192 Scientific Intelligence. 



moon have developed by fission from an original mass, and that 

 tidal friction has been an important factor in their evolution. 

 Indeed, they are so uniformly contradictory to its implications 

 as to bring it into serious question, if not to compel us to cease 

 to consider it as even a possibility " (p. 133). 



Moulton's resultSj based mostly upon the fundamental equa- 

 tions of moment of momentum and energy of the earth-moon 

 system, are thus seen to be an independent proof in harmony with 

 the two preceding, that no changes of importance in the rate of 

 earth rotation have taken place within the period of known earth 

 history, going, however, still further, and pointing to their always 

 having been separate masses. 



The next paper consists of '•' Notes on the Possibility of Fission 

 of a Contracting Rotating Fluid Mass" by F. R. Moulton. In 

 the summary at the end of the paper it is stated : 



"The problem under consideration is that of the fission of 

 celestial bodies because of rapid rotation when they are not dis- 

 turbed by important external forces. The attack is made through 

 well-known results concerning the figures of equilibrium and 

 conditions as to stability of rotating homogeneous incompressible 

 fluids. It is recalled that for slow rotation a nearly spherical 

 oblate spheroid is a stable form of equilibrium ; that for greater 

 rates of rotation the corresponding figure is more oblate ; that 

 when the eccentricity of a meridian section becomes 0'8] 3 the 

 figure loses its stability and at this point a stable line of three- 

 axis ellipsoids branches ; that when the longest axis of the ellip- 

 soid becomes about three times the axis of rotation a new series, 

 known as the pear-shaped figures (or better, perhaps, the cucum- 

 ber-shaped figures) branches, and that before this point is reached 

 there is no possibility of fission. We are almost entirely ignorant 

 as to what may happen after this point is passed, and it must be 

 remembered that it has not been proved that in any. case fission 

 into two stable bodies is possible. 



"The celestial bodies differ from those just considered in two 

 important respects. In the first place their densities increase 

 toward their centers. For a given rate of rotation and mean 

 density this central condensation makes them more nearly spheri- 

 cal, as is shown both by theory and by comparison of the 

 observed figures of the planets with the computed forms of 

 corresponding homogeneous masses. In the case of Saturn, for 

 example, the eccentricity computed on the hypothesis of homo- 

 geneity is 0*607 while the observed value is only 0*409. It seems 

 certain that this central condensation tends toward stability. The 

 second important difference between the ideal homogeneous 

 incompressible fluids and the celestial bodies is that the latter are 

 compressible. This latter factor, at least under certain circum- 

 stances, tends toward instability. 



" The opposing quantitative effects of central density and com- 

 pressibility undoubtedly differ greatly in different masses and 

 can not be easily determined in any case. However, if we may 



