooling the air before passing it down the mine. 



When still greater depths of shaft are in contem- 

 plation further methods of cooling in addition to these 

 would probably be found necessary ; for instance, the 

 carrying of the heat upwards by means of brine cir- 

 culated in a closed ring formed of steel pipes with a 

 rising and descending column. Though the columns 

 might be carried the whole depth of twelve miles, the 

 hydraulic pressure at the bottom would be about 

 12 tons per square inch, and entail very costly 

 pipes of great strength to resist the pressure. .\ 

 cheaper plan would be to work in stages, each ring 

 covering a stage of from two to three miles of the 

 shaft, the heat being transferred from the top of one 

 brine ring to the bottom of the ring above by surface- 

 heat exchangers and refrigerating machinery to 

 neutralise the heat drop on transfer. These may be 

 called heat pumps, and would be driven electrically. 



As the depth of the shaft increases, the pressure of 

 the air upon the miners will be about doubled for 

 everv three miles, but what is more serious is the 

 increase in temperature of the air itself caused 

 bv the adiabatic compression due to gravity, by which 

 it will be raised about 100° F. For these reasons it 

 will be necessary to place airtight partitions across 

 the shaft at every mile or two, and to carry on the 

 ventilation through these by means of a pump to 

 deliver the foul air upwards and an expander to allcw 

 the fresh air to descend. These two machines would 

 be linked together, and the difference in their power 

 supplied by an electric motor. (This method has been 

 often used with water, and is equally applicable to 



sir-) 

 .At each partition heat exchangers and refrigerating 



machinerv similar to those used for the brine would 

 be placed. .Another and preferable plan would be to 

 place numerous heat exchangers between the ascend- 

 ing columns of air to transfer heat from one to the 

 other. The air would, in this case, not itself act as a 

 convevor of hejit to the surface, for which the brine 

 columns would be depended upon, but it would enable 

 airlocks every three miles to suffice. .\ further alterna- 

 tive and verv simple method would be to convey liquid 

 air from the surface, and allow it to escape at the 

 part of the shaft requiring cooling. It would ensure 

 gc^d ventilation. 



When sinking the deeper portions of the shaft, 

 sh'f^lds would orobablv be necessary to protect the 

 miners from the splintering of the rock which is 

 caused bv the intense compressive stress, which splits 

 off scnles from the surface, sometimes with consider- 

 able violence. 



In 100.1 the estimate of the time required to sink 

 twelve miles was eightv vears. and was based on the 

 reco'-d^ of that time. With improved machinerv and 

 methods the records have been so much lowered that 

 an estimate of thirtv vears seems now to be reasonable. 



Threlfnll traced the gradual evolution of the theory 

 of the effects of temperature and pressure on the allo- 

 tropic forms of various substances. He described his 



