What two factors help the Martians overcome the gravitational differences between Mars & Earth?

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The atmosphere of the earth, we now know, contains far more oxygen or far less argon (whichever way one likes to put it) than does Mars’. The invigorating influences of this excess of oxygen upon the Martians indisputably did much to counterbalance the increased weight of their bodies. And, in the second place, we all overlooked the fact that such mechanical intelligence as the Martian possessed was quite able to dispense with muscular exertion at a pinch.

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