Since, however, the bullet-proof helmet is worn officially nowadays there seems no reason whatever why the bullet-proof waistcoat should not be adopted officially too. It is very light and very flexible and it is claimed that it is quite effectual in stopping rifle and machine-gun bullets. Thus we see in what different ways inventions grow. Some are warlike from first to last, like the gun and the torpedo, but we find a vast range of peaceful things growing into implements of warfare, as the farmer's tractor has been developed into the tank, while not less interesting are the old ideas revived and adapted to modern needs, exemplified by the hand-grenade and the chain armour. CHAPTER XXIV AEROPLANES Of all the great inventions perhaps the most striking because of the suddenness with which they have come upon us are those relating to the navigation of the air. Until a few years ago "to fly" was taken to typify the impossible. Now we see men flying every day and there is scarcely anyone who has not had a friend or relative in the Flying Corps. Recent experience, too, has shown that this one invention has revolutionized warfare in several important departments, particularly in the use of very heavy long-range artillery. Huge guns, hidden in a hollow or behind a hill, have been set to throw shells on to an unseen target, while a man in an aeroplane above watches the result and signals back by wireless. Thus by the aid of aircraft the power of artillery has been immensely increased. Again, aircraft have superseded cavalry for reconnaissance purposes, that is to say, for finding out the enemy's strength and preparedness. Only a few years ago a General who needed information as to his foe would send forward a screen of cavalrymen who would cautiously creep forward until, judging by what they could see and by what sort of a reception they got, they were able to form some idea of the foe's arrangements. Nowadays, however, the airmen sail over his head and take photographs of him and his positions. A careful commander to-day not only screens his men and his guns from view along the land but he also tries his best to make them invisible from above. And, speaking of inventions, the soldiers have shown a degree of ingenuity in making themselves and their guns invisible which almost merits a volume to itself. The airman, therefore, goes up and sails over the enemy. He may be simply observing for some particular unit of artillery, or he may be sent to find out things generally--nothing in particular, but anything which seems likely to be of use. He looks out intently and carefully, moreover he not only looks with his own eyes: as has just been mentioned, he takes photographs, which can be developed on his return and studied minutely at leisure. He may, or may not, according to circumstances, send back reports of an urgent nature by wireless telegraphy.