William Gilbert's Quadrant for measuring the variation in the magnetic pole (as opposed to true north). It consists of a plank of wood, about 24 by 16 inches, with semicircles drawn on it. In the centre is a brass post (seen in strange perspective, with the sharp pointed tip). A pointer (here at 45 degrees) is hinged about the base of the post. The oblong box in the middle of the semicircles is a compass. It is set at due magnetic north and true north can be calculated from the shadow cast by the sun from the brass post. The illustration was published in Gilbert's book De Magnete, first published in 1600.