device for establishing a horizontal plane. It consists of a small glass tube containing alcohol or similar liquid and an air bubble; the tube is sealed and fixed horizontally in a wooden or metallic block or frame with a smooth lower surface. The glass tube is slightly bowed, and adjustment to the horizontal is indicated by movement of the bubble. The device is on a level surface when the bubble is in the middle of the glass tube. The level's sensitivity is proportional to the radius of the curvature of the glass.