The duties of naval airships will be of a different nature. Already a point kept in view in their design is ability to “keep the air” for a considerable period, and with what are in these days “large airships” of the Zeppelin type (to which the ill-fated Naval Airship No. 1 _Mayfly_ belonged) there seems no reason why an airship should not be kept in the air for three or four days already. The fuel problem is not very difficult, because a great deal can already be done without the use of the engines, or with only partial use of them. It is also more than probable that with a view to further economy some kind of sails, combined with sea-anchors, will be evolved, whereby the ship might become able to sail in the air nearly as well as the old three-deckers, or, at any rate, as well as the masted ironclads, sailed in the water. The difficulty of “keeping the air” is the inevitable leakage of gas, but as leakage nowadays is infinitesimally less than it once was, the assumption is that as the years go on it will eventually be reduced to almost a minus quantity. Gales will be met by “bulk” and efficient anchors, on the principle that the gale which swamps a fishing-boat or blows over a haystack has no effect on a Dreadnought or a cathedral. Ability to keep the air will enable all Fleets to be accompanied by airships, which would detect mines and perhaps submarines, and with their ability to adapt their speeds at will, the presumption is that they would be able to destroy submarines by bombs. A further and very important duty would be the detection of torpedo attacks at night. Experiments carried out in Austria some few years ago with a captive balloon proved conclusively that except in cases of thick fog any vessels in motion are easily detected at a distance of ten or twelve miles. It is not merely the tell-tale flames in the funnels which betray attacking vessels; their wakes are always clearly visible, and as a general rule the vessels themselves, no matter how dark the night. Bomb-dropping from an airship must be a more serious matter than from aeroplanes, as so much more in the way of explosives could be carried. The chance of being hit, however, would probably be so much greater that it was (when war broke out) unlikely that any airships would be risked for such purposes. Nor is it very probable that naval airships will for some time to come attack each other, if they can possibly avoid it, the reason being that for a good many years they will be comparatively few in number, and the attack would have, in most cases, to be delivered in the presence of a fleet, which would make the attack, to say the least of it, very hazardous.