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.