B. THE PLACE OF ORIGIN OF THE MOON.
Astronomers generally believe that the rnnon was once a part of the earth from which it has been separated by fission when in a somewhat plastic condition. The original material had greater volume than when the separation took place: and by condensation the speed of rotation increased until by centrifugal force the moon was born. Professor G. H. Darwin conceives the earth to have been drawn out to be pear-shaped—and by continued distortion a sort of knob formed at the small end, and eventually separated. I do not understand that Darwin favored the idea that this separa tion could take place except that the plasticity approached liquid ity, in which case no mark would be left at the point of separa tion. Professor W. H. Pickering conceives that the earth was so solid at this time that its main topographical features were the same as now, and that the mass eliminated left behind a scar, which corresponds to the bed of the Pacific Ocean. As it con cerns the history of Hawaii, it has seemed best to refer to the sub ject here.
Upon examining an artificial globe having the land and water represented upon it, and placing it so that the pole will be located one thousand miles northeast of New Zealand, very little land will be seen, and the outline of the Pacific will be circular—most of the land will occupy the other hemisphere. What can be more natural than that the Pacific depression occupies the place where the moon sloughed off ? The volume of the moon is equivalent to a solid whose surface is equal to that of all our terrestrial oceans, and whose depth is thirty-six miles. Supposing the crust of the earth to have been thirty-six miles thick, three-quarters of it could have been carried away to form the moon, and the remainder might have been torn in two for the continental masses, which floated upon the surface as two islands.
The specific gravity of the earth as a whole is about 5.6; of the moon 3.4; the surface of the continents 2.7; the upper half of Mauna Kea 2.1 ; the lower half 3.7; and this lower por
tion is conceived to represent the specific gravity of the liquid upon which the hypothetical islands floated. The average gravity for the whole of Mauna Kea is 2.9. The gravities of the various basalts of Hawaii range from 2.82 to 3.2o. Be cause the gravities of the moon and of the heavier lavas are nearly alike, Professor Pickering concludes that the doctrine of the derivation of the origin of the moon from the Pacific ocean is substantiated. That the lunar and Hawaiian volcanoes are very much alike, as set forth earlier in this volume, does not affect the present question.
Prof. Pickering enters into specific explanation of the con tinental and insular forms, which to us are less convincing than the speculations of W. L. Green, who would object to the derivation of the moon from the Pacific alone because of the near approach of the earth to fluidity. The surface would not have been rigid enough to retain the distinctions of continent and ocean at this early period, and he has clearly explained a better view of the origin of the earth's physical features.
It seems to us that a consideration of the protuberant mass of the earth manifested in the equatorial regions, throws light upon the degree to which a modulation may take place. In former days it was claimed that the flattening of the poles proved igneous fluidity in the earth's early history. After stat ing that there had not been an appreciable shortening of the earth's diameter for the past two thousand years, or since astronomic observations began to be taken, the late Professor Benjamin Pierce remarked that were the earth solid this equatorial bulge would have been formed by the pressure of the agencies that caused it to exist. If so, how much less could the so-called scar of the Pacific have maintained its irregulari ties since that early period when the two spheres separated? Hence it does not appear to us that the theory of the Pacific scar maintained by Professor Pickering can be substantiated.