1885.] 



Conductivity of Mocks, Sfc. 



163 



temperature of the locality not being known. They also differed 

 amongst themselves from taking different surface temperatures, and 

 starting from different datum levels. To these he endeavour's to 

 assign uniform and corrected values. 



The essential differences in the results in the several tables depend, 

 however, upon dissimilar geological conditions, which unequally affect 

 the conductivity of the strata, and disturbing causes of different 

 orders. In the mines the latter are — 



1. The currents established by ventilation and convection. 



2. The circulation of underground waters. 



3. Chemical reactions. 



4. The working operations. 

 And in artesian wells — 



1. The pressure of the water on the thermometers. 



2. Convection currents in the column of water. 



In the later experiments pressure has been thoroughly guarded 

 against, but against the subtle influence of the other causes, though 

 long known, it is more difficult to guard. 



Goal Mines. — The author then proceeds seriatim with each subject, 

 commencing with coal mines. In these he shows that ventilation and 

 convection currents have rendered many of the results unreliable, as 

 he shows to have been the case in the well-known instance of the 

 Dukin field coal pit. The circulation of air in coal pits varies from 

 5000 to 150,000 cubic feet per minute, and tables are given to show- 

 how this" variously affects the temperature of the coal at different dis- 

 tances from the shaft though on the same level. As a rule, the deeper 

 the pit the more active is the ventilation, and therefore the more 

 rapid the cooling of the underground strata. In some pits the in- 

 draughted air has been known to form ice, not only in the shaft, but 

 icicles in the mine near the shaft. 



The cooling effects of ventilation are shown to begin immediately 

 that the faces of the rock and coal are exposed, and as the hotter 

 (and deeper) the pit, and the more gassy the coal, the more active is 

 the ventilation, so these surfaces rapidly undergo a cooling until 

 an equilibrium is established between the normal underground 

 temperature and the temperature of the air in the gallery. Judg- 

 ing by the effects of the diurnal variations on the surface of the 

 ground, it is clear that when there is a difference of 10° to 12° or 

 more between the air in the gallery and the normal temperature of 

 the rock, an exposure even of a few days must tell on the surfaces 

 of both coal and rock to the depth of the 3 to 4 feet — the usual depth 

 of the holes in which the thermometers are placed. The designation 

 of " fresh open faces " is no security, as that may mean a day or a 

 week, or more. The author considers also that so far from the length 

 and permanence of the experiment affording security, he is satisfied 



