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.
Perhaps the most important single invention ever made was the invention of gunpowder.

Why?
Because it put into the hands of man a tremendous force, compressed into a very small volume, which he could use instantaneously or refrain from using at his will.

Its first use was in war; and in war has been its main employment ever since.
War gives the best field for the activity of gunpowder, because in war, we always wish to exert a great force at a definite point at a given instant; usually in order to _penetrate_ the bodies of men, or some defensive work that protects them.

Gunpowder is the principal agent used in war up to the present date.