magnes sive de arte magnetica

Athanasius Kircher – Instrument for Measuring Magnetic Inclination, “Magnes, sive De Arte Magnetica”, 1643.

Magnetic inclination is the angle made with the horizontal by the Earth’s magnetic field lines. This angle varies at different points on the Earth’s surface. Positive values of inclination indicate that the magnetic field of the Earth is pointing downward, into the Earth, at the point of measurement, and negative values indicate that it is pointing upward.