At La Chapelle, in an important well made for the water-supply 
of Paris, observations have been taken of the temperature at different 
depths, as shown in the subjoined table :— 
Depth in Temperature Depth in Temperature 
Metres. (Fahr.}. Metres. (Fahr.). 
aS Rey RR Gs O25 500... . .. | , ee 
Re ss kat ee ONO 600°. < . \ Se 
BOOr ae So eR eb GaSe 660°. -.. ce 
TC OF ice ia as ee ere eo 
In drawing attention to the temperature-observations at the Rose 
Bridge colliery—the deepest mine in Great Britain—Professor 
Everett points out that, assuming the surface temperature to be 
49° Fahr., in the first 558 yards the rate of rise of temperature is 
1° for 57°7 feet; in the next 257 yards it is 1° in 48°2 feet; in the 
portion between 605 and 671 yards—a distance of only 198 feet— 
it is 1° in 33 feet; in the lowest portion of 432 feet it is 1° in 54 
feet.2. When such irregularities occur in the same vertical shaft, it 
is not surprising that the average should vary so much in different 
laces. 
E There can be little doubt that one main cause of these variations 
is to be sought in the different thermal conductivities of the rocks 
of the earth’s crust. The first accurate measurements of the con- 
ducting powers of rocks were made by the late Professor J. D. Forbes 
at Edinburgh (1837-1845). He selected three sites for his ther- 
mometers, one in “trap-rock”’ (a porphyrite of Lower Carboniferous 
age), one in loose sand, and one in sandstone, each set of instruments 
being sunk to depths of 3, 6, 12 and 24 French feet from the surface. 
He found that the wave of summer heat reached the bulb of the 
deepest instrument (24 feet) on 4th January in the trap-rock, on 
25th December in the sand, and on 3rd November in the sandstone, 
the trap-rock being the worst conductor and the solid sandstone by 
far the best.’ 
The British Association has recently appointed a committee to 
investigate this subject in greater detail. Already some important 
determinations have been made by it regarding the absolute con- 
ductivities of various rocks. Asa rule, the lighter and more porous 
rocks offer the greatest resistance to the passage of heat, while the 
more dense and crystalline offer the least resistance. The resistance 
of opaque white quartz is expressed by the number 114, that of — 
basalt by 273, while that of cannel coal stands very much higher at 
1538, or more than thirteen times that of quartz.‘ 
It is evident also that, from the texture and structure of most 
rocks, the conductivity must vary in different directions through the 
' “Report of Committee on Underground Temperature,” Brit. Assoc. Rep., 1873, 
p. 254. ; , 
* “Report of Committee on Underground Temperature,” Brit. Assoc. Rep. for 
1870, p. 31. 
* Trans. Roy. Soc, Edin, xvi. p. 211. 
' Herschel and Lebour, Brit. Assoc. Iep., 1875, p. 59. 
* 
. 
48 GEOGNOSY. " [Book Il. 

