7. Maritime commerce is much less assailable than in former times, because the introduction of steam has confined its course to definite trade routes of extremely narrow width, and has almost denuded the sea of commerce outside these limits.
8. The modern commerce destroyer is confined to a comparatively narrow radius of action by the inexorable limits of her coal supply. If she destroys her prizes she must forgo the prize money and find accommodation for the crews and passengers of the ships destroyed. If she sends them into port she must deplete her engine-room complement and thereby gravely impair her own efficiency.
9. Torpedo craft are of little or no use for commerce destruction except in certain well-defined areas where special measures can be taken for checking their depredations.
Of course all this depends on the one fundamental assumption that the commerce to be defended belongs to a Power which can, and does, command the sea. On no other condition can maritime commerce be defended at all.
CHAPTER VIII
THE DIFFERENTIATION OF NAVAL FORCE
A warship, considered in the abstract, may be defined as a vessel employed, and generally constructed, for the purpose of conveying across the seas to the place of conflict, the weapons that are to be used in conflict, the men who are to use them, and all such stores, whether of food or other supplies, as will give to the vessel as large a measure of enduring mobility as is compatible with her displacement. If we confine our attention to the period posterior to the employment of the gun on shipboard as the principal weapon of offence, and if we regard the torpedo as a particular kind of projectile, and the tube from which it is discharged as a particular kind of gun, we may condense this definition into the modern formula that a warship is a floating gun-carriage. With the methods and implements of sea warfare anterior to the introduction of the gun we need not concern ourselves. They belong to the archæology of the subject. It suffices to point out that in all periods of naval warfare the nature of the principal weapon employed, and to some extent that of the motive power available, have not only governed the structure of the ship and determined the practicable limit of its displacement, but have also exercised a dominant influence over the ordering of fleets and their disposition in action. Sea tactics have never been more elaborate than they were in the last days of the galley period which came to an end with the Battle of Lepanto in 1571, less than a score of years before the defeat of the Armada in 1588. But the substitution of sails for oars as the motive power of the warship and the more general employment of the gun as the principal weapon of offence necessarily entailed radical changes in the tactical methods which had been slowly evolved during the galley period. At first all was confusion and a sea-fight was reduced for a time to a very disorderly and tumultuous affair. "We went down in no order," wrote an officer who was present at Trafalgar, "but every man to take his bird." This is a very inaccurate and even more unintelligent account of the tactics pursued at Trafalgar; but it might very well stand for a picturesque summary of the tactical confusion which prevailed at the period of the Armada and for half a century afterwards.
Gradually, however, order was again evolved out of the prevailing chaos. But it was not the old order. It was a new order based on the predominance of the gun and its disposition on board the ship. To go down in no order and for each man to take his bird would mean that each ship, whether large or small, would be free as far as circumstances permitted to select an adversary not disproportioned in strength to herself, so that there was no very pressing need for the fleet to consist of homogeneous units, nor for the elimination of comparatively small craft from a general engagement. But in the course of the Dutch Wars the practice was slowly evolved of fighting in a compact or close-hauled line, the ships being ranged in a line ahead—that is, each succeeding ship following in the wake of her next ahead—in order to give free play to the guns disposed mainly on the broadside, and being, for purposes of mutual support, disposed as closely to each other as was compatible with individual freedom of evolution and manœuvre. This disposition necessarily involved the exclusion from the line of battle of all vessels below a certain average or standard of fighting strength, since it was no longer possible for "every man to take his bird" and a weak ship might find herself in conflict with an adversary of overpowering strength in the enemy's line. Hence the main fighting forces of naval belligerents came in time to be composed entirely of "ships fit to lie in a line," as Torrington phrased it, of "capital ships," as they were frequently called in former days, of "line of battle ships" or "ships of the line," as afterwards they were more commonly called, or of "battleships" as is nowadays the accepted appellation. Other elements of naval force not "fit to lie in a line" were also required, as I am about to show, and took different forms at different times, but the root of the whole evolution lies in the elimination of the non-capital ship from the main fighting line. In a very instructive chapter of his Naval Warfare, Admiral Colomb has traced the whole course of this gradual "Differentiation of Naval Force." But for my purpose it suffices to cite the briefer exposition of a French writer quoted by Admiral Mahan in his Influence of Sea Power upon History:—
"With the increase of the power of the ship of war, and with the perfecting of its sea and warlike qualities, there has come an equal progress in the art of utilizing them.... As naval evolutions become more skilful, their importance grows from day to day. To these evolutions there is needed a base, a point from which they depart and to which they return. A fleet of warships must always be ready to meet an enemy; logically, therefore, this point of departure for naval evolutions must be the order of battle. Now since the disappearance of galleys, almost all the artillery is found upon the sides of a ship of war. Hence it is the beam that must necessarily and always be turned toward the enemy. On the other hand it is necessary that the sight of the latter must never be interrupted by a friendly ship. Only one formation allows the ships of the same fleet to satisfy fully these conditions. That formation is the line ahead. The line, therefore, is imposed as the only order of battle, and consequently as the basis of all fleet tactics. In order that this line of battle, this long thin line of guns, may not be injured or broken at some point weaker than the rest, there is at the same time felt to be the necessity of putting in it only ships which, if not of equal force, have at least equally strong sides. Logically it follows, at the same moment in which the line ahead became definitely the order for battle, there was established the distinction between the 'ships of the line' alone destined for a place therein, and the lighter ships meant for other uses."