and the Resistance, Elasticity, and Weight of Solar Mther. 213 



20. The hypothesis here proposed is therefore briefly this. 

 The solar system is full of aether which has weight. It is massed 

 most abundantly round the bodies of that system which have 

 the most mass : chiefly round the sun. Spreading from the sun 

 in all directions, it diminishes in density. In its passage round 

 the sun the earth encounters the resistance of this sether. 

 Although the side of the earth nearest to the sun performs the 

 shorter path, yet it encounters sether of greater density than is 

 met with by the remoter side. Consequently the earth revolves 

 on its axis in the same direction as its orbital rotation. 



21. The aether, to the undulations of which the phenomena 

 of light and heat are usually attributed, is supposed to pervade 

 space, both where matter is present and where it is not ; it is 

 supposed to be coextensive with space, and, though not necessarily 

 even in a state of rest, of uniform density throughout. It is 

 clear that the penetrative resistance offered by a medium of this 

 kind to the passage of the earth through it, would effect an axial 

 rotation in the earth in the same direction as that wrought by a 

 merely superficial one such as friction. In the latter case, how- 

 ever, the resistance to a body's translation varies with the plane 

 orbitolinear projection of its surface ; in the former, with the 

 volume of the body, possibly with its density, and so with its'mass. 



22. No attempt is made in the foregoing to account for the 

 inclination of the axis of the earth to the plane of its orbit. 



23. If inequality of sethereal resistance to different parts of the 

 earth be the real and only cause of the earth's axial rotation, 

 since the axial velocity of the earth is known, any one of the 

 three following factors could be found if the other two were known. 



(a) The age of the earth's orbit. 



(b) Coefficient of resistance between aether and earth. 



(c) Law of elasticity of sether. 



24. The direction of the axis of a comet to its orbit has been 

 regarded as evidence against the existence of a resisting sether 

 medium, because it is supposed that such a medium would 

 retard the nucleus less than the tail, on account of the smaller 

 size of the former. This objection would be just, if the resistance 

 were a purely superficial or frictional one. But if the sether per- 

 vades the substance of the comet, it is probable that the inter- 

 penetrative resistance between the comet and the sether may 

 depend also upon the density of the matter of the comet ; so 

 that though the nucleus be smaller than the tail, yet, being- 

 denser, it may have about the same mass, and therefore suffer 

 about the same resistance from the sether. 



Royal College, Mauritius, 

 January 19, 1866. 



Phil. Mac/. S. 4. Vol. 31. No. 20S. March 1866. Q 



