58 SCIENCE PROGRESS 



its orbit, and it is possible that, under the influence of the 

 resisting medium, this would ultimately lead to its becoming a 

 satellite. The mechanism of the process has not been worked 

 out in detail, and, in view of the extremely complicated nature 

 of the problem, it would be very dangerous to predict whether 

 it is feasible. All the satellites in the system are supposed to 

 have been captured in this way by their primaries. In both 

 hypotheses the satellites are considered to have approached their 

 primaries after becoming associated with them owing to the 

 secular effect of the resisting medium. 



The Theory of Tidal Friction. — All the theories so far men- 

 tioned agree in the fact that each commences with a particular 

 distribution of matter, and tries to predict the course of the 

 changes that would follow if this were left to itself. The 

 success or failure of such hypotheses to lead to a system 

 resembling the present solar system is the measure of their 

 truth or falsehood. The method is thus essentially one of trial 

 and error, and when a theory is found unsatisfactory, the next 

 step is to modify it in such a way as to avoid the defects that 

 have been detected. In this way a succession of different 

 hypotheses may be obtained, each giving a better representation 

 of the facts than the previous one. Destructive criticism may 

 thus be of positive value. Such a method must necessarily 

 yield the truth very slowly, and must further involve a large 

 number of assumptions concerning the initial conditions ; in 

 addition, the set of initial conditions that leads to the correct 

 final state may not be unique. 



The Theory of Tidal Friction, due to Sir G. H. Darwin, 1 is 

 of a totally different character. It starts with the present 

 conditions, and by means of a single highly plausible hypothesis 

 obtains relations that the properties of the system must have 

 satisfied at any epoch, provided only that this is not too remote 

 for the calculation to be possible, and that no unknown causes 

 have operated that could invalidate the work. The initial 

 conditions thus obtained are then unique, and the only way of 

 disproving the hypothesis would be to discover some new agency 

 of sufficient magnitude to upset the course of the involution. 

 Whatever hypothesis may ultimately be found to account for the 

 present solar system, the Theory of Tidal Friction must therefore 

 form a part of it. 



1 Scientific Papers, vol. ii. 



