The spirit level was invented by Melchisedech Thevnot, who was a wealthy Frenchman that served as a royal librarian for King Louis XIV. But he was also an amateur mathematician and scientist. He noticed that for a bubble inside a tube of water to position itself at the exact center, it needed to be on a surface that was exactly horizontal. Improvements were made to this principle since its discovery in 1661, but it wasn’t until the early 18th Century that the spirit level experienced widespread use.