Q. How does a spirit level work?

Answer

The liquid (and bubble) tube give the illusion of being cylindrical, but it isn't. It's slightly concave in shape. If the level is perfectly horizontal, the bubble will go toward the tube's center since its top will be somewhat higher there than at either end. The peak will be somewhat off-center if the level is not perfectly vertical. These vials have a bubble in them because they were only partially filled with liquid, typically a colorful spirit or alcohol. They're curved ever-so-slightly upward, allowing the bubble to rest comfortably in the middle at the highest point. The bubble moves away from the designated center at relatively small angles.