A spirit or bubble level, often just called a level, is a measuring instrument that indicates whether an underlying surface is flat (“level”) or angled. The tool was invented sometime before 1661 by the French scientist Melchisédec Thévenot. It works using a bubble of air entrapped within a tubular or circular transparent container otherwise filled with liquid, typically alcoholic spirit, hence its name. The bubble’s position demonstrates the angle of the surface upon which the instrument is placed; if the surface is flat, the bubble floats in the middle, but moves to the left or right if the surface is angled.
