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.  The submarine was designed to accomplish a clear and definite purpose—­a secret under-water attack on an enemy’s ship in the vicinity.  It has succeeded so well in its limited mission that some intelligent people declare that we need submarines only—­ignoring the fact that, even if submarines could successfully prevent actual invasion, they could not carry on operations at a distance from their base of supplies.  It is true that submarines may be made so large that they can steam at great speed from place to place, as capital ships steam now, carry large supplies of fuel and food, house their crews hygienically, and need no “mother ship” or tender.  But if submarines achieve such size, they will be more expensive to build and run than battleships—­and will be, in fact, submersible battleships.  In other words, the submarine cannot displace the battleship, but may be developed and evolved into a new and highly specialized type of battleship.

The necessity for operating at long distances from a base carries with it the necessity for supplying more fuel than even a battleship can carry; and this means that colliers must be provided.  In most countries, the merchant service is so large that colliers can be taken from it, but in the United States no adequate merchant marine exists, and so it is found necessary to build navy colliers and have them in the fleet.  The necessity for continuously supplying food and ammunition to the fleet necessitates supply ships and ammunition ships; but the problem of supplying food and ammunition is not so difficult as that of supplying fuel, for the reason that they are consumed more slowly.

In order to take care of the sick and wounded, and prevent them from hampering the activities of the well, hospital ships are needed.  Hospital ships should, of course, be designed for that purpose before being constructed; but usually hospital ships were originally passenger ships, and were adapted to hospital uses later.

The menace of the destroyer—­owing to the sea-worthiness which this type has now achieved, and to the great range which the torpedo has acquired—­has brought about the necessity of providing external protection to the battleships; and this is supplied by a “screen” of cruisers and destroyers, whose duty is to keep enemy destroyers and (so far as is practicable) the submarines at a safe distance.