Within the memory of many of us the size of combat armies was largely determined by the efficiency and scope of their signal systems, it being essential that the forces in the field should be kept within a size which permitted of communication being maintained between all units by means of runners, riders, or visual signals. Those were the days when messengers, often chosen by lot, crawled through the enemy’s lines at night in order to bring reinforcements to beleaguered garrisons; when stories of ambush and massacre or urgent appeals for ammunition and food were brought to headquarters by weary riders clinging to the manes of reeking ponies; or when, in the Indian country, cavalry columns communicated with each other by means of heliograph messages flashed from mountain-top to mountain-top, or signal-fires curling slowly skyward. But all this changed with the introduction of the telegraph and the telephone, the communications of an army thereafter being limited only by the amount of its wire. A far greater change came, however, with the introduction of the radio or wireless, whose area of operations is limited only by the power of the sending apparatus. Now it should be kept in mind that each of the systems of military signalling which I have already enumerated—telegraphs, telephones, radios, panels, lamps, flags, pigeons, runners, dogs, and the rest—is an adjunct to the others—when one fails, another is employed to get the message through. If the wires of the field telegraph and telephone are cut by a barrage, the radio is employed; if a shell knocks out the radio set, the message is intrusted to a pigeon; should the pigeon fail, a runner attempts to take it through; and if the runner is killed, the message can be communicated, either by means of rockets or by cloth panels spread upon the ground, to the aviators circling overhead.