The Chinese used a gravity meter to correct for local deviations in sea level.
中国人用重力仪来校正海平面的局部偏差。
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Normal swells, known technically as gravity waves, are created by wind pushing the surface of the sea up and gravity then pulling it down, causing it to bounce.
They say that well before dinosaurs or, really, much of anything roamed the earth, rivers were just vast expanses of water heeding only gravity and heading straight to sea.