roads used by the military. When war broke out, the French airmen received explicit instructions not to bombard any town for fear of inflicting harm upon civilians, but the Germans were not so scrupulous, and their defiance of the dictates of humanity forced a change in the policy of their Allies, if only in self-defence. The aeroplane could be used also as a link for communicating with armies and their staffs, particularly in the case of a besieged army or town. And, finally, the man-bird is admirable in the capacity of aerial policeman; he can watch the clouds and he can prevent the passage of the enemy pilots. Not that it is possible to suppose that one force of aeroplanes, however numerous, can completely occupy the heavens, for the skies are broad; but bold aviators ever on the watch, patrolling the sky in constant relays (as was the case in the aerial defence of Paris against the Zeppelins), will generally succeed, whatever measures are taken against them, in overtopping the adversary; and the French aviator is remarkably good in that sort of warfare where native audacity and resource are in demand—that is why the Frenchman is so superb a performer in the air. None the less it is impossible quite to bar out the enemy. The clouds may always hide a foe; the fog is ever the possible lurking ground of the hostile airship. But although the barrage system (so successfully applied on solid earth in the _tir de barrage_) cannot absolutely prevent a Zeppelin attack upon a wide-spreading town, yet the aerial dam has given good results in the war of the air. The procedure is to institute a barrage of aeroplanes over against a certain locality—a certain restricted space. The enemy is marked down and prevented from passing. Undesirable visitors are invited to "move on," and they do not wait for a repetition of the request! I have heard of one hardy airman who, charged to watch the heavens against the passage of the adversary, so manoeuvred that the thick heavy clouds which hung in the sky were positively useful to him as a screen. Noting that at one point there was a clear space in the dense curtain of fog, he placed himself there and watched as a look-out might in the embrasure of a fort. None came to challenge his vigilance.