The inclinometer is an instrument for measuring the inclination, ie the angle that the north pole direction of a magnetic needle, free to move around a horizontal axis and laying in the plane of a magnetic meridian, forms with the horizontal plane.
The needle is horizontal only at the magnetic equator, while in every other place on earth takes an angle to the horizontal plane: down (up) in the northern (southern) hemisphere with a value of the angle that increases with latitude and is vertical at the magnetic poles.
Usually the inclinometer has a sharp needle, located within a vertical ring having a diameter between 15 and 25 cm, and this one can turn onto two agate flats, or, if the instrument is for use on ships, onto two agate cups.
The scale originally was inside the box, then was placed out of it with two vernier microscopes, thereby providing a better reading. In some instruments, the frame supporting the needle is vertical, in others horizontal, making the former more effective in the equatorial areas and the second in polar ones.
The rack is provided with a device to raise or lower the needle from its supports. The vertical ring, arranged as an alidade, rotates about a lower horizontal ring, which rests on three screws. The latter ring may have an inclinometer or two spirit levels.
The instrument provides directly the inclination only if the needle swings in a vertical plane, which coincides with the magnetic meridian, since only in this plane the instrument is sensitive to the total magnetic intensity. In other planes it is necessary to perform at least two measurements: either in two vertical planes whose azimuths are mutually perpendicular, or two planes, one horizontal and the other one vertical and perpendicular to the magnetic meridian; the latter method requires however a different instrument. In both cases, the tilt value need be calculated and is not given directly.
The discovery of inclination, in 1544, is due to Hartmann: he was performing measurements of declination, and noticed that the North needle tilts below the horizon. The first instrument is due to Robert Norman and the first report of a measurement dates to 1576.
The reason why the inclination was discovered so late compared to the declination is also due to the fact that the first compasses were floating. Only when they were placed on a pin and made perfectly horizontal it was possible to grasp that their inclination was not due to a flaw of the suspension, but to the existence of a vertical component of Earth's magnetic field.
In Italy the first measurement dates to 1640 in Rome, done by Athanasius Kircher (1601-1680); only a few measurements are known in the eighteenth century, they became systematic only in the nineteenth century.
The reason for doing regular measurements so late is probably due to the little practical use attributed to this type of measure, that resulted in the determination of latitude, that was also calculated with the sextant. Only when the measurement became useful to study Earth's geophysics, then it became more common.