It is somewhat of a commonplace among writers upon the Art of War, that with it, as with Art in general, the leading principles remain unimpaired from age to age. When recognized and truly mastered, not held by a passive acquiescence in the statements of another, but really appropriated, so as to enter decisively into a man's habit of thought, forming in that direction the fibre of his mind, they not only illuminate conditions apparently novel, by revealing the essential analogies between them and the past, but they supply the clue by which the intricacies of the present can best be threaded. Nothing could be more utterly superficial, for instance, than the remark of a popular writer that "the days of tacks and sheets"--of sailing ships, that is--"have no value as lessons for the days of steam and armor." Contrast with such an utterance the saying of the great master of the art,--Napoleon: "If a man will surprise the secrets of warfare, let him study the campaigns of Hannibal and of Caesar, as well as those of Frederick the Great and my own." Comprehension of warfare, therefore, consists, first, in the apprehension and acceptance--the mental grasp--of a few simple general principles, elucidated and formulated by admitted authorities upon the subject, and, second, in copious illustration of these principles by the application of them to numerous specific instances, drawn from actual experiences of war--from history. Such illustration, adequately developed by exposition of facts and of principles in the several cases, pointing out, where necessary, substantial identity underlying superficial diversity, establishes gradually a body of precedents, which reinforce, by all the weight of cumulative authority, the principle that they illuminate. Thus is laid the substantial foundation upon which the Art of War securely rests. It is perhaps advisable--though it should be needless--to say that, when a student has achieved such comprehension, when his mind has mastered the principles, and his memory is richly stored with well-ordered precedents, he is, in war, as in all other active pursuits of life, but at the beginning of his labors. He has girded on his armor, but he has not yet proved it,--far less is qualified to boast as one about to put it off after a good life's fight. It remains yet to be seen whether he has the gifts and the manhood to use that which he has laboriously acquired, or whether, as happens with many other men apparently well qualified, and actually well furnished with the raw material of knowledge in various professions, he will be unable to turn power into success. This question trial alone can decide in each individual case; but while experience thus forces all to realize that knowledge does not necessarily imply capacity to use it, that there may be foundation upon which no superstructure will be raised, few--and those not the wisest--are inclined to dispute that antecedent