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 Recent Researches on Space, Time, and Force.

Celestial mechanics, which has hitherto been based on the Newtonian laws of motion, is profoundly affected by the discoveries which have been made in recent years regarding measurements of space, time, and force.

One of the oldest and most perplexing questions regarding space is whether any definite meaning can be attached to the terms "absolute rest" and "absolute velocity." Newton in the laws of motion speaks of "rest" and "motion" relative to some presupposed frame of reference, but this frame of reference need not itself be absolutely at rest, for Newton’s laws are valid if the frame of reference is any one of an infinite number of frames which have uniform velocities of translation relative to each other. It is therefore hopeless to look to purely dynamical considerations for guidance in the recognition of absolute rest.

Throughout the nineteenth century it was thought that absolute motion in space, or at any rate absolute motion relative to the general body of stars, could be determined by the astronomical study of proper motions; and the Sun was supposed to have "an absolute velocity of about fifteen miles a second towards a point in the constellation Hercules." It is needless to relate here how this result has been overthrown by the labours of Kapteyn. But even when the failure of dynamics and astronomy to reveal absolute motion was admitted, it was still hoped that the solution might be found by aid of the theories of light and electricity. For suppose a luminous or electro-magnetic disturbance to be set up at a definite