At the time (1542) when Jean Rotz presented Henry VIII with a variation compass, and an accompanying treatise1 on its making and use, the question of magnetic variation was an acute one. It formed part of a wider controversy concerning the general validity of the sea-chart which had been raised among seamen. As to this, Dr. Pedro Nuñez, writing a treatise in 1537 addressed to his friend and pupil the Infante Dom Luys of Portugal, said that there were skilled pilots who derided the chart and declared it to be ‘a mais falsa causa do mundo’—the falsest thing in the world. Hastening to its defence, Nuñez dealt at length with the navigating errors introduced by ignoring the convergence of the meridians, and showed besides that a rhumb, or line of constant bearing, was a spiral curve on the globe. He dealt only incidentally with the variation of the compass needle and how to measure it, and this may explain why Rotz made this second source of error his main theme. The parallel N.–S. lines and E.–W. lines of the compass roses making a rectangular net on the chart are completely misleading, he says, for only the ‘diametral line’ (agonic line, line of no variation) actually runs through the poles of the world, the others lie at angles one with the other according to the magnetic variation.