For a long time physicists have admitted that the ether as a whole
must be considered as being immovable and capable of serving, so to
speak, as a support for the axes of Galileo, in relation to which axes
the principle of inertia is applicable,--or better still, as M.
Painleve has shown, they alone allow us to render obedience to the
principle of causality.
But if it were so, we might apparently hope, by experiments in
electromagnetism, to obtain absolute motion, and to place in evidence
the translation of the earth relatively to the ether. But all the
researches attempted by the most ingenious physicists towards this end
have always failed, and this tends towards the idea held by many
geometricians that these negative results are not due to imperfections
in the experiments, but have a deep and general cause. Now Lorentz has
endeavoured to find the conditions in which the electromagnetic theory
proposed by him might agree with the postulate of the complete
impossibility of determining absolute motion. It is necessary, in
order to realise this concord, to imagine that a mobile system
contracts very slightly in the direction of its translation to a
degree proportioned to the square of the ratio of the velocity of
transport to that of light.
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