Earth's Superficial Temperature. 19 



Again, if the cooling of the earth were to continue for an inde- 

 finite period of time, assuming the temperature of external space, 

 the sun, and the earth's atmosphere, to remain as at present, the 

 superficial temperature would approximate indefinitely near to a 

 certain limit. The difference between that limit and the earth's 

 present superficial temperature is the effect due to the remains of 

 the primitive heat. Now theory gives us a simple relation be- 

 tween the amount of this effect and the rate of increase above- 

 mentioned as we descend below the earth's surface.* Consequently, 

 knowing the one, we can immediately determine the other, and 

 thus, having ascertained the above rate of increase, we know the 

 amount of superficial temperature which is now due to the earth^s 

 primeval heat, assuming always that heat to be the cause of the 

 existing internal temperature of the globe. This amount is thus 

 proved not to exceed about the ^ th of a centesimal degree, so nearly 

 has the earth's superficial temperature approximated to that ulti- 

 mate limit beyond which it could never descend, supposing exter- 

 nal conditions to remain the same. It was calculated by Poisson 

 that, to reduce the superficial temperature by one half of the above 

 amount, or g 1 ^ th of a centesimal degree, it would require the enor- 

 mous period of one hundred thousand millions of years. It would, 

 doubtless, require us to go back into the past some such immense 

 period as this to arrive at the epoch when the superficial tempera- 

 ture should have exceeded its present amount by even one or two 

 degrees. At the same time the rate of increase of temperature in 

 descending beneath the surface would be much more rapid than at 

 present. If the superficial temperature amounted to 2° C. above 

 its ultimate limit, instead of being ^th of a degree, the rate at 

 which the temperature would increase in descending would be about 

 sixty times as great as at present, i. e., there would be an increase 

 of 1° C. for little more than one foot of depth. 



It must be recollected that this state of terrestrial temperature, 

 if due to the cause we are considering, could only have existed at 

 times which, even in a geological sense, must have been extremely 

 remote. The important peculiarity of this state of the earth would 

 seem to consist in the simultaneous existence of a superficial tem- 

 perature, and therefore of climatal conditions, very nearly the same 

 as at present, with an internal temperature at the depth of a few 

 hundred feet and upwards, immensely greater than at present. If 

 we suppose the process of sedimentary deposition to have been then 

 going on, we may understand how great an effect might be pro- 

 duced by this internal temperature in the metamorphism of the 

 earlier sedimentary beds. 



* If /denote the excess of the present superficial temperature above the final 

 limit to which the temperature would descend in an indefinite period of time, 

 and g the rate of increase of temperature mentioned in the text, we have 



— =6, whore h is nearly equal to unity. 



9 b2 



