"[E]lectro-magnetic measurements have been made from which we can calculate by dynamical principles the velocity of propagation of small magnetic disturbances in the supposed electro-magnetic medium. This velocity is great, from 288 to 314 millions of metres per second... Now the velocity of light, according to Foucault's experiments, is 298... But if the luminiferous and the electro-magnetic media occupy the same place, and transmit disturbances at the same velocity, what reason have we to distinguish the one from the other? By considering them as the same, we avoid at least the reproach of filling space twice over with different kinds of æther."