The two armies are spread out, approaching each other. Each Commander has thrown forward a screen of men. These act, so to speak, as "feelers," seeking to come into touch with the enemy. Behind this screen of outposts comes the real strength of the army. Neither Commander knows how, when, or at what point, his opponent will develop his main attack. So they grope towards each other, any authentic news of a definite movement of troops being eagerly awaited. If, as the result of any information brought him, one Commander-in-Chief can anticipate his opponent’s chief move, he may—by that stroke alone—succeed in winning the battle which ensues. Thus it is that a Commander-in-Chief sits at his Headquarters, with maps in front of him, asking himself one vital question: "At what point, behind the wide-flung screen of his outposts, is my enemy developing his main attack?" The cavalry scouts, and the scouts on foot, do their work as best they can. They strive, as far as is practicable, to pierce the barrier of men which the enemy has thrown forward. The task of these scouts is dangerous; it is laborious; and it is slow. It is also haphazard. But, from the fragmentary news that is brought back to him, a Commander-in-Chief has to act as best he can. Some of his scouts succeed; others return with nothing at all. There are serious gaps in the intelligence; much of it may be contradictory. Yet upon such intelligence as this a Commander-in-Chief has acted in the past, and will have to act in the future, unless he has the aeroplane scout placed at his disposal. II. Work of a squadron of air-scouts described—Tasks of the pilot and observer—Combined reconnaissance by many machines—Effect of aeroplanes upon tactics. Having indicated the difficulties of the Commander-in-Chief, who has no aeroplane service at his disposal, it is now legitimate to show what can be accomplished with the aid of this new "arm." We will imagine, for the sake of argument, that an action is imminent, and that the Commander-in-Chief is anxious to know, without delay, from what direction he may expect the enemy to mass his troops for a main attack. So he calls into consultation the Commander of the aeroplane depot. This depot—as has been explained in a previous section—will probably be established at a suitable point near the main body of the troops, and will be maintained at the spot chosen, until a move on the part of the army necessitates a change of quarters. To the Commander of the aeroplane depot the Commander-in-Chief will explain the points, in regard to the general plan of campaign, upon which he requires enlightenment.