278 



THE POPULAR SCIENCE MONTHLY.— SUPPLEMENT. 



changed the moon's rate of turning on her axis. 

 One was the gradual contraction of the moon's 

 frame in cooling. This would have made her 

 turn more quickly on her axis. The other was 

 the continually gathering-in of meteoric matter 

 from without, which was a process taking place 

 probably far more rapidly then than now, seeing 

 that the meteoric systems now remaining are the 

 merest residue of a residue compared with those 

 existing hundreds of millions of years ago. This 

 process would tend to make the moon turn more 

 slowly upon her axis. However, the former pro- 

 cess would probably operate far more effective- 

 ly, and thus the moon would on the whole 

 have acquired a more rapid rate of rotation, and 

 the coincidence between rotation and revolution 

 existing when she first had separate existence 

 would have disappeared. But there was all the 

 time a force at work to check the tendency to 

 change in this respect. The earth was there, ex- 

 erting that very force which we have already de- 

 scribed in considering, another theory — a force 

 competent, we may infer, to check the tendency 

 to a slow increase in the moon's rate of rotation, 

 and to preserve that relation which existed when 

 the moon was first formed. We say that the 

 competence of this force may be inferred — mean- 

 ing that the observed coincidence between the 

 moon's rate of turning round upon her axis and 

 her rate of revolution around the earth shows 

 that the force was sufficient for that purpose. A 

 similar force exerted by the sun upon the earth 

 since she was first separately formed has not 

 proved competent, as we know, to make the 

 earth turn on her axis in the same time exactly 

 that she travels round the sun ; that is, in a 

 year. Nor have any of the planets been forced 

 to behave in this way. But we can readily un- 

 derstand that a great difference should exist 

 between the formation of a planet which, hav- 

 ing an enormously high temperature when first 

 formed, would have an enormous amount of con- 

 traction to undergo ; and the formation of a sub- 

 ordinate orb like the moon, which, though no 

 doubt intensely hot when first thrown off 1 by 

 the contracting earth, cannot have been nearly 

 so hot as a planet at the corresponding stage of 

 its existence. On the whole, there are (so it 

 seems to us) good reasons for believing that that 

 peculiar law of the moon's motion which causes 

 the same lunar hemisphere to be constantly 



1 "We here use the words " thrown off" as equivalent 

 to "left behind." The theory that the moon was thrown 

 off by the earth, or the earth by the sun, is altogether in- 

 consistent with mechanical possibilities. 



turned earthward had its origin during the birth 

 itself of our satellite. We may, indeed, find in 

 that peculiarity one of the strongest arguments 

 in favor of the theory that our solar system 

 reached its present condition by a process of 

 development, since on no other theory can a 

 satisfactory solution be obtained of the most 

 striking peculiarity of the moon's motions. 



But the inhabitants of earth are more directly 

 interested — not for their own sake, but for the 

 sake of their remote descendants — in the subject 

 of the moon's present airless and waterless con- 

 dition, regarded as the result of systematic pro- 

 cesses of change. If we can ascertain what those 

 processes may have been, and if we should find 

 that similar processes are taking place, however 

 slowly, on the earth, then the moon's present 

 condition has in a sense the same sort of interest 

 for us that a man in the full vigor of life might be 

 supposed to find in the study of the condition of 

 aged persons, if through some strange chance he 

 had never had an opportunity of observing earlier 

 the effects of old age upon the human frame. The 

 inhabitant of earth who contemplates the moon's 

 present wretched condition may be disposed — 

 like Lydia van den Bosch when she saw Madame 

 Bernstein's shaky hands and hobbling gait — to 

 hope we " sha'n't be like her when we're old, 

 anyhow ; " but the probabilities are in favor of a 

 young world following in the same path which 

 those now old have followed, and so reaching the 

 same condition. If the moon is really a much 

 older world than the earth — and we have seen 

 that in all probability she is — then she presents 

 to us a picture of the condition which our earth 

 will hereafter attain. 



We had occasion in the article on the Moon, 

 referred to above, to notice the theory advanced 

 by Frankland in this country respecting the way 

 in which the lunar air and seas have been caused 

 to disappear ; but we did not then enter into any 

 very careful discussion of that theory, our pur- 

 pose leading us to consider other matters. But 

 in this place the theory must occupy a larger 

 share of our attention. In passing, we may re- 

 mark that the originator of the theory was See- 

 man, the German geologist ; but it was inde- 

 pendently advanced by Frankland in England, 

 Stanislas Meunier in France, and Sterry Hunt in 

 America. 



In the first place, it is to be noted that no 

 other theory seems available. Of three others 

 which have been advanced, only one, Hansen's, 

 according to which the seas and atmosphere of 

 the moon have been drawn by lunar gravity to 



