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TEE POPULAR SCIENCE MONTELY.— SUPPLEMENT. 



sloping near the shore-line, and nearly level else- 

 where, a large proportion of the water might be 

 drawn off, and the ocean-surface still remain al- 

 most as great as before. We may assume as a 

 mean and sufficiently probable hypothesis that 

 the lunar oceans had a relative surface equal to 

 between one-half and one third of the present 

 relative surface of the terrestrial oceans. That is 

 to say, our oceans covering about 72 hundredths 

 of the entire surface of the earth, we may assume 

 that the lunar oceans covered between 36 and 

 24 hundredths of the entire surface of the moon. 

 It will be seen presently that some importance 

 attaches to this question of the probable surface 

 of the seas on the moon, a portion of the evidence 

 for the theory we are examining depending on 

 this relation. 



Let us next consider in what way the with- 

 drawal of the lunar oceans into the moon's inte- 

 rior probably took place. On this point, Frank- 

 land's presentation of the theory is undoubtedly 

 defective. In fact, it has been the weakness of 

 the theory in this respect, as presented in Eng- 

 land, which has in all probability prevented it 

 from receiving the attention here which it fairly 

 deserves. " The cooling of the moon's mass 

 must," said Frankland, " in accordance with all 

 analogy, have been attended with contraction, 

 which can scarcely be conceived as occurring 

 without the development of a cavernous structure 

 in the interior. Much of the cavernous structure 

 would doubtless communicate, by means of fis- 

 sures, with the surface, and thus there would be 

 provided an internal receptacle for the ocean, 

 from the depths of which even the burning sun 

 of the long lunar day would be totally unable to 

 dislodge more than traces of its vapor." And he 

 proceeds thus to analyze the amount of space 

 which would be rendered available for the re- 

 treat of the lunar oceans: "Assuming the solid 

 mass of the moon to contract on cooling at the 

 same rate as granite, its refrigeration through 

 only ISO of the Fahrenheit thermometer (the 

 difference between the boiling and the freez- 

 ing points) would create cellular space equal to 

 nearly 14,500,000 cubic miles, which would be 

 more than sufficient to ingulf the whole of the 

 lunar oceans, supposing them to bear the same 

 proportion to the mass of the moon as our own 

 oceans bear to that of the earth." 



But in reality no such cavernous structure 

 could possibly be developed in the interior of a 

 planet like the moon. Frankland's mistake, here, 

 is similar to that made by Brewster and others, 

 who have suggested that possibly the small mean 



density of the outer planets might be due to 

 the existence of great void spaces in the interi- 

 or of those bodies. So soon, however, as we 

 make the roughest calculation of the pressures 

 existing in the interior of even a small planet 

 like the moon, we perceive that there could be 

 no cavities. The most solid materials — steel, ad- 

 amant, platinum — become plastic under pressures 

 far less than those brought into action by the at- 

 tractive energy of a planet's mass upon all parts 

 of its interior, except those not far from the sur- 

 face. Be it noticed that it is not, as some seem 

 to suppose who have written on this subject, the 

 force of gravity at different depths which has to 

 be considered. That diminishes as the centre of 

 the planet is approached. What we have really 

 to consider is the pressure produced by the 

 weight of the superincumbent mass above any 

 given level, and this of course becomes greater 

 and greater as the depth below the surface in- 

 creases. If the rigidity of the solid substances 

 forming the solid crust of a planet were such 

 that any amount of pressure could be borne with- 

 out impairing it, then of course the various lay- 

 ers of the crust would form a series of arches, 

 stronger and stronger with approach to the cen- 

 tre, because of the increased compression, and 

 therefore the increased density of their substance. 

 There is no a priori reason, perhaps, why this 

 should not be so. Compression, for example, 

 might increase the rigidity or force-resisting 

 power of the materials of the earth's substance 

 in such sort that mines might be dug to any 

 depth, and horizontal tunneling carried out from 

 the lowest parts of any mine. But experiment 

 shows that the fact is otherwise. Under great 

 pressures the most solid substances become 

 plastic. Steel behaved like a liquid in Tres- 

 ca's experiments, affording the most conclusive 

 evidence that at a depth of ten or twelve miles 

 no steel walls, however massive, could defend a 

 cavernous space from the surrounding pressures, 

 which would simply crush in the steel until it 

 formed one solid mass without interstices — at 

 least with no interstices which could be seen if the 

 steel were afterward brought up from that depth 

 to be cut open and examined. It will be readily 

 understood that at the depth of ten or twelve 

 miles there can be no caverns into which the 

 water of the oceans could be bodily withdrawn. 

 Extending similar considerations to the moon, 

 we perceive that there can be no caverns in the 

 moon's interior at a greater depth than 60 or 70, 

 or at the utmost 100 miles. Now, 100 miles is 

 less than the- twentieth part of the moon's diam- 



