ASTRONOMY 35; 



ill order to agree with the physical estimates, and that this 

 increase is consistent with the geological evidence. On the 

 other hand, Prof. Sollas dissented from this view and was not 

 prepared to accept such an increase. 



Dr. Jeffreys stated that from considerations of the tem- 

 perature distribution downwards in the earth's crust, allowing 

 for the radio-active content, and also from the tidal theory of 

 the origin of the solar system, he had separately derived con- 

 cordant estimates of about 2,000 million years since the solidifi- 

 cation of the earth's crust. Thus, with revised data, two of 

 Lord Kelvin's methods of reasoning have been brought into 

 agreement with the results derived from other physical methods. 

 Lord Kelvin's third method — the contraction hypothesis — is not 

 valid on account of the existence of other sources of stellar energy. 



It will have been gathered that, on the whole, there is now 

 a satisfactory agreement between the results of arguments 

 based upon astronomical, physical and geological considerations. 

 These arguments combine in indicating an age of the earth, 

 since solidification, of about 1,000 million years. 



The Theory of Saturn's Rings. — An important contribution 

 to this theory has been made by Dr. G. R. Goldsborough in a 

 paper entitled " The Influence of Satellites upon the Form of 

 Saturn's Rings " {Phil. Trans., Sec. A, 223, pp. 101-30, 1921). 

 It was shown by Maxwell in 1856 that the rings could only 

 be stable for small disturbances if they were composed of 

 meteorites sufficiently small. Spectroscopic observations have 

 confirmed this result. From this point of view, the observed 

 divisions in the rings have been attributed to the disturbing 

 action of the satellites of Saturn. It has been supposed that in 

 the positions in which a single particle moving in a circular 

 orbit would have a period commensurate with one of the 

 satellites, the disturbing action would gradually increase by a 

 resonance effect until in time a ring would be swept clear of 

 particles. Lowell, at the Flagstaff Observatory, has in fact 

 found that, in addition to the well-known and easily observable 

 rings, there are a large number of additional fine divisions in 

 the rings which occur at intervals corresponding to periods 

 commensurable with that of the satellite Mimas. This theory 

 has never received adequate theoretical investigation, in spite 

 of the fact that some doubt was thrown upon it by Tisserand. 

 Moreover, the theory in this form is not complete, as the mutual 

 attractions of the particles composing the ring are left out of 

 consideration. The paper by Goldsborough, previously referred 

 to, supplies this lack. He considers the effect of a satelhte upon 

 a number of particles forming a single ring round the planet 

 and subject to their mutual gravitational attraction, as well as to 

 that of Saturn and of the satellite. 



