Clay pole on the Interior of the Earth. 385 
whole globe had so fai" cooled down that its original fluid- 
ity was lost and it became viscid or slaggy. The sinking 
of the cooled crust then ceased because convection-currents 
could no longer travel between the centre and the circumfer- 
ence. Free motion between one part of the sphere and another 
was henceforth impossible and all fui'ther cooling must take 
place by the slower process of radiation from the surface and 
conduction from the interior. Even by this means some further 
refrigeration must have occurred in the long interval that has 
since elapsed so that the centre itself is now rather lower in 
temperature than it was when solidification took place. 
On this view then the interior of the earth consists of a mass 
of matter intensely hot but less so than some writers have as- 
serted. As already mentioned there is abundant reason to 
believe that it is solid. But it is under enormous pressure and 
this pressure has the effect of raising its temperature of solidifi- 
cation or in other words of its freezing-point. It is consequently 
solid at a temperature at which it could not remain solid were 
the pressure removed. That is to say, a portion of the central 
mass if suddenly transferred to the surface would instantly 
become liquid merely from relief of pressure. 
On this view the internal temperature must increase from the 
surface downward to the very centre. But the inference that 
this increase continues at the same rate as that which we find 
near the surface would be very hasty. As was remarked at the 
outset, the greatest depth yet reached by an auger is infinitesi- 
mal compared with the 4000 miles that separate us from the 
centre, and to assume therefore that the law of increase which it 
reveals will be found to hold good at great depths would be 
not only illogical but, in view of the considerations above stated 
regarding the condition of the central mass, it would be exceed- 
ingly improbable. It is more in consonance with them to be- 
lieve that the rate of increase of temperature downward will be 
found to steadily diminish until it becomes insignificant, and for 
practical purposes from that point down to the centre the tem- 
perature may be considered constant. 
The interior of the earth on this view then is a sphere of 
matter whose nature is unknown but which is kept solid solely 
by the pressure of the mass above it. It is therefore always on 
