206 T. Mellard Reade — A Cooling and Shrinking Globe, 



in whicli lie arrives at practically the same results as myself. He 

 treats the problem mathematically, and locates the level-of-no-strain 

 five miles below the surface after 174 million years of cooling. 

 Attached to the paper is an Appendix by Prof. George Darwin, in 

 which he calculates that after 100 million years of cooling the level- 

 of-no-strain would be two miles deep. The Eev. Osmond Fisher 

 has devoted a considerable amount of attention to the subject, and 

 arrives at even a less result. I also showed that, whether we look 

 at the earth's nucleus as solid or liquid, the level-of-no-strain would 

 still be located in the hard shell or within the crust. Mr. Fisher 

 has mathematicall}'^ investigated the problem on the assumption that 

 there is a liquid zone between a hard outward crust and a solid 

 nucleus, and places the level-of-no-strain on this hypothesis of the 

 condition of the earth's interior at four miles deep. Some people 

 find a difficulty in taking in these conceptions. Perhaps it will 

 help them to think of a ball of plastic clay drying. If it be dried 

 slowly it parts with its moisture so equably that it becomes hard and 

 solid without cracking. If, on the contrary, the drying is forced, 

 it becomes fissured all over, because the surface portions contract 

 more rapidly than the interior portions. This illustrates the con- 

 tracting shell of the earth ; but the contracting shell of the earth 

 does not necessarily fissure, because of the weight of strata above, 

 which keeps it solid by what I have called '• compressive extension." 

 The clay ball, however, gives no illustration of the shell of the 

 earth, which is in compression because its matter does not gravitate 

 towards its own centre but towards that of the earth. 



Irregular condition of the Earth's Cooling. 



The earth is, however, not an inert homogeneous ball, cooling 

 equably into space, so it will be necessary to trace out the various 

 modifying influences acting upon its surface. Chief of these is 

 denudation and sedimentation. We may assume that in the planet's 

 early history, volcanic action brought certain portions of the earth, 

 either by upheaval or accumulation of ashes and lava, above the 

 surface of the water as land. Then the land areas came under the 

 joint influences of marine action and what is known as " subaerial 

 denudation." The result was that large areas of the crust became 

 covered with thick layers of sediment, lava flows, and ashes. What 

 would be the effect of this padding upon the outflow of heat? 



In 1834 Babbage, reasoning upon the causes which have produced 

 the oscillations of level of the Temple of Jupiter Serapis, in the 

 Bay of Bale, pointed out that the addition of sediment to any part 

 of the earth's crust must raise the temperature of the portion of the 

 crust it covers.^ 



1 These consequences are worked out in a very ingenious manner, considering the 

 time at which it was written, by Babbage in the 9th Bridgwater Treatise, in the 

 Appendix Note F, pp. 182-201. Sir John Herschel's views, having a somewhat 

 similar bearing, with the addition of what is now called Isostacy, are also given in 

 a letter to Lyell, dated 1836, and published in Note 1, pp. 202-217. 



I had not read these remarkably luminous sketches of a theory of elevation and 

 subsidences until long after the publication of my " Origin of Mountain Ranges." 

 The ideas are far in advance of the times, but are incomplete as not accounting for 

 lateral pressure. Columnar expansion is the only thing considered. 



