THE EVOLUTION OF SATELLITES. 117 



the final couditiou resembles the case of au egg lyiug on its side, which 

 only rocks a little when we disturb it. So if the moon were slightly 

 disturbed from her final condition, she would continue to describe very 

 nearly the same path round the earth, and would not assume some 

 entirely new lorm of orbit. 



It is by methods of rigorous argument that the moon is traced back 

 to the initial unstable condition when she revolved close to the earth. 

 But the argument here breaks down, and calculation is incompetent to 

 tell us what occurred before, and how she attained that unstable mode 

 of motion. We can only speculate as to the preceding history, but 

 there is some basis for our speculation, for I say that if a planet, sucli 

 as the earth, made each rotation in a period of three hours, it would 

 very nearly tly to x)ieces. The attraction of gravity would be barely 

 strong enough to hold it together, just as the cohesive strength of iron 

 is insufficient to hold a fly wheel together if it is spun too fast. There 

 is, of course, an important distinction between the case of the ruptured 

 fly wheel and the supposed break-up of the earth, for when the fly 

 wheel breaks the pieces are hurled apart as soon as the force of 

 cohesion fails, whereas when a planet breaks up, through too rapid 

 rotation, gravity must continue to hold the pieces together after they 

 have ceased to form parts of a single body. 



Hence, we have grounds for conjecturing that the moon is composed 

 of fragments of the primitive planet which we now c.all the earth, which 

 detached themselves when the planet spun very swiftly, and afterwards 

 became consolidated. It surpasses the powers of mathematical calcu- 

 lation to trace the details of the process of this rupture and subsequent 

 consolidation, but we can hardly doubt that the system would pass 

 through a period of turbulence before order was reestablished in the 

 formation of a satellite, 



I have said that rapid rotation was probably the cause of the birth 

 of the moon, but this statement needs qualification. There are certain 

 considerations which prevent us from ascertaining the common period 

 of revolution of the moon and the earth with accuracy. It may lie 

 between three and five hours. I think that such a speed might not, 

 perhaps, be quite sufficient to cause the planet to break up. Is it pos- 

 sible, then, to suggest any other cause which might have cooperated 

 with the tendency to instability of the rotating planet? I think that 

 there is such a cause, and though we are here dealing with guesswork, 

 I will hazard the suggestion. 



The primitive planet, before the birth of the moon, was rotating 

 rapidly with reference to the sun, and it must, therefore, have been 

 agitated by tidal oscillations due to the sun's attraction. Now, the 

 magnitude of these solar tides is much influenced by the speed of rota- 

 tion of the planet, and mathematical reasoning appears to show that 

 when the day was about three or four hours in length the oscillations 

 must have been very great, although the sun stood no nearer to the 



