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222 day after day for weeks, disclosed a curious shift, which, though slight, was unmistakable. Upon thought the cause suggested itself, and on being subjected to calculation proved equal to such accounting. In this

singular systematic sway stood revealed the libration in longitude caused by the eccentricity of the planet's orbit.

Mercury revolves about the Sun in an ellipse more eccentric than that of any other principal planet. At times he is half as far off again from him as he is at others. When near, he travels faster than when far.