The horizontal and vertical angles are indicated on the scales. Used in construction to ensure two points are level. The level is hooked to a string spanning two points. Ensure objects are perfectly vertical. The bobs hang from a string.
Gravity ensures they hang straight down.
Plumb bobs have been used in surveying and building since Egyptian times and maybe earlier. Measures vertical angles. This level sits on a plane table which is adjusted to the horizontal using the bubble. An object is sighted through the level and the vertical angle is read off the scale. Measures horizontal angles for marine navigation or surveys of coastlines.