57 G Mr. H. Jeffreys on the Compression 



in order to continue to fit the inner ones. At the same time 

 they showed that the inner parts are being stretched. The 

 boundary between the parts in process of extension and 

 compression respectively is known as the " level of no 

 strain/' Initially it was at the surface, and has moved 

 steadily downwards. The amount of compression geologically 

 available is not, however, the integral amount since the time 

 when the earth became solid, for if we consider a layer at a 

 definite depth, in the earlier part of the interval stretching 

 was taking place there, and compression has only taken 

 place since the layer of no strain was at that depth. In 

 calculating the available compression we must then take the 

 amount that has occurred since that date, and ignore the 

 stretching that took place at first. That this is true can be 

 seen from the analogy of a football which has been inflated 

 to such an extent that the outer case is just taut. If it is 

 partially deflated, the size of the bladder decreases, and we 

 have a condition resembling that of an outer layer too large 

 to fit the inner parts. The case then crumples up and 

 departs from the spherical form. On the other hand, if the 

 ball is inflated further, the case only expands a little and 

 remains spherical. Thus compression can produce folding, 

 but tension cannot. In the case of the earth, the effect of 

 long-continued extension is to produce permanent set, thus 

 relieving the strain without departure from symmetry, and 

 even a small subsequent compression can cause folding. 

 Sir Gr. H. Darwin showed that the effect would be sufficient 

 to shorten every great circle of the earth by a quantity of 

 the order of 100 miles. This estimate was probably exces- 

 sive for the form of the theory that he adopted, his initial 

 temperature and his assumed coefficient of expansion being 

 both too high. Nevertheless it appeared inadequate to 

 satisfy the requirements of geology. A later paper * by 

 Davison showed that when the increase of the coefficient of 

 expansion with temperature was taken into account, a 

 considerable improvement was obtained, but the compression 

 was still insufficient. 



Both of these estimates rested on the theory of the cooling 

 of the earth given by Lord Kelvin, who treated the earth as 

 a simple body, initially at a uniform temperature, and losing 

 its heat from the surface. The recent discovery of radio- 

 activity has, however, shown that so much heat is actually 

 being generated within the crust of the earth, that if the 

 same amount per unit volume were being produced through- 

 out the mass, the temperature gradient at the surface would 

 * Phil. Mag. xli. pp. 133-138 (1896). 



