For this reason a battleship needs other protection than that imparted by its cellular subdivision.
This is given by its "torpedo defense battery" of minor guns of about 5-inch calibre.
By reason of the great vulnerability of all ships to attack below the water-line, the torpedo was invented and developed.

In its original form, the torpedo was motionless in the water, either anchored to the ground, or floating on the surface, and was in fact what now is called a "mine." But forty-eight years ago an Englishman named Whitehead invented the automobile, auto-steering, torpedo, which still bears his name.

This torpedo is used in all the navies, and is launched on its mission from battleships, battle cruisers, destroyers, submarines, and other craft of various kinds.
Most torpedoes are to be found in destroyers--long, fast, frail vessels, averaging about 700 tons displacement, that are intended to dash at enemy ships at night, or under other favorable conditions, launch their torpedoes, and hurry away.

The torpedo is "a weapon of opportunity." It has had a long, slow fight for its existence; but its success during the present war has established it firmly in naval warfare.
The submarine has followed the destroyer, and some people think will supplant it; though its relatively slow speed prevents those dashes that are the destroyer's role.

The submarine is, however, a kind of destroyer that is submersible, in which the necessities of submersibility preclude great speed.