ON THE RATE OF INCREASE OF UNDERG ROUND TEMPERATURE. 179 



scale projected, from which readings were taken from time to time nntil 

 the temperature became constant. The final reading had to be corrected 

 not only for rise of zero but also for the temperature of the quicksilver in 

 the thermometer tube which extends from the opening to the bottom of 

 the bore-hole. Another very notable correction was required for the 

 more or less oblique position of the thermometer; for the hydrostatic 

 pressure of the quicksilver presses out the glass bulb so far that without 

 change of temperature the long thermometer reads from o- 4 to l o- less 

 in the vertical than in the horizontal position. 



" After about from three to ten days, the reading of a thermometer 

 luted into a bore-hole ceased to alter. 



" Separate trials with thermometers of similar construction, but different 

 length, showed moreover that, after months, the temperature of the rock 

 at about a metre deep was still unchanged. This is obviously owing to 

 the small difference of temperature between the rock and the surrounding 

 air. 



"From the observations at No. 8 and No. 15, in Table III., it is seen 

 that the temperature at the bottom of the bore-hole was sometimes a little 

 lower and sometimes a little higher than nearer its mouth. 



" This mode of observing gave correct results, but was laborious and 

 costly, not only on account of the necessity of making special bore-holes 

 for the purpose, but because almost every experiment cost a thermometer. 

 The projecting end was often maliciously broken off, and on account of 

 the swelling of the wooden case it almost never happened that at the end 

 of an experiment a thermometer was drawn out again uninjured. 



" Hermann and Pfister remedied this latter evil by surrounding the 

 thermometer tube, from the bulb to the scale, with a glass case, and this 

 with a steel jacket. This arrangement, however, involves not only con- 

 duction through the steel, but also continual interchange of heat by 

 currents of air in the glass case, from the mouth to the bottom of the hole. 

 For these reasons the observations made with these thermometers could 

 not be employed without intricate corrections. 



" Later I tried a Thomson's maximum thermometer,* kindly placed at 

 my disposal by Professor Everett, which (after previous strong cooling) 

 was left for several days at the bottom of the bore-hole, closed air-tight. 

 The results agreed with those obtained by other methods ; but who can 

 guarantee that the higher temperature prevailing in a newly-bored hole 

 is always just so much depressed by the cold mass of the thermometer 

 and its copper case, that the rock temperature alone determines the final 

 indication of the maximum thermometer. 



" This consideration induced me to employ for rock-temperature 

 observations (and they also serve for air and water observations) the 

 above-mentioned short thermometers with insulated bulbs, the first of 

 which Professor Everett caused to be made by Negretti and Zambra 

 for this express purpose. These thermometers, enclosed in a metal 

 box provided with a handle, are thrust to the bottom of the bore-hole, which 

 is at least a metre deep. To the handle is fastened a strong cord reach- 

 ing to the mouth of the hole, by which it can be drawn out again at 

 the end of the trial. The bore-hole, from the thermometer to the 

 mouth, is stopped with greased rag or other similar material, as air- 

 tight as possible. After two or three days, the thermometers have 



* It was one of the protected Negretti maximum thermometers constructed for 

 the Committee. 



N 2 



