Prof. W. Thomson on the Secular Cooling of the Earth. 5 



temperature instituted by Principal Forbes there, I find that, if 

 the supposed transit through a hotter region of space took place 

 between 1250 and 5000 years ago, the temperature of that sup- 

 posed region must have been from 25° to 50° F. above the pre- 

 sent mean temperature of the earth's surface, to account for the 

 present general rate of underground increase of temperature, 

 taken as 1° F. in 50 feet downwards. Human history negatives 

 this supposition. Again, geologists and astronomers will, I 

 presume, admit that the earth cannot, 20,000 years ago, have 

 been in a region of space 100° F. warmer than its present sur- 

 face. But if the transition from a hot region to a cold region, 

 supposed by Poisson, took place more than 20,000 years ago, 

 the excess of temperature must have been more than 100° F., 

 and must therefore have destroyed animal and vegetable life. 

 Hence, the further back and the hotter we can suppose Poisson's 

 hot region, the better for the geologists who require the longest 

 periods ; but the best for their view is Leibnitz's theory, which 

 simply supposes the earth to have been at one time an incan- 

 descent liquid, without explaining how it got into that state. If 

 we suppose the temperature of melting rock to be about 10,000° 

 F. (an extremely high estimate), the consolidation may have 

 taken place 200,000,000 years ago. Or, if we suppose the tem- 

 perature of melting rock to be 7000° F. (which is more nearly 

 what it is generally assumed to be), we may suppose the conso- 

 lidation to have taken place 98,000,000 years ago. 



11. These estimates are founded on the Fourier solution de- 

 monstrated below. The greatest variation we have to make on 

 them, to take into account the differences in the ratios of con- 

 ductivities to specific heats of the three Edinburgh rocks, is to 

 reduce them to nearly half, or to increase them by rather more 

 than half. A reduction of the Greenwich underground observa- 

 tions, recently communicated to me by Professor Everett of 

 Windsor, Nova Scotia, gives for the Greenwich rocks a quality 

 intermediate between those of the Edinburgh rocks. But we are 

 very ignorant as to the effects of high temperatures in altering 

 the conductivities and specific heats of rocks, and as to their 

 latent heat of fusion. We must therefore allow very wide limits 

 in such an estimate as I have attempted to make ; but I think 

 we may with much probability say that the consolidation cannot 

 have taken place less than 20,000,000 years ago, or we should 

 have more underground heat than we actually have; nor more 

 than 400,000,000 years ago, or we should not have so much as 

 the least observed underground increment of temperature. That 

 is to say, I conclude that Leibnitz's epoch of " emergence " of 

 the " consistentior status " was probably between those dates. 



12. The mathematical theory on which these estimates are 



