There can no longer be any doubt that light-waves consist of electric vibrations in the all -pervading ether, and that the latter possesses the properties of an insulator and a magnetic medium...From the standpoint of theoretical science it is perhaps even more important to be able to understand how apparent actions-at-a-distance really consist in a propagation of an action from one layer of an intervening medium to the next. Gravitation still remains an unsolved puzzle ; as yet a satisfactory explanation of it has not been forthcoming, and we are still compelled to treat it as a pure action-at-a distance." Helmholtz introducing Hertz's The Principles of Mechanics.
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