A monitor is a relatively small warship that is neither fast nor strongly armored but carries disproportionately large guns. They were used by some navies from the 1860s, during the First World War and with limited use in the Second World War.
USS Monitor, the first monitor (1861)
HMS Marshal Ney used a surplus 15-inch gun battleship turret.
Officers of a Union monitor, probably USS Sangamon, photographed during the American Civil War
Huáscar anchored in the harbour at Talcahuano
John Ericsson was a Swedish-American inventor. He was active in England and the United States.
John Ericsson
John Ericsson
Novelty, Braithwaite and Ericsson's entry for the Rainhill Trials. Illustration from The Mechanics Magazine, 1829.
German drawing (1833) of the steam locomotive Wilhelm IV with scale in feet, built by "Braithwaite und Ericsson".