212 Prof. F. Guthrie on the Axial Rotation of the Earth, 



between two parallel planes tangental to opposite ends of a dia- 

 meter, and if the friction on one plane were greater than that 

 on the other, the sphere would acquire a rotation in the direc- 

 tion it would have if rolling on the plane whose friction is 

 greatest. 



13. The actual orbital motion of the earth being elliptical 

 (nearly circular), the part of the surface of the earth (for super- 

 ficial resistance) whose resistance to the medium would tend to 

 cause rotation in the same direction as the orbital motion of the 

 earth, and the part whose resistance would tend to cause rota- 

 tion in the opposite axial rotatory direction, are the two parts 

 cut off by an elliptical (nearly circular) cylinder, one of whose 

 focal axes passes through the sun, and whose plane radius vec- 

 tor reaches from the centre of the sun to the centre of the earth. 

 For penetrative resistance, mutatis mutandis. 



14. This principle may be easily illustrated experimentally in 

 the following manner: — A heavy spherical ball is hung by a fibre 

 of silk in a wide shallow cylindrical vessel of water. The water 

 is made to gyrate slowly and steadily by the rotation in it of a 

 little cardboard screw worked on the end of a stick by a thread 

 wound round its axis. As soon as the water is in motion the 

 ball begins to rotate on a vertical axis in the same direction. 

 The orbital motion of the earth is here represented by the gyra- 

 tion of the water in the cylinder. 



15. The actual axial rotation of the earth is in the same di- 

 rection as its orbital rotation. Hence^if its axial rotation be due 

 to the resistance of the aether, the resistance on the part of the 

 earth nearest the sun must be greater than that on the remoter 

 side. 



16. This cannot be the case if the aether offers uniform resist- 

 ance throughout, but it would result from the aether which lies 

 nearer the sun being denser than the remoter aether. 



17. Such a difference would result from the possession by the 

 aether of weight and elasticity. 



18. The idea of compressibility and elasticity is inseparable from 

 that of aether; or rather the necessity for an aether has been 

 felt in order to explain the phenomena of light and heat by 

 means of travelling states of unequal compression and the re- 

 covery from such states — called undulation. Further, there can 

 be no such thing as density, either constant or variable, and 

 therefore no such thing as undulation without matter. Nor is 

 it possible to conceive matter without weight. 



19. Hence it is no extension of the ordinary conception of 

 the aethereal medium to suppose that the aethereal medium ex- 

 tends throughout all space, indeed, but is more dense in the 

 neighbourhood where other matter most abounds. 



