The observer in a sea-plane, under favourable conditions, can also detect the movements of a sub-marine, his height above the water enabling him to see beneath its surface and discern the shape of the vessel, even when it is submerged. This would be of value in war, as proved in manœuvres with our fleets. A sea-plane, patrolling the entrance to a harbour, can send a warning by wireless should hostile submarines--invisible to all save the aerial watcher--seek to steal in upon a destructive raid. The air-patrol would play its part, too, if an enemy attempted to land troops. Flying above transports as they neared the coast, the planes would drop bombs upon them. Many of the patrols, also, would be equipped with wireless, and would scout in search of an enemy’s fleet, reporting constantly to the land stations as they flew. [Illustration: FIG. 86.--Sea-plane to carry a crew of seven. A. Hull upon which the machine floats when in the sea; B.B.B. Wheels upon which it may move when on land, and which fold upward when it is on the water; C. Pilot’s controlling wheel; D.D. Main sustaining planes; E. Four-bladed propeller driven by chain-gearing from engine within the hull.] A coastal sea-plane, as now planned, is a craft having, say, two engines, each devolving 120 h.p., with a wing span of some 80 feet, and an accommodation in its hull for three men--the pilot, a combatant with a machine-gun, and an observer with an installation of wireless. But types are changing constantly, and the tendency is to build larger craft. A machine weighing a couple of tons is shown in Fig. 86, and a novelty in regard to it is that it has wheels upon either side of its boat-shaped car, upon which it can move on land, and which fold upward when it rests upon the water. Long flights over-sea, in search, say, of the assembling point of an enemy’s fleet, would be undertaken by the naval airships. They will be relied upon, for example, to scour the North Sea by day and night; and, poised high above the water, they would be able to locate an enemy’s fleet when at an immense distance.