If one may contrast for a moment the present attempts at aerial navigation with the concurrent attempts at submarine navigation, one quickly sees how terribly the æronaut is handicapped as compared with the under-water sailor. The advantage of the dense medium which the sea offers to the submarine navigator is precisely the same as it has offered from the beginning of time to the surface navigator, and nothing new is needed to sustain the submarine ship, whereas the unhappy man who seeks to navigate the air has to obtain from a medium of extraordinary levity the support necessary for keeping him aloft. The difference between the specific gravity of air (of which ships are full) and of water is so great by comparison between the specific gravity of any gas available for filling aerial-ships and that of air, that the problem of the submarinist is easy indeed compared with the other. But it is in the face of this initial and enormous difficulty that the æronauts of to-day have apparently persuaded themselves that they can successfully float their balloon-ship in mid-air, and propel it not only against the rapid tides of the air in which it floats, but also drive it at a good additional speed. When men are to be found capable of committing their fortunes, and even their lives, to navigation of this kind, it is not surprising to find that the far easier problem of navigating the seas beneath the surface has won the attention and the effort of enterprising men. They certainly have chosen, if the humbler, also the more practical and promising field of operation. I doubt not that they have likewise chosen the more fruitful field. It is worthy of remark that it is once again in connection with the arts of war that a great extension of human progress has been commenced. But for the temptation of gaining equality with, and even mastery over, our possible foes, the art of submarine navigation would certainly not have been attracting the attention of some of our best and most scientific men, who are once again eagerly developing— “Those dire implements Which sombre science with unpitying pains, That love of neither man nor God restrains, To warring foes presents.” One can only be thankful that the world is so constituted and so ruled, that out of seeming evil often comes great good to men. I have not been asked to say anything of the book with which these lines are associated. I may nevertheless remark that I have had an opportunity of hastily looking through the author’s proofsheets, and have formed the opinion that it is a most timely and highly instructive work, and one which gives to the non-technical world an extremely good review of all that has been done in the way of submarine war vessels, while the technical man into whose hands it may come will be compelled, by its great interest and by its clever record of facts, to read every page of it.