THERMAL CONDUCTIVITIES OF ROCKS 



273 



in, three heaters and fifteen thermo-junctions were installed as soon as the 

 drilling was finished. As the casing was not excluding the water it was 

 withdrawn. 



The presence of water in the hole makes it impossible to make satisfactory 

 thermal conductivity measurements with buried heaters as the water content 

 of the clay around the hole has been completely altered. As specimens of 

 the clay had been taken every 10 ft. with precautions to prevent them being 

 affected by the water, laboratory measurements of conductivity can be made. 



When attempts were made to measure temperatures with the thermo- 

 junctions completely inconsistent results were obtained. This was traced 

 to the leads having become damp from water condensed in the tube. This 

 dampness caused the copper and constantan wires to act as a small battery. 



30 



20 



5 



a 



00 



HEATER 



TURNED 



ON 



-o 0—0- 



-0—0 o- 



J I I I I 



2l-4cm 



36-6 cm 



62-3 cm 



2 4 6 8 10 12 14 16 18 20 22 



DAYS 



Fig. I. — -Temperature of junctions above that of the bottom of hole. 



A loo-ohm copper resistance thermometer was therefore constructed in 

 a water-tight steel case 8 mm. in diameter. Its behaviour was entirely 

 satisfactory. 



Temperature measurements were made in a |-in. steel pipe at every 10 ft. 

 between the surface and the bottom of the hole (with the hole filled with 

 water) and after it had been filled in with clay. For the lower 30 ft. of the 

 hole the gradient was constant and equal to 33-8 ± 1-5° c./km. for the 

 filled hole and 33-2° c./km. for the unfilled hole. Between 30 and 70 ft. 

 from the surface the mean gradient was about the same as in the lower 

 part of the hole but the individual points deviated by up to 0-05° c. from 

 a straight line (see Fig. 2). Above 30 ft. the annual temperature wave 

 obscures the normal gradient. The measurements have been repeated 

 several times and the departures from a straight line are reproducible and 

 are certainly real. They are presumably due to the circulation of water in 

 one or more of the porous bands that are shown to exist by the entry of 



