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(c) Gravitational field and matter together must satisfy the law

of the conservation of energy (and of impulse).

Finally, the general principle of relativity permits us to

determine the influence of the gravitational field on the course

of all those processes which take place according to known laws

when a gravitational field is absent i.e. which have already been

fitted into the frame of the special theory of relativity. In

this connection we proceed in principle according to the method

which has already been explained for measuring-rods, clocks and

freely moving material points.