8 FIRESIDE SCIENCE. 



ently made in school-books and popular journals. 

 Some years ago experiments were made in a large 

 number of Prussian mines, and it was found that it 

 varied between 1 F. in 21.3 and. in 155.5 feet. 

 Other experiments have been made in mines in 

 this country, Mexico, and South America, which 

 prove that the extremes are still greater. Undoubt- 

 edly much of this variation is due to the differing 

 conducting power, or specific heat of the forma- 

 tions through which the excavations were made ; still 

 it is clear that there is no uniform rate of increase 

 of temperature throughout the earth's crust. If it 

 were true that the rise equals 1 F. for every fifty 

 feet of descent, at a depth of twenty-five miles 

 every known substance would be in a state of fusion. 

 How intense must be the central heat of our globe ! 

 for it is impossible to doubt that these fires exist, 

 notwithstanding the cogent arguments which have 

 been urged against the theory. It is this great 

 central ocean of fire which warms the water of our 

 deep wells, and it is often brought to the surface 

 heated to a point actually above that of boiling 

 water. The Icelandic Geysers afford water heated 

 to 248 F., which is 36 above the boiling point. 

 There are springs in California and also in South 

 America, which supply water heated above 200 F., 

 and a very large number scattered over the surface 

 of the earth, which show a temperature of 175 F. 



