On the Agency of Water in Volcanic Eruptions. 



123 



at which the expansive force of vapour will exceed that of the 

 hydrostatic pressure (especially modified as it is by friction) and the 

 descent of the surface-waters is in all probability stayed.* 



Adopting the conclusion arrived in the paper I recently laid before 

 the Society — viz., of a mean thermometric gradient of 48 feet of 

 depth per 1° F., the following will be the relation between depth and 

 temperature down to a depth of 150,000 feet or 28^- miles, starting 

 with an annual mean surface temperature of 50° Fahr. 



Table of Temperatures at Depths taking the Thermometric Gradient 

 at 48 feet per 1° Fahr. 



Depth. Temperature. 



Surface = 50° Fahr. 



500 feet = 60J 



1,000 „ = 71 



1,500 „ = 81 



2,000 „ = 92 



3,000 „ = 112 



4,000 „ ^ 133 



5,000 „ = 154 



7,776 „ = 212 Boiling point. 



10,000 „ = 258 



15,000 „ = 362 



20,000 „ = 467 



30,000 „ = 625 



34,704 „ = 773 Critical point. 



40,000 „ = 883 



50,000 „ = 1092 



100,000 „ = 2133 



150,000 „ = 3175 



If there are experimental errors, as I consider not improbable, 

 such as would reduce the gradient to 45 feet per degree, the tem- 

 perature at the depth of 150,000 feet would be 3383°, unless the 

 increase of heat modifies the conductivity of the rocks at depths. Or 

 if in centres of crystalline rocks and slates, as in Cornwall, and with 

 a gradient of 40 feet, a temperature of 3000° might be reached at a 

 depth of 120,000 feet or about 23 miles. 



* M. Delesse, in his paper on the water in the interior of the Earth, considered 

 that notwithstanding that water tends to pass into vapour at the high temperature 

 of great depths, the pressure of the overlying strata and the resistance they offer 

 to its return being greater than its tension, would cause it to retain its liquid state. 

 But at a depth which he estimates at about 60,000 feet, and at a temperature of 

 about 1100° F., the overlying pressure (taken at the rock weight), and the elastic 

 force of the vapour of water would be in equilibrium. " Bull. Soc. Greol. de France," 

 2nd Ser., vol. xix (1861), p. 64. 



