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In lectures given during World War II at Harvard, Frank attributed to Mach himself the following graphic expression of Mach's principle:

"When the subway jerks, it's the fixed stars that throw you down."

In commenting on this formulation of the principle, Frank pointed out that Mach chose the subway for his example because it shows that inertial effects are not shielded (by the mass of the earth): The action of distant masses on the subway-rider's mass is direct and instantaneous. It

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