8 Prof. Oliver Lodge on 



inertia; so the axes are sometimes supposed to be generated by 

 particles of matter projected in different directions and subse 

 quently free from force *; or else the axes are made of infinite 

 mass so that no finite force may be able to affect them. 



Now I hold that all such notions as axes of reference are 

 artificial scaffolding, necessary for the numerical specification 

 of a velocity, but not at all necessary for the apprehension 

 of what is meant by a uniform velocity. It is in the 

 specification of any absolute velocity that the difficulties cited 

 about an origin and axes of reference legitimately occur. It 

 is in fact impossible to specify the absolute velocity of any- 

 thing, because we have literally no criterion of rest. We 

 shall, I believe, hereafter find it convenient to postulate the 

 Ether as a body absolutely at rest; but none of these physical 

 or geometrical complications should enter into an axiomatic 

 statement. 



All that the first law asserts is that the motion of a body 

 not acted on by force is uniform in magnitude and direction. 

 There is no need to attempt the impossible and say what that 

 magnitude and direction absolutely are. Whatever they are 

 they remain constant. If asked to prove this statement, we 

 should at once decline, and throw the burden of disproof on 

 the doubter. This is what Maxwell doesf when he says 

 (virtually) : — If the speed and direction of a freely moving 

 body vary they must vary in some definite manner ; very well, 

 tell me in what manner they are varying. You cannot, 

 unless you can show me absolutely fixed lines of reference. 



The fact is that the conception of uniform motion is based 

 upon a simple primary muscular sensation, or at any rate upon 

 a succession of such sensations ; everybody understands what 

 it means, so far as it is possible to understand anything in this 

 material universe, and the sense in which it is understood is 

 amply sufficient as a basis for a physical superstructure. The 

 first law is a true axiom, and its boldest and simplest form is not 

 only the best, but is the only one that can with any justice 

 be called axiomatic. How can one appeal to the experience of 

 the human race with reference to coordinate axes of infinite 

 or any other mass ? How can we utilize as axes the 

 trajectory of particles free from force, without tacitly assuming 

 the first law continually ? The whole attempt to complicate 

 the statement of the first law of motion seems to me absurd. 



* Thomson and Tait, vol. i. Part I. (1879), § 249. But although these 

 writeis do propose to use such axes to fix direction, or, better, the invariable 

 plane of a rotating system, §§ 267, 245, they quite logically deduce these 

 things from Newton's laws, and do not use them in the statement of those 

 laws. 



t Matter and Motion, art. xli, p. 36 e 



