AN ESSAY ON THE MATHEMATICAL PRINCIPLES OF PHYSICS. 339 



matter must be consistent with those of which experiments on sensible bodies 

 have made us cognizant. 



The world, according to Professor Challis, is made up of atoms and aether. 

 The atoms are spheres, unalterable in magnitude, and endowed with inertia, but 

 with no other property whatever. The aether is a perfect fluid, endowed with 

 inertia, and exerting a pressure proportional to its density. It is truly con- 

 tinuous (and therefore does not consist of atoms), and it fills up all the 

 interstices of the atoms. 



Here, then, we have set before us with perfect clearness the two con- 

 stituents of the universe : the atoms, which we can picture in our minds as 

 so many marbles ; and the aether, which behaves exactly as air would do if 

 Boyle's law were strictly accurate, if its temperature were invariable, if it were 

 destitute of viscosity, and if gravity did not act on it. 



We have no difficulty, therefore, in forming an adequate conception of the 

 properties of the elements from which we have to construct a world. The 

 hypothesis is at least an honest one. It attributes to the elements of things 

 no properties except those which we can clearly define. It stands, therefore, 

 on a different scientific level from those waxen hypotheses in which the atoms 

 are endowed with a new system of attractive or repulsive forces whenever a 

 new phenomenon has to be explained. 



But the task still before us is a herculean one. It is no less than to 

 explain all actions between bodies or parts of bodies, whether in apparent 

 contact or at stellar distances, by the motions of this all-embracing aether, and 

 the pressure thence resulting. 



One kind of motion of the aether is evidently a wave-motion, like that of 

 sound-waves in air. How will such waves affect an atom ? Will they propel 

 it forward like the driftwood which is flung upon the shore, or will they draw 

 it back like the shingle which is carried out by the returning wave ? Or will 

 they make it oscillate about a fixed position without any advance or recession 

 on the whole ? 



We have no intention of going through the calculations necessary to solve 

 this problem. They are not contained in this Essay, and Professor Challis 

 admits that he has been unable to determine the absolute amount of the 

 constant term which indicates the permanent effect of the waves on an atom. 

 This is unfortunate, as it gives us no immediate prospect of making those 

 numerical comparisons with observed facts which are necessary for the verification 



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