NA TURE 



[April 19, 1906 



LETTERS TO THE EDITOR. 

 [The Editor does not hold himself responsible for opinions 

 expressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of Nature. 

 No notice is taken of anonymous communications.] 



The Existence of Absolute Motion. 



The article of Prof. Schuster's in your number of 

 March 15, entitled "A Plea for Absolute Motion," is 

 very interesting, but I think there are several conceptions 

 contained therein which will not bear analysis. Partly in 

 reference to his article, therefore, but also because the 

 question is such an important orie, I think it may be well 

 to consider as definitely as may be what direct observational 

 or experimental evidence we have for a zero point of 

 motion belonging to space alone, and to which all motions 

 of material bodies may be referred. 



Not to be entangled in the snare which is as old as 

 human discussion, I define my terms for present use. By 

 space I mean vacuum in the ordinary sense, that which 

 exists in interplanetary space, that to which we approach 

 in our laboratories, nothing more or less. We have good 

 reason to believe that in the visible universe no other kind 

 of space exists. This is not, I believe, Prof. Schuster's 

 use of the word, but I shall try to show that it is the 

 only proper scientific use. 



By absolute motion I mean motion considered with 

 reference to this space alone. 



The first evidence is furnished by the observed orbits of 

 binary stars. If the velocity of light is dependent on the 

 motion of the source, light which left the star when its 

 motion was toward the earth must of necessity reach us 

 sooner than the light emitted when this approach com- 

 ponent of the star's velocity was smaller or negative. 

 The observed orbit would therefore be distorted in a per- 

 fectly definite manner. The fact that this distortion is not 

 observed proves that the velocity of light is not dependent 

 upon the velocity of the source, and must therefore depend 

 upon some positional property of space alone. 



The conclusion is vividly represented as follows : — 

 Imagine a hollow sphere in space with a light source at 

 its centre. In general, a light wave leaving the centre 

 will not reach all parts of the surface in equal times. 

 There exists, however, one. motion of the sphere for which 

 this condition is fulfilled, and this state, which is abso- 

 lutel) independent of all existing bodies, has a funda- 

 mental claim to be called absolute rest, because it depends 

 on space alone. 



Further evidence is furnished by the laws of electro- 

 dynamics. The magnetic effect of electric convection is 

 generally considered to be now beyond question. From it 

 we know that the electromagnetic attraction between two 

 like point charges moving together is a function of their 

 velocity. Since there is no relative motion of the two 

 and they may be considered alone in space, the motion is 

 with respect to space alone. The state of absolute rest is 

 found when the electromagnetic attraction is zero for all 

 directions of the line joining the two charges. 



The evidence furnished by the Kaufmann experiment on 

 the mass of a rapidly moving (lection, indicating as it 

 does a limiting velocity in space, also implies the existence 

 of absolute motion. 



The evidence is not so good as in the other cases, be- 

 cause the effect is complicated by the existence of an 

 outside magnetic field with reference to which the electron 

 moves. 



I foresee Prof. Schuster's objection to the above. What 

 I have considered he will call motion relative to the ether, 

 while his argument was based on space in a philosophical 

 sense. I have carefully avoided the term ether. It seems 

 to me the word has nothing to do with the discussion. 

 The universe, out to the furthermost visible star, is of 

 such a nature as to be traversed by light. With space in 

 any other sense we have nothing whatever to do, because 

 it does not exist in the visible universe. Even if such 

 " space " did exist outside the visible universe, it is 

 difficult to understand bow our observational data could 

 have any bearing on the matter. 



Finally, if any more argument is necessary to show that 



NO 1903, VOL. 73] 



the only space we can consider is that which surrounds 

 us in the universe, it might be derived from the funda- 

 mental notion of space perception. Our perception of 

 space is brought about through various sensations, sensa- 

 tions which are caused by events which do not take place 

 in a hypothetical space, non-existent so far as we know, 

 but in the real space which surrounds us. Our verv use 

 of the word therefore arises out of experience, and to think 

 of another space is to form only what Mr. Spencer would 

 have called "a symbolic conception." Indeed, I fear if 

 this fundamental standpoint of perception were strictly- 

 adhered to, those arguing from the standpoint of another 

 space would have great difficulty in making themselves 

 clear. We cannot be too careful, it seems to me, in con- 

 sidering the origin of our fundamental conceptions. 



At any rate, real space, as has been pointed out, 

 possesses a positional, or perhaps better a motional, 

 attribute, and so gives us a basis, founded on experience, 

 for a conception of absolute motion. 



, Daniel Comstock. 



Zurich, Switzerland, April 3. 



The Magnetic Inertia of a Charged Conductor in a 

 Field of Force. 



I think there is, in Another Place, possibly some mis- 

 understanding concerning the inertia of a moving charged 

 conductor due to the magnetic energy set up by its motion. 

 It depends upon the distribution of the electrification, and 

 may vary from a minimum up to infinity. No question of 

 distortion due to high speed is involved, so the theory is 

 quite simple. Say a sphere of radius a has any distribu- 

 tion of surface charge. For simplicity, let it be sym- 

 metrical round the axis of motion, so that the surface 

 density is representable by the sum of any number of 

 zonal harmonic distributions. The corresponding magnetic 

 fields follow. Their magnetic energies are all independent, 

 so that the actual magnetic energy is the sum of the 

 separate magnetic energies. 



The really practical case, which is also very simple, is 

 when the conductor has a charge Q and moves in a uniform 

 electric field F. Then the surface density is 



ir = Q/47r<z- + 3<-Fcos0, (1) 



where 6 is the polar angle. The magnetic force is 



H = H, + H, = (Qu/4-nr") sin + 3c Ffatyr 8 ) fin 6 cos 8. (2) 

 The magnetic energy is SJ/tH 2 , and by integration conies 

 to 



T = hll[fii r-'/6Tra + iac -F'W] . ( 



The magnetic inertia is therefore »n = in,(i + ft), where 

 ?ii, is the value for the uniform charge, or m, = fiQ 2 /6na, 

 and 



« = »»/i5*. if« = (3F)(4™ 2 ' Q). (4) 



] his 11 is the ratio of the induced electric force at the 

 pole to the undisturbed force. If n=i, F is just large 

 enough to make the surface density be zero at one pole. 

 Then /i=i 47. This is the increased inertia due to the 

 disturbance of the distribution of the charge. The 

 " equation of motion " under F is 



FQ= {m + m^i + /;)}«, (5) 



where m is the mass of the body. The whole is subject to 

 1I1. restriction of small ujv and small acceleration, so 

 that the electric and magnetic fields sensibly travel with 

 the charge. Nor need F be constant in space or in time, 

 provided it does not vary too rapidly in relation to the 

 size of the conductor. In slow motion the magnetic 

 energy is the fraction ir'v 2 of that part of the electric 

 energy that depends upon the transverse electric force. 



\pril 3. Oliver Heavkide. 



Old Customs and Festivals. 

 My mother, now in her eightieth year, was led by a 

 recent article by Sir Norman Lockyer in Nature to relate 

 some reminiscences of some of the festivals formerly cele- 

 brated in Newton-on-Ayr. One of these seems to point 

 to ancient human sacrifices. In her mother's school-days, 

 the pupils of Newton-on-Ayr annually elected a king and a 

 queen on Candlemas Day. On " Pase Friday " (Good 



