THERMAL CONDUCTIVITIES OF ROCKS 275 



Iranian Co. the work was then abandoned. Measurements in the higher 

 parts of the hole would have been valueless as they had been cooled for 

 months by the circulation of drilling mud. Similar difficulties were met 

 with in some measurements made by the Anglo-Iranian at Portsdown. 



This experience shows how difficult it is to obtain satisfactory measure- 

 ments in rotary drilled holes. Even at the bottom of the hole measure- 

 ments cannot be taken till some days after drilling has stopped ; and, since 

 the hole is not lined, it is very likely to become blocked during this time 

 and can only be cleared by lowering a drilling tool for which it is necessary 

 to circulate mud which completely upsets the temperature equilibrium 

 again. Even if the hole should remain clear the measurements cause niuch 

 expense to the company drilling the hole, since the Mines Dept. insisted 

 that such holes should be plugged with cement at various depths and as 

 the drilling rig cannot be removed till this has been done, it must stand 

 idle during the temperature measurements. 



Measurements are therefore only possible if the hole is lined and the 

 lining left in for some time after drilling. One such case has occurred. 

 The Anglo-Iranian drilled a hole at Pevensey 842 ft. deep, cased it to 500 ft. 

 and left it for some weeks. Temperature measurements were made at 

 765 ft. and 772 ft. (the hole was blocked below this). The temperatures 

 found at these depths were 16-2° c. and 16-3° c. There was fluid in the 

 hole only below about 700 ft. and in any case it had not been left long enough 

 for satisfactory measurements to be made in the upper part. The gradient 

 has therefore to be deduced by a comparison of the temperatures at the 

 bottom with the surface temperature. The mean air temperature at 

 Pevensey is stated by the Meteorological Office to be 10-3° c.^ The mean 

 temperature of the ground a few feet down is normally o- 8° c. above that of 

 the air .2 We therefore deduce a gradient of 5 ■ 1° c. in 668 ft. or 25° c./km. 



In order that it should be possible to make temperature measurements 

 in any bores that became available two inverted maximum thermometers 

 and a winch for lowering them on piano wire have been purchased. As 

 the thermometers used at Kingsclere showed a great tendency to shake 

 down on raising them from the bottom of the hole, an investigation was 

 made of this source of error with the new thermometers. At Pevensey 

 the two thermometers were lowered opposite ways up so that shaking would 

 affect them in opposite directions and their positions in the container were 

 reversed for the second run. The results agreed to less than o-i° c. As 

 a further check a weight thermometer was constructed, but there has so far 

 been no opportunity to use it in a bore hole. Messrs. Negretti and Zambra 

 state that when maximum thermometers have been much used the constric- 

 tion becomes worn and they tend to shake down more easily. The new 

 thermometers are so difficult to shake down that it was necessary to construct 

 a whirling case to do so. 



4. Laboratory Measurements of Conductivity. — In order to measure the 

 conductivity of specimens of rock from bores an apparatus has been con- 

 structed by Mr. Benfield of the Cambridge Department of Geodesy and 

 Geophysics^ with the advice of Dr. Ezer Griffiths. This apparatus con- 

 sists of two brass bars one inch in diameter between the ends of which a 

 disc-like specimen is placed. The top end of the upper brass bar is heated 

 electrically and the bottom end of the lower is cooled by a stream of water. 



1 This is deduced from the mean air temperature at Eastbourne from 1888 to 

 1935. 



2 J. Koenigsberger and M. Muhlberg, Neues. Jahrb. f. Min., beilage bd. 31, 1 15, 

 1911. Everett, 2nd Rep. Roy. Comm. on Coal Supplies, 2, 292, 1904. 



» Mr. Benfield also made the temperature measurements in the Pevensey bore. 



