INTERNAL CONDITION OF THE EARTH. 311 



does exist undiminished at considerable depths, 

 falls to the ground. 



The accompanying diagrams, drawn to scale 

 {Natural Philosophy > Thomson and Tait, Vol. I., 

 Appendix D) show the rate of augmentation, and 

 the augmentation of underground temperature as 

 we proceed downwards, on the assumption that the 

 true rate of augmentation of temperature, after the 

 superficial layer affected by summer heat and winter 

 cold has been passed, is i Fah. per 51 feet at the 

 beginning, and that the diminution in rate of aug- 

 mentation of temperature downwards through the 

 upper crust is 10 per cent, per 30 kilometres, or 

 100,000 feet. Fig. I shows the varying rate of 

 augmentation of temperature downwards to a 

 depth of 1 80 kilometres. Distances measured in 

 the direction OX are depths below the surface, the 

 unit length being 400,000 feet. Thus the position of 

 the point P' in the diagram represents the state as 

 to rate of augmentation of temperature at about 7 

 of 400,000 feet, or 57,000 feet, below the surface. 

 Ordinates measured outward on the diagram from 

 the line OX represent rate of augmentation of 



