682 SCIENCE PROGRESS 



a lens-shaped mass is first formed, matter then being thrown off from the 

 equator. The limit separating the two modes is found with some degree of 

 approximation for each problem. 



Chapter VIII is devoted to the consideration of the evolution of gaseous 

 masses, which are continually emitting radiation, rotation now being assumed. 

 The results obtained give a very plausible explanation of Russell's theory 

 of the order of stellar evolution, and this theory is adopted in the re- 

 mainder of the book. It is generally accepted as correct by astronomers 

 to-day. 



The theoretical discussion suggests almost naturally the probable mode 

 of formation of spiral nebulae, which is considered in Chapter IX. A slight 

 external gravitational field is sufficient to account for the ejection of matter 

 in two opposite filaments, instead of being thrown off in the form of a ring ; 

 it is further shown that, with very probable assumptions, the mass of the 

 separate nuclei will be comparable with the mass of our sun. 



The second type of structure discussed is the star-cluster. This prac- 

 tically forms a separate section by itself, not being dependent on the previous 

 chapters. The discussion is, in fact, based upon the methods of general 

 stellar dynamics. It is shown that the phenomena in our stellar universe 

 indicate that it is not in a statistically steady state, but that there is a toler- 

 able approximation to such a state. This being so, certain conclusions as 

 to stellar motions in the system can be deduced which are in reasonably 

 good accordance with observation. 



The evolution of binary and multiple stars is next considered. This 

 subject bristles with difficulties, but some important results of wide gener- 

 ality are obtained. It is shown that, if a binary star is formed by fission, 

 although the spectral type and eccentricity will vary as the evolution pro- 

 gresses, yet in general the period remains of the same order. It had pre- 

 viously been thought that period and eccentricity increased together, since 

 observation appeared to indicate this. The effect of close encounters with 

 other stars is also discussed, and it is found that, in the present state of the 

 universe, these can have only an infinitesimal effect upon the orbits of binary 

 stars ; but that, if the stars were at some previous time much more closely 

 packed than they are at present, the effect would have been to increase not 

 only the eccentricity and linear dimensions, but also the period. The 

 shorter periods of the B-type binary stars are therefore probably to be 

 accounted for by supposing them to have been formed comparatively re- 

 cently, so that the effect of encounters has not materially increased their 

 periods. 



The most difficult of all the structures found in the heavens to account 

 for is our own solar system. Jeans discusses the possible causes at some 

 length. It is concluded that it is very improbable that the rotational theory 

 can account for its formation, but it is shown that the tidal theory offers 

 a not impossible explanation provided it be granted that the outer planets 

 were partially fluid at, or shortly after, their birth. Jeans concludes that 

 " the theory is beset with difficulties and in some respects appears to be 

 definitely unsatisfactory ; ... it appears more acceptable than the rota- 

 tional theory, or any other theory so far offered of the genesis of the solar 

 system." 



We have endeavoured to give the reader a general idea of the contents 

 of the book ; space does not permit a critical discussion of the conclusions 

 arrived at. In some measure, however, the author disarms criticism by 

 the frank admission that some of the conclusions are not free from objec- 

 tion ; yet alternative conclusions seem to be open to even more serious 

 objections. The volume contains at once the most comprehensive and 

 also the most thorough discussion yet made of the method of evolution 

 of the various types of structure known to us in the heavens. A very large 



