65 
accurately determined. From 231 to 685 yards, the tempera- 
ture increases from 57 f° F. to 75i°. This is a mean 
increase of 1° in 76'8 feet, which does not widely differ from 
the results of other observers. Walferdin and Arago found 
an increase of 1° in 59 feet; at Rehme, in an Artesian well 
760 yards deep, the increase was l°in 54*7 feet; De La Rive 
and Marcet found an increase of 1° in 51 feet, at Geneva. 
Other experiments have given 1° in 71 feet. The observa- 
tions' are affected by the varying conductivity of the rocks, and 
by the percolation of water. The Author has exhibited upon 
a diagram, in which the ordinates are depths, and the abscissae 
temperatures, the results obtained, between the depths of 231 
and 717 yards. The strata of the mine are also shown in 
section. Additional to these, the Author gives a Table of 
similar results in another pit at the same colliery, taken 
between the depths of 167|- and 467 yards, and showing an 
increase of temperature of 1° in 106 feet of descent. 
Assuming as an hypothesis, that the law thus found for a 
depth of 790 yards, continues to operate at greater depths, we 
arrive at the conclusion that at 2-| miles from the surface, a 
temperature of 212° would be reached, and at forty miles a 
temperature of 3,000°, which we may suppose sufficient to 
melt the hardest rocks. The Author then discusses the effect 
of pressure and increased conductivity of the rocks in modifying 
this result. If the fusing point increased 1°*3 F. for every 
500 lbs. pressure, as is the case with wax, spermaceti, &c., the 
depth would be increased from 40 to 65 miles before the fluid nu- 
cleus would be reached; but as the same increase is not observed 
with tin and barytes, the influence of pressure on the thickness 
of the crust cannot yet be determined. Again, Mr. Hop- 
kins has shown that the conductivity of the dense igneous 
rocks is twice as great as that of the superficial sedimentary 
deposits of clay, sand, chalk, &c. And these close grained 
ioneous rocks are those which we believe must most resemble 
the strata at great depths. Now, if the conductivity of the 
