1868. | the Temperature of the Earth’s Crust. 15 
feet, the boiling point of water would not be reached under 12,720 
feet ; and while on the one hand it is doubtful whether metamor- 
phism would take place in ordinary strata at this temperature, it is 
seldom we meet with rocks which we are certain had originally been 
buried at much greater depths. 
Whether this increase of temperature is continuous for any 
considerable distance in reference to the semi-diameter of the earth, 
or whether it increases or diminishes according to definite laws, are 
questions which are probably beyond solution by actual experiment, 
for, in the words of Humboldt, the question of the internal central 
heat, as a mathematical problem, “ yields rather negative than positive 
results.” The experimental evidences, however, as far as they come 
within the range of investigation, all point to one conclusion. They 
are also of several kinds, derived from observations of the tempera- 
ture of the water springing from different depths through artesian 
borings—those obtained from testing the temperature of the water 
issuing from coal-seams and fissures in mines, and those obtained 
from observations made during the sinking of mining shafts both 
through wet and dry strata. It is on the experimental evidences we 
propose here to dwell, and taking some examples from authorities 
within our reach, to present the reader with a succinct account of 
what has already been achieved, and afterwards to offer some sug- 
gestions as to the best manner, in our opinion, for pursuing further 
investigations. 
One of the most remarkable and carefully-observed cases of 
artesian borings is that of the Puits de Grenelle, near Paris. The 
sinking of this bore-hole was watched by Arago till 1840, down to a 
depth of 1,657 feet, when the borer had left the Chalk formation, 
and was beginning to penetrate the Gault. The series of observ- 
ations were completed by Walferdin in 1847. ‘The surface of the 
basin of the well at Grenelle lies at an elevation of 119 feet above 
the sea, and the borings extend to a depth of 1794-6 feet from the 
surface. The water which rises from the Lower Greensand 
formation is of a temperature of 81:95° Fahr., and the increase is 
at the rate of 1° Fahr. for every 59 feet.* 
The next boring we shall describe is that of Neu Saltzwerk, in 
Westphalia, and situated 231 feet above the level of the sea at 
Amsterdam. It penetrated to an absolute depth of 2,281 feet from 
the surface. The salt-spring lies, therefore, at a depth of 2,052 
feet below the level of the sea, a relative depth which is, perhaps, 
the greatest that has yet been reached. The temperature of the 
brine is 91:04° Fahr., and as the mean annual temperature of the 
air at these works is about 49°3°, we may assume there is an increase 
of 1° Fahr. for every 54°68 feet.| This boring is 487 feet deeper 
* «Cosmos.’ Trans of Otté and Dallas, v. 1. v., p. 35-6. + Ibid., p. 36-7. 
