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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. This entry comes from
Encyclopædia Britannica Concise. For the full entry on level,
visit Britannica.com.