This instrument is used to identify horizontal lines in topography and geodesy, the latter meaning the study of the shape and size of the Earth and the positions of points on or above its surface. It has a solid metal frame on which there are 2 eyepieces, with their horizontal lines of vision arranged so that each corresponds to the lens of the other. The upper section has a suspension point and plumb line, while the lower section has a silver arc on which degree increments are engraved. When the wire crosses the center of the scale, the eyepiece optical axes are perfectly horizontal.