If the country to the front of the troops offers a wide view, a spy sent on in advance can make fires, whose number and arrangement has a previously agreed meaning. Among the enemy's outposts the making of fires will seldom be practicable, since it would at once arouse suspicion. In these cases the spy can make use of houses which can be seen from a distance on all sides, since the appearance and disappearance of a light in the windows, or the opening and shutting of the shutters, can have a recognised sense and meaning for the troops from whom the spy is sent. Optical communication of news is seldom possible in wooded country, or at any rate only if the troops advance, and not if they retreat or remain stationary. In these cases the spy sent in advance can inform the troops by various agreed signs, such as breaking or tying together small trees and branches, moving turf, writing agreed-on signs with chalk on bark of trees, fences, houses, and big stones or rocks. Smugglers in some Russian districts use little stones, which they put down by the way on single big stones, heaps of earth, and leaves; as the number and position of the stones with regard to one another varies, so the smugglers mutually impart valuable information to each other. This method should be borne in mind for adoption by an army in the field. 2. _Agreed communications by writing_.--In many cases secret communication by writing can be carried on in the guise of quite ordinary correspondence, in which the speech is of trade and family matters, etc, so that a stranger, who was not initiated into the secret meanings of the special words, would attribute no special meaning to them. Such letters would be addressed to persons who are prominent through their business in official positions--lesser state officials and merchants, for instance. In July 1887, at the Imperial Court of Justice at Leipzig, the trial took place of the Alsatians--Klein and Greber for espionage. In the indictment of the public prosecutor it was said, among other things, that the defendant Klein had never been directly in correspondence with the Bureau at whose head was Colonel Vincent, but he knew that his despatches had been communicated to the War Office from which Colonel Vincent took his orders. In order to arouse no suspicion, the correspondence was given the appearance of letters which passed between relatives; herein lay the reason for the constant repetition of Christian names, and the good wishes for Uncle, Aunt, and other persons who certainly did not exist, or under whose names personalities would be understood that were very well-known to the French secret service.