Heat of Granite and Basalt at High Temperatures. 79 



disks separated from the rock disks by thin sheets of asbestos, 

 cemented to the copper as shown. The whole was placed in 

 the centre of a round metal vessel, 40 cm. in diameter by 



Fig. 9. 



1 





B 







D 





1 





C 







E 





1 -. 





G 



12 cm. deep, filled with quartz sand. In these circum- 

 stances the flow of heat through the rocks would be almost 

 uniform and perpendicular to the plane faces of the disks 

 ■especially in the central parts, so the conductivities could be 

 found if the temperatures of the rock-faces were known and 

 the heat flux per square centimetre. 



The temperatures were measured by means of thermo- 

 couples composed of the same wires as before used. One 

 couple measured the sum of the two temperature differences 

 across the two granite disks and another the sum for the two 

 basalt disks, so that even if the heat flux upward were not 

 quite equal to the flux downward, the conductivities could be 

 found since the sum of the two fluxes was known. As the 

 couple wires were only 0*1 mm. in diameter and the junctions 

 were flattened to the same thickness, the couples were placed 

 between the rock-faces, which were ground plane, without any 

 grooves, and all the disks and the heater cemented together, 

 the junctions being thus kept tight against their faces. It 

 was hoped that in this way the well-known difficulty of 

 ascertaining the temperature of a face across which heat is 

 flowing would be overcome, and that the error caused by the 

 layer of cement in which the couples were situated would be 

 negligible, as this layer was only about 0*1 mm. thick. 



The heating element was made of nichrome ribbon, 

 3*2 mm. wide by 0'55 mm. thick, wound zigzag as shown in 

 the lower part of the figure. The successive parallel strips 

 of the heater were separated by asbestos card, about 1 mm. 

 thick, the whole being cemented together with caamentium. 

 Sheets of asbestos millboard 1*5 mm. thick were cemented 

 over the two faces of the disk : the surfaces were thinly 



