488 SCIENCE PROGRESS 



if circulating within the " hour-glass " region enclosing both 

 bodies, finally come to one or other of the closed folds and 

 acquire a constant of relative energy such that it cannot again 

 escape therefrom. There seems reason to believe that light v^^ill 

 be thrown upon the problem of the origin of the Zodiacal Light, 

 and possibly of the " Gegenschein " also. Though the investi- 

 gations of Sir George Darwin are confined to cases of motion in 

 one plane, and indeed the circumstances of our own system are 

 such that this is very nearly true, yet it is very easy to extend 

 these results to space in general by regarding the curves as 

 contour lines of a surface, whereby a notion of the surface may 

 be obtained. Lord Kelvin was of opinion that all possible orbits 

 for any value of the constant of energy are essentially unstable, 

 but Darwin has given reasons for supposing that this instability 

 being a very slow process " the whole history of a planetary 

 system may be comprised in the interval required for the insta- 

 bility to become manifest." 



