Intelligent readers of this little Manual will perceive at once that it pretends to be nothing more than an introduction, quite elementary in character, to the study of naval warfare, its history, and its principles as displayed in its history. As such, I trust it may be found useful by those of my countrymen who desire to approach the naval problems which are constantly being brought to their notice and consideration with sound judgment and an intelligent grasp of the principles involved in their solution. It is the result of much study and of a sustained intimacy with the sea service, both afloat and ashore, such as few civilians have been privileged to enjoy in greater measure. Even so, I should have thought it right, as a civilian, to offer some apology for undertaking to deal with so highly technical and professional a subject, were I not happily relieved of that obligation by the kindness of my friend Rear-Admiral Sir Charles L. Ottley, who has, at the instance of the Editors of this series, contributed to this volume an Introduction in which my qualifications are set forth with an appreciation which I cannot but regard as far too flattering. It would ill become me to add a single word—unless it were of deprecation—to credentials expounded on such high authority.

I should hope that readers who have found this volume useful to them will not confine their studies to it. Abundant materials for a deeper and more comprehensive study of the subject will be found in the several works incidentally mentioned or quoted in my text, and in the writings of those other contemporary authors with whom Sir Charles Ottley has done me the high honour to associate myself. In these several works further guidance to a still more sustained study of the subject will be found, and in this regard I would specially mention the admirable Short History of the Royal Navy, by Mr David Hannay—two volumes which, in addition to their other and more conspicuous merits, contain a well-selected list of authorities to be consulted prefixed to each chapter. These references, which in truth cover the whole subject, will, I trust, better serve the purpose of the advanced or advancing student than any such Bibliography as I could compile on a scale commensurate with the form and purpose of the present Manual.

Readers of my other writings on naval topics will, perhaps, observe that in one or two cases, where the same topics had to be discussed, I have not hesitated to reproduce, with or without modification, the language I had previously employed. This has been done deliberately. The topics so treated fell naturally and, indeed, necessarily within the scope of the present volume. To exclude them because I had discussed them elsewhere was impossible. Wherever I found I could improve the language previously employed in the direction of greater lucidity and precision I have done so to the best of my ability, so that the passages in question are close paraphrases rather than mere transcripts of those which occur elsewhere. But I have not attempted to disguise or weaken by paraphrase any passages which still seemed to me to convey my meaning better than any other words I could choose.

Changes in the methods, though not in the principles, of naval warfare are in these days so rapid and often so sudden that one or two topics have emerged into public prominence even since the present volume was in type. I desire therefore to take this opportunity of adding a few supplementary remarks on them. The first, and possibly in the long run the most far-reaching of these topics, is that of aviation, which I have only mentioned incidentally in the text. That aviation is still in its infancy is a truism. But to forecast the scope and direction of its evolution is as yet impossible. For the moment it may perhaps be said that its offensive capacity—its capacity, that is, to determine or even materially to affect the larger issues of naval warfare—is inconsiderable. I say nothing of the future, whether immediate or remote. Any day may witness developments which will give entirely new aspects to the whole problem. In the meanwhile the chief functions of aircraft in war will probably be, for some time to come, those of scouting, observation, and the collection and transmission of intelligence not obtainable by any other means. Offensive functions of a more direct and formidable character will doubtless be developed in time, and may be developed soon; but as I am no prophet I cannot attempt to forecast the direction of the evolution, to determine its limits, or to indicate its probable effects on the methods of naval warfare as expounded in the following pages. I will, however, advance two propositions which will not, I believe, be gainsaid by competent authorities. They are true for the moment, though how long they may remain true I do not know. One is that no aircraft yet constructed can take or keep the air in all conditions of weather. The number of days in the year in which it can do so in safety can only be represented by the formula 365-x, in which x is as yet an unknown quantity, though it is no doubt a quantity which will diminish as the art of aviation is developed. The other is that there is as yet no known method of navigating an aircraft with accuracy and precision out of sight of land. The air-currents by which it is affected are imperceptible to those embarked, variable and indeterminate in their force and direction, and quite incapable of being charted beforehand. In these conditions an airman who sought to steer by compass alone, say, from Bermuda to New York, might perchance find himself either at Halifax, on the one hand, or at Charleston on the other.

In my chapter on "Invasion" no mention is made of those subsidiary forms of military enterprise across the sea which are known as raids. I have treated invasion as an enterprise having for its object the subjugation of the country invaded, or at least the subjection of its people and their rulers to the enemy's will. As such it requires a force commensurate in numbers with the object to be attained, and it stands to reason that this force must needs be so large that its chances of evading the vigilance of an enemy who is in effective command of the sea must always be infinitesimal. A raid, on the other hand, is an enterprise of much lesser magnitude and much smaller moment. Its method is to elude the enemy's naval guard at this or that point of his territory; and, having done so, its confusion and to do as much harm as they can—which may be considerable before their sea communications are severed by the defending naval force assumed to be still in effective command of the sea affected. If that command is maintained, the troops engaged in the raid must inevitably be reduced sooner or later to the condition of a forlorn hope which has failed. If, on the other hand, that command is overthrown, then the troops aforesaid may prove to be the advanced guard of an invasion to follow. Thus, although a successful raid may sometimes be carried out in the teeth of an adverse command of the sea, yet it cannot be converted into an invasion until that adverse command has been assailed and overthrown. It is thus essentially fugitive in character, possibly very effective as a diversion, certain to be mortifying to the belligerent assailed, and not at all unlikely to cause him much injury and even more alarm, but quite incapable of deciding the larger issues of the conflict so long as his command of the sea remains unchallenged. It is perhaps expedient to say this much on the subject, because the programme of the Naval Manoeuvres of this year is known to have included a series of raids of this fugitive character. Whether, or to what extent, any of these operations were adjudged to have been successful I do not know. I am only concerned to point out that, whether successful or not, their utmost success can throw little or no light on the problem of invasion unless in the course of the same operations the defenders' command of the sea was adjudged to have been overthrown.

