822 
IklE. W. HOPKINS’S EXPEEIZHENTAL EESEAPvCHES 
§ II. Cojnparison of Theoretical Deductions from the j/receding Experimental JResults. 
with Observations on Terrestrial Temperature at vanous depths. 
12. A considerable number of observations have been made (as is well known) to 
ascertain the temperature of the earth at considerable depths beneath its surface, and 
the law according to which that temperature increases in descending. The existence of 
increased heat at considerable depths has long been estabhshed beyond all doubt, and 
the law of increase in a considerable number of localities may be considered as approxi- 
mately and somewhat roughly determined to be — that the increase of temperature above 
that of the mean temperature at the surface m any proposed locality, is proportional to 
the depth beneath the surface. 
Now I shall not attempt to discuss the numerous observations by which this approxi- 
mate law has been established, but shall merely cite a few instances in which the 
observations may be fully relied on, and in which also the great depth to which they 
were extended leaves less liability to serious error than in many other cases. 
The Puits de Grenelle at Paris is an Artesian well, which extends to the depth of 546 
metres, through strata which must all have low conductive powers. It first peneti’ates 
through upwards of 41-5 metres of the older tertiaries; afterwards thi’ough the chalk to 
the depth of nearly 500 metres, and finally through argillaceous beds for the remaining 
depth. The whole process and its results are very fully described in the sixth volume 
of Aeago’s Works (p. 399). Commencing at the depth of the caves of the Observatory 
at Paris, where the temperature is constant, and equal to I1°‘T C., the increase of 
temperature for the whole depth was at the rate of 1° Fahr. for 60 feet. The following 
Table will also explain the extent of the deviation from uniformity in the progressive 
increase of temperature in descending (p. 388). It gives for difterent depths the increase 
of depth corresponding to an increase of 1° C. 
Depths. 
metres, metres. 
From 28 to 66 
From 66 to 173 
From 173 to 248 
From 248 to 298 
From 298 to 40(t 
From 400 to 505 
From 505 to 548 
Increase of Depth for 1° C. 
metres. 
31-1 
30-6 
20-8 
22-8 
62-5 
38-9 . 
33-0 
This Table exhibits a remarkable anomaly in the rate of increase in the central beds of 
the chalk, for which there appears to be no adequate reason in the natine of the beds 
themselves as described by Arago (p. 426). 
Arago also gives an account, sent to him by Humboldt, of an Artesian well at Neu- 
Saltzwerk in Westphalia. It had reached the depth of 644'5 metres, nearly 100 metres 
