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motion; that is, moving in a straight line with equal velocity.
"Gradually I was led to the idea, seeming a very paradox in science, that it might apply equally to all moving systems, even of difform motion, and thus I developed the conception of general relativity which forms the second part of my theory."
As summarized by an American astronomer, Professor Henry Norris Russell, of Princeton, in the Scientific American for November 29, Einstein's contribution amounts to this:
"The central fact which has been proved — and which is of great interest and importance — is that the natural phenomena involving gravitation and inertia (such as the mo-
