_Controllability_.--If one watches a fleet of ships moving on the sea, he gets an impression of tremendous power. But if he watches Niagara, or a thunder-storm, he also gets an impression of tremendous power. But the tremendous power of Niagara, or the thunder-storm, is a power that belongs to Niagara or the thunder-storm, and not to man. Man cannot control the power of Niagara or the thunder-storm; but he can control the power of a fleet. Speaking, then, from the standpoint of the human being, one may say that the fleet has the element of controllability, while Niagara and the thunder-storm have not. One man can make the fleet go faster or slower or stop; he can increase its power of motion or decrease it at his will; he can reduce it to zero. He cannot do so with the forces of nature. _Directability_.--Not only can one man control the power of the fleet, he can also direct it; that is, can turn it to the right or the left as much as he wishes. But one man cannot change the direction of motion of Niagara or the lightning-bolt. _Power, Controllability, and Directability_.--We may say, then, that a fleet combines the three elements of mechanical power, controllability, and directability. _The Unit of Military Power_.--This is an enormous power that has come into the hands of the naval nations; but it has come so newly that we do not appreciate it yet. One reason why we do not and cannot appreciate it correctly is that no units have been established by which to measure it. To supply this deficiency, the author begs leave to point out that, since the military power of every nation has until recently been its army, of which the unit has been the soldier, whose power has rested wholly in his musket, the musket has actually been the unit of military power. In all history, the statement of the number of men in each army has been put forward by historians as giving the most accurate idea of their fighting value; and in modern times, nearly all of these men have been armed with muskets only. It has been said already that the main reason why the invention of gunpowder was so important was that it put into the hands of man a tremendous mechanical power compressed into a very small space, which man could use or not use at his will. This idea may be expressed by saying that gunpowder combines power and great controllability. But it was soon discovered that this gunpowder, put into a tube with a bullet in front of it, could discharge that bullet in any given direction. A musket was the result, and it combined the three requisites of a weapon--mechanical power, controllability, and directability. While the loaded gun is perhaps the clearest example of the combination of the three factors we are speaking of, the moving ship supplies the next best example. It has very much greater mechanical power; and in proportion to its mass, almost as much controllability and directability.