INTERNAL CONDITION OF THE EARTH. 309 



temperature as we proceed inwards is uniform, or 

 nearly so, and that as we penetrate mile after 

 mile we should still find the temperature increasing 

 with equable rapidity. Thus, if we take metrical 

 measurement and the centigrade scale of ther- 

 momctry, we find an increase in temperature as 

 we go downward of i C. per 30 metres of descent. 

 This, if we suppose the rate of augmentation of 

 temperature uniform would give 1000 degrees at 

 30,000 metres, and 3333 degrees at a depth of 100 

 kilometres. One hundred kilometres is about 60 

 statute miles ; and thus at 60 miles below the 

 surface we should have a temperature of about 

 3000 degrees centigrade. This is a temperature 

 far higher than would be required to melt any of 

 the ordinary rock materials at ordinary pressure. 

 But what I want to ask now is, " What reason 

 have we for supposing that, as we proceed farther 

 and farther in, the temperature does go on in- 

 creasing at this rate ? " A very fallacious answer 

 has been given : " No falling off in the rate of 

 " increase of underground temperature has been 

 " found at the greatest depth to which observa- 



