n.
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.