"Amongst the many other ingenious contrivances frequently alluded to in his book, Gilbert mentions the versorium, an iron needle moving freely upon a point, with which he was enabled to measure excited electricity. He is besides the inventor of "two most ingenious and necessarie Instruments for Sea men to find out thereby the latitude of any place upon sea or land, in the darkest night, that is without the helpe of Sunne, Moone or Starre." These instruments are described in Thomas Blunderville's quarto work entitled "The Theoriques of the seven Planets, shewing their diverse motions... printed at London 1602.""