History and Development
The spirit level (or bubble level) was invented by Frenchman Melchisedech Thevenot in approximately 1660.
His invention was what is commonly called a bull's eye level. This is a circular flat bottomed device with liquid under a slightly domed glass face.
The dome is not completely full of liquid, thus leaving a bubble, when the bubble is in the top dead centre of the dome it indicates it is level.
The earliest surviving examples date to the beginning of the 18th Century however it is highly likely that bull's eye levels were in use in France and elsewhere long before the turn of the century. They are still widely used today in surveying and machinery leveling.
In the 1920s, Henry Ziemann the founder of Empire Level, invented the modern level with a single vial. These vials, common on most ordinary levels today, feature a slightly curved tube which is partially filled with a liquid leaving a bubble of air. The liquid is usually a colored 'spirit' (derived from oil).