It is an instrument used for measuring the magnetic inclination (which is to say the angle which at equilibrium is formed with the horizontal plane by a magnetic needle allowed to rotate on the plane of the magnetic meridian of the location, around a horizontal axis passing through its barycentre).
The instrument presented here is all made of brass except for the magnetic needle and is composed of an azimuthal circle graduated from 0 to 360 degrees and a vertical circle for measuring inclination which is divided into four quadrants, each of which graduated from 0 to 90 degrees. There is a small air bubble level on the azimuthal circle.
By means of a small steel transversal pin passing through its barycentre, the magnetic needle is placed on two horizontal blades so that the pin, on which the needle is free to turn, is perpendicular to the plane of the vertical graduated circle and passes through its centre.
The entire instrument is fixed to a sturdy tripod with levelling screws by means of a vertical support in the shape of a truncated cone which fits with negligible attrition into a seat in the centre of the tripod.
The instrument lacks the fork needed to raise the magnetic needle when not in use and for placing it exactly in the centre when its use is required.
Daguin (1878), T. III, p. 69
Drion-Fernet (1883), p. 434
Ganot (1864), p. 491
Segnini-Vergara Caffarelli (1990), p. 160