1907.] AND CONTRACTION OF THE EARTH. 217 



In the case of the earth there is excellent reason to conclude that the 

 surface temperature never much exceeded 2000° Fahr. 



The experiments of Professors Riicker and Roberts-Austen at the 

 Royal College of Science, London, on basalt or dolerite of Rowley 

 Regis, undertaken at the suggestion of the Rev. O. Fisher, showed 

 that this rock was completely fused at 1688° Fahr. Basalt is a typi- 

 cal rock of the earth's crust, prevalent in nearly all volcanic districts ; 

 and it seems probable that a temperature of 2000° Fahr. would there- 

 fore not only fuse all the principal rocks of the earth's surface but 

 also reduce many of them to a state of vapor. 



The fact that the other planets of our solar system are not at 

 present self-luminous, though the larger masses are known to have 

 high internal temperature, tells against the theory of a very high sur- 

 face temperature also in the case of the earth. For although we 

 view' the other planets at only one stage of their existence, and there- 

 fore cannot fully judge of their conditions at other cosmical epochs, 

 yet the absence of self -luminosity when so much heat is known to be 

 stored up within these planets can only indicate that a great lowering 

 of temperature always takes place near the surface, as is also true in 

 a sphere of monatomic gas. Thus it is not probable that even at the 

 maximum the surface temperature of such masses would be very 

 high. There are clear indications that Lord Kelvin's estimate of 

 7000° Fahr. is much too great ; and in all probability we shall not be 

 far wrong in using 2000° as the most acceptable value in all calcula- 

 tions on the secular cooling of the terrestrial globe. The temperature 

 will increase with the depth, but for a shallow layer we may take it 

 to be uniform ; the temperature of 2000° then would not correspond 

 exactly to the surface, but rather to the average of a thin sheet form- 

 ing the boundary of the molten mass. As the outer layer was no 

 doubt agitated from beneath, it would both lack in uniformity of 

 temperature, and also be constantly changing, so that a mean tem- 

 perature of 2000° Fahr. seems to be the closest approximation we can 

 make to the true conditions. 



Using F = 4000° in formula (20), we get /^ 33,208,850 years; 

 which is the age of the earth on this hypothesis. When V = 2500° 

 Fahr., we find t^ 12,972,200 years. And if F = 2000° Fahr., the 

 result is / = 8,302,210 years, a comparatively short duration. As the 



