The _Warrior_, completed in 1861, was built entirely of iron, protected at vital points by armour-plating four and a half inches in thickness, which, at the time, was supposed to render her invulnerable. She was the precursor of a class of enormous fighting machines, which, however ungainly in appearance, have increased the sea-power of Britain to an incalculable extent. But, alas, for the four and a half inches of armour-plating! Developments in gunnery called for increased thickness of protective armour. The rivalry betwixt gun and armour-plate, keenly contested for years, has not yet been definitely settled; but when ships’ guns are actually in use weighing 110 tons and over, capable of throwing a shot of 1,800 lbs. with crushing effect a distance of twelve miles, and, on the other hand, when ships are to be found carrying twenty-four inches of protective iron and steel plating, it seems as if the climax had been nearly reached. In the meantime the insignificant-looking “torpedo destroyer” is coming to the front as one of the most formidable instruments of marine warfare. Although only about 200 feet long, with a displacement of perhaps 250 tons, they have yet a motive power of 5,000 to 6,000 horse-power, and a speed of from 25 to 35 knots an hour. Some of these destroyers are supposed to be strong enough to deal a death-blow to a first-class battle-ship, and all of them are swift enough to overhaul the fastest cruiser on the ocean. The estimation in which they are held by the Admiralty is apparent from the fact that already upwards of one hundred of them are in commission, and many more are being built. Twenty-five destroyers, it is said, can be built for the cost price of one battle-ship, and in actual warfare there would be exposed the same number of lives in fifteen destroyers as in one battle-ship.