The question of the Earth's magnetic field and, in particular, magnetic declination was addressed in Book 2 of Magnes. It was one of great interest in Kircher's time, because it was thought that an understanding of it would help resolve the problem of longitude. The Jesuit order had scholars working in many different parts of the world and was thus able to collect observations on declination. Kircher collected forty-three of these and combined them with observations from a network of other scholars including Mersenne to produce three tables and two lists with a total of 518 values. He proposed that a magnetic map of the Earth could be made with this data, although he did not attempt to do this himself.