402 Scientific Proceedings, Royal Dublin Society. 



consisting of shallow waves each a quarter of a mile long, were to 

 traverse the medium, the blocks would appear to act on one 

 another in a certain definite way. This corresponds to the way 

 in which Professor Mac Cullagh's elements of volume, his dxdydz's, 

 are in his formulae assumed to act. But if the whole space were 

 divided into much smaller blocks, suppose into cubes of half an 

 inch, great differences would be found to prevail between these 

 small blocks, and equally great differences in the way they act on one 

 another : and the difference would become more striking if the sub- 

 division were carried so far as to render the blocks small in compari- 

 son with the thickness of the wires that represent vortex filaments. 



If now we further conceive small vortex tangles travelling 

 about in this medium, the long vortex filaments opening to let 

 them pass, and acting in front sideways and behind upon them 

 in such a way as to urge them equally forwards and backwards, so 

 long as their journey is along a straight path with uniform speed — 

 we shall have a first sketch of what constitutes ponderable matter 

 and the luminiferous ether, according to these speculations. 



The particular hypotheses which are here described may perhaps 

 not have quite hit the mark ; but, though we have as yet only a 

 glimmering of this great subject, it is pretty certain that either 

 these hypotheses, or something like them, are the true ultimate 

 account of material Nature. 



We must, therefore, carefully distinguish between the elemental 

 and the luminiferous ethers. The elemental ether, until motions 

 create differences in it, is absolutely alike and undistinguishable in 

 all its parts, and in the mathematical investigation of motions in 

 it, wherever in any of the equations of dynamics an element of 

 mass appears, we must write everywhere the element of volume 

 instead. It is itself the integral of these elements of volume ; in 

 other words, it is space under a new aspect. In the geometrical 

 way of conceiving space, the parts into which it may be conceived 

 to be divided are thought of as they would be if at rest relatively 

 to one another. In the kinematical way of conceiving space, 

 which alone is in accordance ivith what objectively exists, we are to 

 recognise that each portion of volume is pervaded by the motions 

 that actually subsist within it, and that it can travel about carry- 

 ing those motions with it. In fact the volume occupied by a 

 block of iron differs from an equal volume occupied by air, only 



