Concentrated forces, therefore, are those upon which warfare depends for efficient control, and for efficient energy in the operations of war. They have two chief essential characteristics: force, which is gained by concentration of numbers; and mobility, which is the ability to carry the force rapidly, as well as effectively, from the centre to any point of the outlying field where action, offensive or defensive, becomes necessary. It is essential to keep in mind both these factors, and to study them in their true mutual relations of priority, in order and in importance,--force first, mobility second; for the force does not exist for the mobility, but the mobility for the force, which it subserves. Force without mobility is useful; even though limited, as in coast fortifications; mobility without force is almost useless for the greater purposes of war. Consequently, when it is found, as is frequently the case, that one must yield somewhat, in order to the full development of the other, it is extreme mobility, extreme speed, which must give way to greater force.
This caution may seem superfluous, but it is not so; for in the popular fancy, and in the appreciation of the technical expert, and to some extent also in the official mind as well,--owing to that peculiar fad of the day which lays all stress on machinery,--mobility, speed, is considered the most important characteristic in every kind of ship of war. Let the reader ask himself what is the most pronounced impression left upon his mind by newspaper accounts of a new ship. Is it not that she is expected to make so many knots? Compared with that, what does the average man know of the fighting she can do, when she has reached the end of that preposterously misleading performance called her trial trip? The error is of the nature of a half-truth, the most dangerous of errors; for it is true that, as compared with land forces, the great characteristic of navies is mobility; but it is not true that, between different classes of naval vessels, the swiftest are the most efficient for control of the sea. Force is for that the determining element.
Keeping these relations of force and mobility constantly in mind, there is a further consideration, easily evident, but which needs to be distinctly stated and remembered. When a ship is once built, she cannot be divided. If you have on land concentrated ten thousand men, you can detach any fraction of them you wish for a particular purpose; you can send one man or ten, or a company, or a regiment. You can, in short, make of them any fresh combination you choose. With ships, the least you can send is one ship, and the smallest you have may be more than you wish to spare. From this (as well as for other reasons) arises a necessity for ships of different classes and sizes, which must be determined beforehand. The determination must be reached not merely by _a priori_ reasoning, as though the problem were wholly new; but regard must be had to the experience of the past,--to the teaching of history. History is experience, and as such underlies progress, just as the cognate idea, experiment, underlies scientific advance.