The instrument, composed of an air-bubble level on a brass triangle, is used in measuring the slope of the flat supporting surface with respect to the horizontal plane.
One of the two sides of the triangle, which are at right angles to each other, 11 centimeters long and with a brass angle, acts as the base of the instrument.
The level, 8 cm in length, is assembled with one end near the vertex of one of the acute angles and is free to rotate around that end and can be set at any slope from 0 to 52 sexagesimal degrees. The other acts as an index on a circular scale divided into sexagesimal degrees and is equipped with a vernier giving the reading of 10 minutes.
If the slope of a flat surface with respect to the horizontal plane is less than 45 degrees, the slope is given by the angle by which the level must rotate in order to be horizontal when the base of the instrument is resting on that surface.
If the slope of the surface is more than 45 degrees, the slope is given by the complement of the angle by which the level must rotate in order to make it horizontal when the side normal to the base of the instrument is placed on the surface.