Position of the air-sea boundary, to which all terrestrial elevations and submarine depths are referred. The sea level at any location changes constantly with changes in tides, atmospheric pressure, and wind conditions. Longer-term changes are influenced by changes in the Earth's climates. Consequently, the level is better defined as mean sea level, the height of the sea surface averaged over all stages of the tide over a long period of time.
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Device for establishing a horizontal plane. It consists of a small, sealed glass tube containing liquid and an air bubble; the tube is fixed horizontally in a block or frame with a smooth lower surface. When the bubble is in the middle of the glass tube, the device is on a level surface; adjustment to the horizontal is indicated by movement of the bubble. The glass tube is slightly bowed, and the level's sensitivity is proportional to the radius of curvature.
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