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498 . This will doubtless lead to further experimental tests; and even apart from direct tests, one may regard the evidence for the principle as being strengthened if it introduces simplification and harmony into the theory of phenomena which are apparently remote from those that led originally to its adoption. As the dimensions of all bodies are altered by motion through the ether, it is plain that such motion must be taken into account in the exact theory of even purely dynamical phenomena. As such applications are not very familiar and present some points of interest, it seems not altogether superfluous to consider a few very simple dynamical cases from this point of view. The Torsion Pendulum. Suppose a bar of length $$\scriptstyle{\text{L}}$$ (when at rest) hung up by a torsion wire in the ordinary way. Let the apparatus be carried by the earth through the ether with the velocity $$\scriptstyle{v}$$ in a direction perpendicular to the wire; and let us consider the period of the pendulum when the bar is clamped to the wire in two different positions: (1) with its length perpendicular to the earth's motion, and (2) parallel to the direction of motion. By the principle of relativity the two periods must be equal. As the length of the bar in the first position is $$\scriptstyle{\text{L}}$$ and in the second position $$\scriptstyle{\sqrt{1-\beta^{2}}\text{L}}$$, it appears at first sight that the mass of every particle of the bar should be greater in position (2) when it is moving perpendicularly to the earth's motion than in (1) when it is moving parallel to it. This would make the