118 THE LUMINIFEROUS ^THER. 



becomes easily intelligible, if we consider that the resistance experi- 

 enced would tend to check its motion, and so prevent it from getting 

 away so far from tlie sun at aphelion, and would consequently bring it 

 more nearly into the condition of a planet circulating round the sun in 

 a smaller orbit. 



Many years ago I asked the highest authority in this country on 

 physical astronomy, the late Prof. Adams, what he thought of the evi- 

 dence afforded by Encke's comet for the existence of a retarding force, 

 such as nught arise from the aitlier. He said to me that he thought we 

 did not know enough as to whether there might not possibly be a planet 

 or planets within the orbit of Mercury which would account for it in a 

 different way. But quite independently of such a supposition it is 

 worthy of note that the remarkable phenomena presented by the tails 

 of comets render it by no means unlikely that even without the presence 

 of a resisting medium, and without the disturbing force arising from 

 the attraction of an unknown planet situated so near to the sun as not 

 to have been seen hitherto, the motion of the head of a comet might 

 not be (piite the same as that of a simple body representing the nucleus, 

 and being subject to the gravitation of the sun and planets and noth- 

 ing else. It appears that the tails consist of some kind of miitter 

 driven from the comet with an enormons velocity by a sort of re|)ulsion 

 emanating from the sun. If the nucleus loses in this manner at each 

 ])erihelion passage an exceedingly small portion of its mass, which is 

 repelled from the sun, it is possible that the residiu* may expericnci* an 

 attraction towards the sun over and above that due to gra\itati(»n, and 

 that possibly this may be the cause of the observed acceleration in the 

 tinn^ of passing perihelion even though there be no resistance on the 

 ])art of the jetlier. So that the (juestion of resistance or no resistance 

 must be left an open one. 



The supposition that the icther would resist in this manner a body 

 moving through it is derived from what we observe in the case of solids 

 moving through fluids, liquid or gaseous, as the case may be. In ordi- 

 nary cases of resistsmce, the main represent,! five of the work ai)par- 

 ently lost in propelling the solid is in the first instance the molecular 

 kinetic energy of the trail of eddies in the wake. The formation of 

 these eddies is however an indirect effect of the internal friction, or 

 if we prefer the term — viscosity, of the fluid. Now the viscosity of gases 

 has been explained on the kinetic theory of gases, and in the case of a 

 liquid we cannot well doubt that it is connected with the constitution 

 of the substance as not being absolutely continuous but molecular. 

 But if the a'ther be either non-molecular, or molecular in some totally 

 different sense from jionderable matter, we cannot with safety infer 

 that the motion of a wSolid through it necessarily inq)lies resistance. 



The luminiferous a'ther touches on another mysterious agent, the 

 nature of which is unknown, although its laws are in many respects 

 known, and it is applied to the every-day wants of life, and its appli- 



