16 REPORT 1875. 



levels and places were selected as were far Irom the workings and from all 

 circumstances which could cause a change of the temperature of the rock. 

 The temperature was observed on thermometers put in bore-holes of 2 feet 

 depth, which were bored in idle rock free of any particles of iron pyrites 

 and far from all lodes. Through the whole time of the experiment, in 

 summer and winter, the temperature of the rock on each level remained, 

 excepting only some very small variations, nearly without change. 



" From the Table it will be seen that the increment of heat by descending 

 in the mines is much smaller than in the mines of other localities. The 

 reason of it may be looked for in the quality of the rock, which, belonging 

 to the beds of the Lower Silurian formation, is very quartzose and free of 

 any particles of iron pyrites. 



" The temperature in the bore-hole remained, by the observations made 

 throughout the whole year 1830, without a change, as the bore-hole (which 

 was closed up with a piece of clay) kept always the equal temperature of the 

 rock. Even in the year 1854-55, when the observations in the higher levels 

 were repeated and the same bore-holes used, the temperature remained the 

 same. 



" The shaft-braces of the different shafts differ very little in height, as you 

 have seen from the sketch sent to you, and all of them are situated on lofty 

 hills. My observations of the increase of heat have aU been made near the 

 Adalbert shaft, on the different levels ; and tlie difference from the tempera- 

 ture on the same levels in the other mines can only be trifling." 



These observations appear to be thoroughly reliable, and to prove con- 

 clusively that the rate of increase in this locality is remarkably slow. Even 

 after applying a large conjectural correction for tlie convexity of the ground, 

 as connected with the fact above stated that all the shaft-braces " are situated 

 on lofty hills," the rate of increase will still remain slower than any that we 

 have hitherto discussed. From the description of tliis rock, considered in 

 connexion with the description given of the rocks iu the Mont-Cenis Tunnel 

 (1871 Report), it would appear that highly quartzose rock is characteiized 

 by a slow rate of increase — an index probably of high conductivity*. 



Further observations will be taken by Herr Grimm with two thermometers 

 which have been supplied to him by the Committee. One of them is a 

 maximum protected Negretti, the other a simple mercurial thermometer 

 with a large bulb. 



Several insti'umcnts of this latter kind have been constructed for the Com- 

 mittee during the past year, with a view to observations similar to those 

 above described by Herr Gi'imm. The objects aimed at in the construction 

 are, slowness of action, combined with facilitj'^ for reading with quickness 

 and certainty in a bad light. 



It was stated in last year's Eeport that M. E. Sadoine, Director-General of 

 the mines of the Societe Cocqueril at Seraing, near Liege, had consented to 

 have observations taken in the mines of that company. A Negretti maximum 

 thermometer was accordingly sent in September 1873, and at a later date 

 (March 1874) a non-registering unprotected thermometer. The following- 

 results, obtained with the maximum thermometer, have been communicated 

 by the chief engineer of the collieries. The observations were made in 

 December 1873. 



* Added September 1875. This inference as to the high conductivity of quartz, pub- 

 lished a year ago, is verified by the direct experiments of Professor Herschel (see Eeport 

 on Conductivity of Rocks in the present volume). Quartz was found to be the best 

 conductor of all the rocks experimented on. 



