156 C. Lloyd Morgan — Geological Time. 



heat is always to flow from the hotter part of the body to the colder. 

 Therefore, as the earth's crust is warmer and warmer as we go 

 farther and farther down, there must be a steady flow of heat 

 onwards from the interior to the surface. The earth is therefore 

 even now losing heat at a certain perfectly measurable and cal- 

 culable rate." This rate has been calculated by Sir William 

 Thomson. Taking the rise of temperature, over the whole earth's 

 surface, " at an average of about one degree for one hundred feet of 

 descent," he concludes " that about ten millions of years ago the 

 surface of the earth had just consolidated, or was just about to con- 

 solidate." " Thus we can say at once to geologists," says Professor 

 Tait, "that granting this premiss, — that physical laws have remained 

 as they are now, and that we know of all the physical laws which 

 have been operating since that time, — we cannot give more scope for 

 their speculations than about ten or (say at most) fifteen millions of 

 years. 



It is hardly necessary to point out how uncertain are the data on 

 which these calculations are based, and with uncertain data the most 

 rigorous and accurate mathematics can do little. As Professor 

 Huxley has remarked, if we throw peascods into our mathematical 

 mill, we cannot expect to extract wheaten flour. In the first place, 

 the laws of the conduction of heat through intensely heated bodies, 

 which are not homogeneous, are by no means well understood, and 

 the earth's crust is, probably, not only heterogeneous in composition, 

 but partially solid and partially liquid. In the second place, the 

 rate of increase of temperature taken 1° for every one hundred 

 feet descent into the earth, though, perhaps, a fair average of known 

 observations, is but a rough generalization. Could we know what 

 is the rate of increase of temperature for every one hundred feet of 

 descent over the vast oceanic areas of our globe, we might have to 

 modify our generalization in a very important degree. I am not 

 aware whether Sir William has, in his estimate, taken into con- 

 sideration the amount of heat — in all probability great — which has 

 been generated in the earth by contraction of her sphere and the 

 consequent lateral pressure, or the amount which, as suggested by 

 Professor Huxley, may have been produced by chemical combination. 



Geologists may, therefore, well hesitate before they accept this 

 limitation to ten or fifteen millions of years. Sir William Thomson 

 himself, however, takes — or until a recent date took — a liberal view 

 of the matter, and granted to geologists about one hundred millions 

 of years. 



Sir William Thomson's second argument depends upon tidal 

 retardation. The fact that the tide waves act as a check upon the 

 rotation of the earth on her axis, is now well known. The earth, 

 in fact, revolves within a friction-brake which has caused a slacken- 

 ing of her rate of rotation, even within historical times ; for partly 

 from this cause the moon " seems to have been moving quicker as 

 time has gone on since the eclipses of the fifth and eighth centuries 

 before our era." Now this action, if continued long enough, will 

 at length cause the tide waves to remain fixed in their position on 



