EARTHQUAKES AND VOLCANOES. 



supplied by so scanty a range of observation must be very limited. 

 Less direct sources of observation, though not less certain and 

 precise, have, however, been opened by the researches of geologists, 

 who have shown that the crust of the earth fractured by eruptions 

 produced by forces acting from the interior outwards has been 

 exposed to view, so that the condition of the external shell, to a 

 depth of about forty thousand feet, or the five-hundredth part of 

 the entire distance from the surface to the centre, can be ascer- 

 tained. 



5. From what has been explained in former numbers of this 

 series, in which the subject of terrestrial heat has been considered, 

 it will be seen' that extended and general thermometric observations 

 made in mines and other deep excavations, and on the temperature 

 of water rising in Artesian wells, prove that in descending to greater 

 and greater depths into the crust of the earth, there is a constant 

 and regular increase of temperature at the rate of about one 

 thermometric degree for every fifty feet of depth, or what is nearly 

 the same, an increase at the rate of 100 per mile. 



6. Now supposing this law of increase to continue without 

 interruption downwards, it would follow that at the depth of forty 

 miles, or the hundredth part of the distance from the surface to 

 the centre, a temperature of 4000 prevails. It is certain that no 

 part of the matter composing the crust of the earth could remain 

 solid at such a temperature, being higher than those at which the 

 most refractory bodies are fused. 



7. No great degree of precision in the numerical data on which 

 this calculation is based is necessary to establish the conclusion to 

 which it leads. "Whatever be the exact rate at which the tempera- 

 ture is augmented in descending, it is beyond all doubt that at a 

 depth of thirty or forty miles it must be such as to reduce to the 

 state of an incandescent liquid the most refractory bodies which 

 enter into the composition of the earth. 



This liquid fire must extend to the very centre of the globe, 

 and from the well understood properties of fluidity, it may be 

 considered certain that an uniform temperature is maintained 

 throughout the liquid mass thus enclosed within the solid sphe- 

 roidal shell. 



Now let us pause for a moment to consider the consequences to 

 which this leads, and to contemplate the spectacle which it offers 

 of the condition of our terrestrial dwelling. 



8. Let us take the extreme estimate of forty miles as the depth 

 below which the matter composing the earth is completely liqui- 

 fied. This depth is the one-hundredth part of the terrestrial radius. 



We are then to regard the earth as a spherical shell of solid 

 matter filled with liquid fire. The thickness of this shell being in 

 US 



