Lightships guided ships of all nations past offshore reefs and sand banks that posed extreme hazards to navigation, where lighthouses could not be built. This book describes their design, construction and evolution over more than two centuries of service. Originally constructed with of and teak, they progressed through iron hulls to steel; their lights burnt whale oil, colza oil, then paraffin before electricity gave the greatest brightness. Fog warnings were given by bells, gongs, reed horns and sirens before reaching maximum power with the diaphone, which could be heard for up to ten miles. Among the illustrations here are examples of these features, most of which resulted from major scientific and engineering advances made in the nineteenth century. By their efforts the sea lanes around Great Britain were made far safer for navigators.