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Now in virtue of its motion in an orbit round the sun, our earth

is comparable with a railway carriage travelling with a velocity

of about 30 kilometres per second. If the principle of relativity

were not valid we should therefore expect that the direction of

motion of the earth at any moment would enter into the laws of

nature, and also that physical systems in their behaviour would

be dependent on the orientation in space with respect to the

earth. For owing to the alteration in direction of the velocity

of revolution of the earth in the course of a year, the earth

cannot be at rest relative to the hypothetical system K0

throughout the whole year. However, the most careful observations

have never revealed such anisotropic properties in terrestrial

physical space, i.e. a physical non-equivalence of different

directions. This is very powerful argument in favour of the

principle of relativity.

VI.

THE THEOREM OF THE ADDITION OF VELOCITIES EMPLOYED IN CLASSICAL

MECHANICS