446 ANNUAL EEPOKT SMITHSONIAN INSTITUTION, 1908. 



to cooling, caused its motion to become more and more accelerated, 

 till finally it reached a point where gravitation was no longer sufficient 

 to maintain the ellipsoidal shape. As inferred by calculation, the 

 subsequent detachment of a drop began thereupon to be preindicated 

 by the development of a pear-shaped appearance. This pear-shaped 

 body was constricted more and more, until the original single planet 

 was divided into a main body, the earth, and a satellite, the moon. 

 As the moon was thus formed of the outer parts of the original body, 

 we infer that it consists of the same material that in the earth was 

 arranged in the form of a rock mantle. The friction of the tides 

 thereupon carried the moon farther and farther away from the earth, 

 and diminished the rate of rotation of the earth. Thus the moon, 

 which to-day describes its orbit at a great distance from the earth, is 

 simply a drop detached from mother earth by the centrifugal force. 



A very interesting question is that regarding the time that has 

 elapsed since the moon was severed from the earth. Keeping in view 

 the magnitude of the forces at work in the tide phenomenon, we can 

 form at least an approximate judgment concerning it. We infer that 

 it took place about ten thousand million years ago. By an " ap- 

 jDroximation " we must understand in this case that the time can 

 hardly have been more than one hundred thousand million years, but 

 very probably more than one thousand million years. Life on the 

 earth can only have begun a good while after the sejDaration of the 

 moon, and yet we are forced to admit that life has lasted several 

 thousand million years. 



Compared with human life, a period of ten thousand million years 

 appears enormous; and yet it is so small that even the moon can not 

 have lost, during that period, any great fraction of the heat that it 

 took away at its birth. Thus we have to infer that the moon, like the 

 earth, is still intensely hot in its interior. As the pressure conditions 

 in the interior of the moon's body must be similar to those existing in 

 the rock mantle of the earth, we arrive at the conclusion that the 

 physical conditions of matter in the rock mantle of the earth are not 

 greatly different from those prevailing in the body of the moon. We 

 saw that the material, too, must be the same. Thus we are presented 

 with an opportunity to a certain degree to test the correctness of the 

 views above developed ; for if they are correct, the rock mantle of the 

 earth and the body of the moon must have very nearly the same den- 

 sity. It was shown a while ago that, according to observations on 

 earthquakes, the thickness of the rock mantle is to be estimated at 

 1,500 kilometers. If with this we combine the known data on the 

 average density of the earth and the degree of its flattening, we may 

 conclude that the material in the rock mantle is on an average 3.4 

 times denser than Avater. The average density of the moon, on the 

 other hand, may be inferred from astronomic observations. And 



