THE LAWS OF MOTION 289 



might be used to determine our frame of reference/ and 

 there is little doubt that this would clear up many of our 

 current difficulties as to inertia and absolute rotation. 

 Meanwhile, we must bear in mind that while the frame of 

 reference and the principle of inertia are quite clear ideas 

 in the conceptual model of corpuscles, they have no exact 

 perceptual equivalents. But no parts, indeed, of our 

 mechanical models have, as we have before noted, exact 

 perceptual equivalents ; all we must ask is : Are they 

 valid as instruments for describing phenomena ? Here 

 the answer must be : Most certainly, if we take our frame 

 as determined by the so-called " fixed " stars. 



With regard to this law of inertia it must probably be 

 conceived as holding from the prime-atom to the particle, 

 but a difficulty comes in when we consider ether-elements. 

 If the prime-atom be a particular type of ether-motion, 

 for example an ether vortex-ring or ether-squirt, then the 

 very existence of the corpuscles of gross " matter " de- 

 pends upon the presence of the ether-elements, not only 

 in their own constitution, but in their immediate neighbour- 

 hood. It becomes, therefore, hopelessly absurd to con- 

 sider what a corpuscle of gross "matter" would do if it 

 were isolated from the influence of ether-elements. The 

 law of inertia for gross " matter " must then flow from 

 the peculiar structure of gross " matter." The mutual 

 presence of ether-elements and of an isolated prime-atom 

 will then be seen to involve the inertia of the latter, but 

 the ether-elements themselves will, while the prime-atom 

 moves uniformly, be varying their motion with due regard 

 to the presence of the prime-atom.^ What the law of 

 inertia is to be considered as meaning when applied to 

 isolated ether- elements, it is again difficult to say. 



1 Actually the ether, is used ; it is the direction of a ray of light in the 

 ether which gives the " fixed " direction, and this light may have left the star 

 millions of years ago, and does not necessarily mark the present direction of 

 the star. Unfortunately it does not persist. 



^ For example, it may be shown that an isolated vortex-ring in an infinite 

 fluid moves without sensible change of size with uniform velocity perpen- 

 dicular to its plane ; on the other hand, the ether-elements vary their velocity 

 according to their position relative to the ring (see A. B. Basset, A Treatise 

 071 Hydrodynamics, vol. ii. pp. 59-62). 



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