Kircher was not the first scholar to write about magnetism. In 1600, William Gilbert published De Magnete ('On the Magnet'), the first modern treatise on magnetism and in 1635, Henry Gellibrand first discovered that magnetic declination changes with time. Among Jesuit scholars, Leonardo Garzoni wrote Trattati della Calamita ('Treatise on the Lodestone') (around 1580) which described the double polarity of magnets. Niccolò Cabeo used Garzoni's manuscript as the basis for his own Philosophia Magnetica. Kircher cited Gilbert, Garzoni and Cabeo in Magnes sive de Arte Magnetica. Magnetism was an important theme in Kircher's 1664 work Mundus Subterraneus as well as his 1667 book Magneticum Naturae Regnum. Many of the magnetic machines lavishly illustrated in the work were later built and displayed in Kircher's museum in Rome.