346 POPULAR SCIENCE MONTHLY 



are necessary; it is necessary to admit that bodies in motion undergo 

 a uniform contraction in the sense of the motion. One of the diame- 

 ters of the earth, for example, is shrunk by one two-hundred-mil- 

 lionth in consequence of our planet's motion, while the other diameter 

 retains its normal length. Thus the last little differences are com- 

 pensated. And then, there is still the hypothesis about forces. 

 Forces, whatever be their origin, gravity as well as elasticity, would 

 be reduced in a certain proportion in a world animated by a uniform 

 translation; or, rather, this would happen for the components perpen- 

 dicular to the translation; the components parallel would not change. 

 Eesume, then, our example of two electrified bodies; these bodies repel 

 each other, but at the same time if all is carried along in a uniform 

 translation, they are equivalent to two parallel currents of the same 

 sense which attract each other. This electrodynamic attraction dimin- 

 ishes, therefore, the electrostatic repulsion, and the total repulsion is 

 feebler than if the two bodies were at rest. But since to measure this 

 repulsion we must balance it by another force, and all these other 

 forces are reduced in the same proportion, we perceive nothing. Thus, 

 all seems arranged, but are all the doubts dissipated? What would 

 happen if one could communicate by non-luminous signals whose 

 velocity of propagation differed from that of light? If, after having 

 adjusted the watches by the optical procedure, we wished to verify the 

 adjustment by the aid of these new signals, we should observe dis- 

 crepancies which would render evident the common translation of the 

 two stations. And are such signals inconceivable, if we admit with 

 Laplace that universal gravitation is transmitted a million times more 

 rapidly than light? 



Thus, the principle of relativity has been valiantly defended in 

 these latter times, but the very energy of the defense proves how serious 

 was the attack. 



Newton's Principle. — Let us speak now of the principle of New- 

 ton, on the equality of action and reaction. This is intimately bound 

 up with the preceding, and it seems indeed that the fall of the one 

 would involve that of the other. Thus we must not be astonished to 

 find here the same difficulties. 



Electrical phenomena, according to the theory of Lorentz, are due 

 to the displacements of little charged particles, called electrons, im- 

 mersed in the medium we call ether. The movements of these elec- 

 trons produce perturbations in the neighboring ether; these perturba- 

 tions propagate themselves in every direction with the velocity of light, 

 and in turn other electrons, originally at rest, are made to vibrate 

 when the perturbation reaches the parts of the ether which touch them. 

 The electrons, therefore, act on one another, but this action is not 

 direct, it is accomplished through the ether as intermediary. Under 



