If commercial sea power was not the power meant by Mahan, then he must have meant naval power. And if one reads the pages of history with patient discrimination, the conviction must grow on him that what really constituted the sea power which had so great an influence on history, was _naval_ power; not the power of simply ships upon the sea, but the power of a navy composed of ships able to fight, manned by men trained to fight, under the command of captains skilled to fight, and led by admirals determined to fight. Trafalgar was not won by the merchant service; nor Mobile, Manila, or Tsushima. If sea power be essentially naval power, it may be interesting to inquire: In what does naval power consist and what are its principal characteristics? If one looks at a fleet of war-ships on the sea, he will be impressed consciously or unconsciously with the idea of power. If he is impressed consciously, he will see that the fleet represents power in the broadest sense--power active and power passive; power to do and power to endure; power to exert force and power to resist it. If he goes further and analyzes the reasons for this impression of power, he will see that it is not merely a mental suggestion, but a realization of the actual existence of tremendous mechanical power, under complete direction and control. In mechanics we get a definition of power, which, like all definitions in mechanics, is clear, definite, and correct. In mechanics, power is the rate at which mechanical work is performed. It is ability to do something in a certain definite time. Now this definition gives us a clear idea of the way in which a navy directly represents power, because the power which a navy exerts is, primarily, mechanical; and any other power which it exerts is secondary and derived wholly from its mechanical power. The power of a gun is due wholly to the mechanical energy of its projectile, which enables it to penetrate a resisting body; and the power of a moving ship is due wholly to the mechanical energy of the burning coal within its furnaces. It may be objected that it is not reasonable to consider a ship's energy of motion as an element of naval power, in the mechanical sense in which we have been using the word "power," for the reason that it could be exerted only by the use of her ram, an infrequent use. To this it may be answered that energy is energy, no matter to what purpose it is applied; that a given projectile going at a given speed has a certain energy, whether it strikes its target or misses it; and that a battleship going at a certain speed must necessarily have a certain definite energy, no matter whether it is devoted to ramming another ship or to carrying itself and its contents from one place to another. Besides the mechanical power exerted by the mere motion of the ship, and often superior to it, there is the power of her guns and torpedoes.