In the event of active service, the German system of communication between spies and the army in the field is very largely on the lines laid down by Klembovski, whose "Military Espionage in Peace and War" is one of the recognised textbooks on espionage. So clearly and well does Klembovski indicate the methods to be followed, that certain passages of his work with regard to this matter of communications are worthy of reproduction as they stand in his original work, in order to assist in a conception of the means employed. It is not always possible, he says, for the spies to report personally to their chief about the results of their investigations; therefore it is of interest to consider the methods of communication by means of which they can forward the required information to the stipulated place. These methods of communication are to be grouped in three divisions, according to their nature: 1. Optical signals; 2. Agreed communications by writing; 3. Despatches in cipher. 1. _Optical signals_.--In operations in heavily wooded districts which obstruct reconnoitring by cavalry, or in the operations of detachments which consist only of infantry, the assurance of guarding against danger which is obtained by means of the despatch of patrols in the direction of the enemy embraces no especially broad stretch in the front of the troops. In such cases the duty of reconnaissance can be allotted to spies, who should collect the simplest and fullest possible information about the enemy. For example: presence or absence of the enemy in certain directions and at certain distances; the approximate strength of his troops; whether the enemy is on the march, at rest, or occupying a position. Such information is especially of importance to such detachments as possess cavalry, for the commander of the detachment can, if he has fresh knowledge of the presence and dispositions of the enemy, execute cavalry reconnaissances far better than he could without the knowledge to be obtained by spies. He can estimate the number and strength of the patrols which he intends to send out, and can give them the correct direction of marching to achieve their purpose, etc. For communicating such information the simplest signals are desirable. Signals should be employed which on the one hand will not arouse the suspicions of the enemy, and on the other hand will not require special instructions and explanations, and will not occupy too much time either in preparation or sending.