I believe it is an Inclinometer. The leveling bubble is a way to either find an angle or create an angle. You had two ways of using it. You could put the flat bottom edge on a surface and then rotate the arm until the bubble reached the center of the tube (which would read out on the gauge your angle); or you could set the angle and lock it at the correct angle then place the edge down and rotate the surface until it reached the proper angle you are needing. These tools are very handy in any type of surveying projects. You can by digital inclinometer or digital protractors that work very well and are .00 accurate.