The longitude at sea, [electronic resource] : not to be found by firing guns, nor by the Most Curious Spring-Clocks or Watches. But the only true method for discovering that valuable secret by the sun, moon or stars, and an Exact Time-Keeper, with such Necessary Improvements, as have not yet been Describ'd by any other Person; and (with respect to the Term of any Ordinary Voyage) may properly be call'd a Perpetual Motion. Now Humbly Proposed, To the Consideration of the Publick, by Case Billingsley.