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.