A breastwork monitor was a modification of the monitor, a warship which was first built in the United States in 1861, designed by John Ericsson and distinguished by the first rotating gun turret, designed by Theodore Timby. The modified design known as a breastwork monitor was introduced by Sir Edward Reed, the Chief Constructor of the Royal Navy between 1863 and 1870. The original monitors were very stable, and difficult to damage by gunfire, because of their very low freeboard. This, however, caused them to behave, some said, as a "half-tide rock", with the ever-present risk of being swamped in a sea should water gain access to the interior through hatches, turret bases or other openings in the deck.