MONITOR (from Lat. monere, to warn, advise), an advisor or counsellor, one who warns another person as to his course of action, also used of things that are more or less personified, as conscience. The word is chiefly applied to senior pupils (also known as " prefects ") in some of the great secondary schools in England; in America to senior students in certain colleges to whom special duties are assigned, particularly that of keeping order; and also to pupil teachers in English elementary schools. It is used in a general way of anything that gives warning, and in this sense is applied to a lizard of the family Monitoridae, or Varanidae, found in Africa and Australia, which is supposed to give warning of the Approach of crocodiles. The name of monitor was also given to a particular kind of ironclad invented for the American navy by Captain John Ericsson (q.v.) in 1862, which had a very low freeboard and revolving gun-turrets. The letter of Ericsson to the assistant secretary of the navy, of the 20th of January 1862 (quoted in the Century Dictionary), gives the inventor's reason for the name. " The impregnable and aggressive character of this structure will admonish the leaders of the Southern Rebellion that the batteries on the banks of their rivers will no longer present barriers to the entrance of the Union forces. The ironclad intruder will thus prove a severe monitor to those leaders . . . ' Downing Street ' will hardly view with indifference this last ' Yankee notion,' this monitor." It is also the name of an ironclad railway truck used for carrying a big gun. In America the raised part of the roof of a railway carriage or omnibus in which the lights or ventilators are placed is known as a monitor roof or top. In mining the word is applied to a jointed nozzle which may be turned in all directions, and is used in hydraulic mining.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)