46 Condition of the Moon's Surface. [January, 



crust sinks together to follow down after the shrinking 

 nucleus, the work expended in mutual crushing and dislo- 

 cation of its parts is transformed into heat, by which, at the 

 places where the crushing sufficiently takes place, the ma- 

 terial of the rock so crushed and of that adjacent to it are 

 heated even to fusion. The access of water to such points 

 determines volcanic eruption. Volcanic heat, therefore, is 

 one result of the secular cooling of a terraqueous globe 

 subject to gravitation, and needs no strange or gratuitous 

 hypothesis as to its origin." 



It is readily seen how important a bearing these conclu- 

 sions have upon the question of the moon's condition. So 

 far, at any rate, as the processes of contraction and the 

 consequent crushing and dislocation of the crust are con- 

 cerned, we see at once that in the case of the moon these 

 processes would take place far more actively than in the 

 earth's case. For the cooling of the moon must have taken 

 place far more rapidly, and the excess of the contraction of 

 the nucleus over that of the crust must have been consi- 

 derably greater. Moreover, although the force of gravity is 

 much less on the moon than on our earth, and therefore the 

 heat developed by any process of contraction correspondingly 

 reduced, yet, on the one hand, this would probably be more 

 than compensated by the greater activity of the lunar con- 

 traction (i.e., by the more rapid reduction of the moon's 

 heat), and on the other, the resistance to be encountered in 

 the formation of elevations by this process would be reduced 



portant series of experiments completed by him : — the one on the actual amount 

 of heat capable of being developed by the crushing of sixteen different species 

 of rocks, chosen so as to be representative of the whole series of known rock- 

 formations from oolites down to the hardest crystalline rocks ; the other, on 

 the coefficients of total contraction between fusion and solidification, at 

 existing mean temperature of the atmosphere, of basic and acid slags analo- 

 gous to melted rocks. The latter experiments were conducted on a very large 

 scale ; and the author points out the great errors of preceding experimenters, 

 Bischoff and others, as to these coefficients. By the aid of these experimental 

 data, he is enabled to test the theory produced when compared with such facts 

 as we possess as to the rate of present cooling of our globe, and the total 

 annual amount of volcanic action taking place upon its surface and within its 

 crust. He shows, by estimates which allow an ample margin to the best data 

 we possess as to the total annual vulcanicity, of all sorts, of our globe at 

 present, .that less than one-fourth of the total heat at present annually lost by 

 our globe is upon his theory sufficient to account for it ; so that the secular 

 cooling, small as it is, now going on, is a sufficient primum mobile, leaving the 

 greater portion still to be dissipated by radiation. The author then brings his 

 views into contact with known facts of vulcanology and seismology, showing 

 their accordance. He also shows that to the heat developed by partial tan- 

 gential thrusts within the solid crust are due those perturbations of hypogeal 

 increment of temperature which Hopkins has shown cannot be referred to a 

 cooling nucleus and to differences of conductivity alone."' 



