65 



accurately determined. From 231 to 685 yards, the tempera- 

 ture increases from 57J° 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 abscissse 

 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 21 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 

 igneous rocks are those which we believe must most resemble 

 the strata at great depths. Now, if the conductivity of the 



