The most important part of our espionage system is conducted in our own territory, by sharpening our own spirit of observation, by seeking to gather everywhere all the many scattered rumors heard about the enemy. Details which at first may seem insignificant, may become extremely useful when compared with other details, collated with other observations and completed by detailed, careful investigations. The service of espionage is merely the application on a broad scale of a vast study of analysis and synthesis. It is nothing more than a police system which, instead of trying to discover the secrets of a small drama, seeks to investigate great causes, seeks to penetrate the essence of that great drama in which the major interests of two great nations, the future happiness of two great peoples are compromised. The two opposing armies even as they try to damage each other by the use of arms, endeavor also to bring ruin on each other by the use of an elaborate system of intrigues and disguises through which each tries to discover the secret of the other and to guard jealously his own. The sources from which information is obtained differ widely. I shall discuss only the principal ones that I may place clearly before the reader the various means at my disposal were I to attempt to do what the Colonel proposed to me. An essential element of modern espionage, one of the elements which has revolutionized all that was done and attempted in past wars, is the aeroplane. The small reconnoitring machine which flies over the enemy defenses at great heights, is almost safe from the fire of enemy anti-aircraft guns, and that the observer may make his observations unmolested and lose no time in defending himself from possible attacking enemy planes, several chasing machines are sent out with him as sort of guardian angels. Nothing should escape the vigilant, educated eye of the observer. His mind, well-acquainted with the enemy situation, his vision, accustomed to the appearance of terrain from on high, examines the roads, searches the railroads, observes both fields and camps, and since at times some detail may escape the notice of the observer, the other eye, the faithful lens of the camera, completes the picture by recording what the observer may have overlooked. These are exact, useful, tangible records of what has been seen; records which can be consulted under any circumstances.