ON THE CONDUCTIVE POWEES OF YAEIOUS SUBSTANCES. 
831 
19. We are now prepared to compare our theoretical results with those obtained by 
observation respecting the terrestrial temperature at various depths beneath the earth’s 
surface, as aheady described. Those observations, as before stated, extend to the greatest 
depths at which satisfactory observations on terrestrial temperature (so far as I am 
acquainted with them) have been made. We see that, according to our theoretical 
results, the rate of increase of temperature in ditferent strata, either in the same or in 
different localities, ought to vary inversely as the conductive powers of the strata; 
whereas the observed rates of increase, in the instances above cited, exhibit a striking 
uniformity, although the observations are made in masses in which the conductive 
powers are unquestionably very different. I would direct attention especially to two of 
these cases, the Puits de Grenelle at Paris, and the coal-shaft at Dnckenfield. In the 
former a great depth of chalk was penetrated, and in the latter a like depth of strata, 
which for the most part are hard arenaceous rocks. The chalk of the northern region 
of France and that of this country are extremely similar in composition and structure ; 
we cannot therefore be much m error, 1 conceive, if we estimate its conductive power at 
from ’25 to *27 (art. 10). Nor should I estimate the conductive powers of the super- 
incumbent beds (of comparatively small thickness) at much higher values. At Ducken- 
field the mass which has been penetrated is more distinctly stratified, and consists of 
beds of a much greater variety than is found in a mass of chalk. They have, however, 
for the most part, the common character of being very siliceous ; and many of the thick- 
est beds, the aggi’egate of which composes a large portion of the mass through which 
the shaft passes, are almost entirely siliceous, and extremely hard and compact. I 
obtained the conductive powers of a considerable number of these rocks. They varied 
between ’51 and '726. 1 estimate their mean value at about ‘62, and the conductive 
power of the aggregate mass of all the strata at not less than -5. The number of dis- 
continuities between successive strata is not sufficient to diminish materially the con- 
ductivity of the whole ; but whatever effect may be produced by them, it is probably 
more than counterbalanced by the effect of moisture in those strata which are less com- 
pact and of smaller conductive power 
According to these estimates, then, the conductive power of the mass through which 
the Puits de Grenelle has penetrated, can scarcely exceed one half of that of the aggre- 
gate of the beds at Dnckenfield. The rate of increase of temperature in the former 
case ought, therefore, according to our theory, to be nearly twice as great as in the latter, 
whereas it is 60 feet for 1° Fahe. at Paris, and only 65 feet at Dnckenfield, instead of 
110 or 120 feet, as the theory would have led us to suppose. 
The instance above mentioned of the Artesian well at Mondorff (art. 12) presents a 
similar discrepancy between theory and observation, for there can be no doubt but that 
the mass of rocks through which that well passes, as above stated, is of considerably 
higher conductivity than the Chalk, though observation there gives a somewhat more 
* The shaft at Dnckenfield is likely, I believe, to attain its ultimate depth of upwards of 2000 feet in a 
short time. I hope to obtain from its lower portion much more complete evidence than I yet possess. 
5 Q 2 
