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Device for determining latitude. The metal bar is attached to the center of the disc. An observation of the rising of a known star is made and its location plotted using the bar against its known declination. The angle read off from the edge of the disc as pointed to by the bar gives the latitude. De Magnete, Magneticisque Corporibus, et de Magno Magnete Tellure (On the Magnet and Magnetic Bodies, and on That Great Magnet the Earth) is a scientific work published in 1600 by William Gilbert. De Magnete was influential because of the inherent interest of its subject matter, but also for the rigorous way in which Gilbert described his experiments and his rejection of ancient theories of magnetism. William Gilbert (May 24, 1544 - November 30, 1603) was an English physician, physicist and natural philosopher. He was the first to argue that the center of the Earth was iron, and he considered an important and related property of magnets was that they can be cut, each forming a new magnet with north and south poles. He invented the first electrical measuring instrument, the electroscope, in the form of a pivoted needle he called the versorium. He died in 1603 at the age of 59. His cause of death is thought to have been the bubonic plague.
