694 Dr. C. V. Burton : Notes 



on 



changes in the translational velocity of the envelope. The 

 translation of the whole system may be accelerated, or even 

 changed impulsively, without the production of any effects 

 which could be detected by means of (ideal) observations 

 confined to the interior of the envelope. On certain suppo- 

 sitions, this result has an interesting theoretical application. 

 We are familiar with the view that the aether, whether free 

 from or encumbered by ordinary matter, constitutes an 

 incompressible plenum having everywhere the same density ;. 

 from which it would follow that, if a translational accelera- 

 tion of any magnitude could by suitable means be imparted 

 . to the whole of the exploitable universe, we should be entirely 

 without means of detecting it. Though changes of velocity,, 

 if actually occurring through so large a region, would almost 

 certainly take place with extreme slowness, yet we are faced 

 by the possibility that, even in regard to constancy of absolute 

 translational motion, no true dynamical criterion may be 

 available; experimental dynamics being more strictly con- 

 cerned with motion relative to the aether. 



3. Since the mere motion of a system through the aether 

 does not cause that system to be the seat of a magnetic field, 

 or of an electrostatic field, it follows that, unless we attach a 

 very real physical significance to absolute motion of the cether in 

 space, neither the magnetic nor the electric vector at any 

 point is to be identified with translational velocity of the aether 

 at that point *. Otherwise our motion through the aether,, 

 being equivalent to a contrary flow of aether with ourselves 

 at rest, would be evidenced to us by the existence of a 

 magnetic or electric field; and if the density of the aether is- 

 very great, the field-intensity arising from a moderate velo- 

 city of flow would be correspondingly great. 



4. For a certain class of cases, the argument may be pre- 

 sented in a somewhat different form. Let us suppose, as in 

 § 2 above, that the aether is incompressible, and that atomic 

 matter is built up of strain-distributions, involving no de- 

 parture from uniform density, which obtains everywhere. 

 Now consider a limited region of the aether through which 

 we can establish a uniform electrostatic field. Let a material 

 system be present in the region considered, and let this 

 system be entirely unconstrained, and initially at rest, sub- 

 ject to no external forces ; then let a uniform electrostatic 



* In the case of the electric vector, there is of course the further 

 objection that electrons would, on the view referred to, be sources and 

 sinks in the medium, involving continual creation and destruction of the 

 stuff of which the medium is made. This, I believe, was first pointed 

 out by von Helmholtz. 



