-axis with respect to each other. This group of so-called Galilei-transformations is distinguished, even in the case in which the direction of motion makes any angle with the co-ordinate axes, by the circumstance that the time-co-ordinate
always becomes transformed by the identity
into the time-values of the second system of reference; it is in this that the absolute character of the time-measures manifests itself in the classical theory. Newton's equations of mechanics do not alter their form if we substitute the co-ordinates
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