In my chapter on "The Differentiation of Naval Force" I endeavoured to define the functions of the so-called "battle-cruiser" and to forecast its special uses in war. At the same time I pointed out that "it is held by some high authorities that the battle-cruiser is in very truth a hybrid and an anomaly, and that no adequate reason for its existence can be given." It would appear that the views of these high authorities have now been adopted, in some measure at least, by the Admiralty. Since the chapter in question was in type it has been officially announced that the battle-cruiser has been placed in temporary, and perhaps permanent, abeyance. Its place is to be taken by a special type of fast battleship, vessels in every way fit to lie in a line and yet, at the same time, endowed with qualities which, without unduly increasing their size and displacement, will enable them to discharge the special functions which I assigned to the battle-cruiser in the line of battle. This is done by employing oil instead of coal as the source of the ship's motive power. The change thus adumbrated would seem to be in the natural order of evolution, and at the same time to be in large measure one rather of nomenclature than of substance. The battle-cruiser, as its name implies, is itself essentially a fast battleship in one aspect and an exceedingly powerful cruiser in another. In the fast battleship which is to replace it, the battle function will be still further developed at the expense of the cruiser function. But its speed will still qualify it to be employed as a cruiser whenever occasion serves or necessity requires, just as the battle-cruiser was qualified to lie in a line and do its special work in a fleet action. The main difference is that the fast battleship is much less likely to be employed as a cruiser than the battle-cruiser was; but I pointed out in the text that the employment even of the battle-cruiser in cruiser functions proper was likely to be only occasional and subsidiary.

The decision to use oil as the exclusive source of the motive power of fast battleships, and of certain types of small cruisers of exceptional speed, is undoubtedly a very significant one. It may be taken to point to a time when oil only will be employed in the propulsion of warships and coal will be discarded altogether. But that consummation can only be reached when the internal combustion engine has been much more highly developed for purposes of marine propulsion than it is at present. At present oil is only employed in large warships for the purpose of producing steam by the external combustion of the oil. But it may be anticipated that a process of evolution, now in its initial stages in the Diesel and other internal combustion engines, will in course of time result in the production of an internal combustion engine capable of propelling the largest ships at any speed that is now attainable by existing methods. When that stage is reached oil will, for economic reasons alone, undoubtedly hold the field for all purposes of propulsion in warships. It is held by some that this country will then be placed at a great disadvantage, inasmuch as it possesses a monopoly of the best steam coal, whereas it has no monopoly of oil at all, and probably no sufficient domestic supply of it to meet the needs of the Fleet in time of war. But oil can be stored as easily as coal and, unlike coal, it does not deteriorate in storage. To bring it in sufficient supplies from abroad in time of war should be no more difficult for a Power which commands the sea than to bring in the supplies of food and raw material on which this country depends at all times for its very existence. Moreover, even if we continued to depend on coal alone, that coal, together with other supplies in large quantities, must, as I have shown in my last chapter, be carried across the seas in a continuous stream to our fleets in distant waters, and one of the great advantages of oil over coal is that it can be transferred with the greatest ease to the warships requiring it at any rendezvous on the high seas, whether in home waters or at the uttermost ends of the globe, which may be most conveniently situated for the conduct of the operations in hand. For these reasons I hold that no serious apprehension need be entertained lest the supply of oil to our warships should fail so long as we hold the command of the sea. If ever we lost the command of the sea we should not be worrying about the supply of oil. Oil or no oil, we should be starving, destitute and defenceless.

It only remains for me to express my gratitude to my friend Sir Charles Ottley, not merely for an Introduction in which I cannot but fear that he has allowed his friendship to get the better of his judgment, but also for his kindness in devoting so much of his scanty leisure to the reading of my proofs and the making of many valuable suggestions thereon. I have also to thank my friend Captain Herbert W. Richmond, R.N., for his unselfish kindness in allowing me to make use of his notes on the Dunkirk campaign which he has closely studied in the original papers preserved at the Admiralty and the Record Office. To my son, Lieutenant H.G. Thursfield, R.N., I am also indebted for many valuable suggestions. Finally, my acknowledgments are due to the Editors of this series and the Syndics of the Cambridge University Press for their uniform courtesy and consideration.

J.R.T.

4th September 1913.