and the Resistance, Elasticity, and Weight of Solar JEther. 211 



in a circle round the centre of the moving circle in the same 

 direction as the moving circle moves in its orbit, and in such a 

 manner as to move round the centre of the moving circle in the 

 same time as the circle performs one orbital revolution, that 

 point remains at the same distance from the centre of the 

 orbit. 



5. Inversely, when a circle moves without axial rotation in a 

 circular orbit round a point, the points of greatest and least dis- 

 tance between the circle and the orbital centre travel round the 

 circumference of the moving circle in such a manner that the 

 line joining them, when produced, passes through the orbital 

 centre. This line completes one revolution as the moving circle 

 completes one revolution. 



6. Hence the successive series of points of maximum distance* 

 traverses an orbital path longer than the path described by the 

 successive series of points of minimum distance by the circum- 

 ference of a circle whose radius is equal to the diameter of the 

 moving circle. 



7. The same is true, mutatis mutandis, for any points in any 

 body moving in any closed orbit. 



8. Suppose that the earth at some time had orbital (by the 

 action of gravity or otherwise) but no axial rotation. 



9. If the space through which the earth travels in its motion 

 round the sun be filled with a uniform medium of any degree of 

 tenuity, which has any inertia or which in any way offers resist- 

 ance to the passage of the earth through it^ either frictional 

 resistance to the earth's surface, or penetrative resistance towards 

 its mass, then orbital motion must necessarily give rise to axial 

 rotation. 



10. Because the succession of points furthest from the sun 

 will be continually subjected to the greatest resistance, and the 

 points nearest to the sun to the least resistance during the earth's 

 orbital motion ; and because all other points of the earth's sur- 

 face (if the resistance be only superficial) or of its mass (if it be 

 penetrative) meet with greater or less opposition, according as 

 they are further from or nearer to the sun. 



11. Hence the resulting effect would be equivalent to a force 

 upon the part of the earth furthest from the sun, tending to 

 make the earth revolve on an axis perpendicular to its orbit- 

 plane and in a direction opposite to the orbital rotation of the 

 earth. 



12. Thus if a sphere were to move (at first without rotation) 



* The successive (with regard to the circumference of the wheel) series 

 of points in which a rolling wheel touches the ground is the point (with 

 regard to position referred to a vertical line through the centre of the 

 wheel) where the wheel touches the ground. 



