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Manual of the Laws of Naval War. Oxford, 9 August 1913.
Section V : On the rights and duties of the belligerent with regard to individuals - Art. 67.
Art. 67. A person can be considered a spy only when, acting clandestinely or on false pretenses, thus concealing his operations, he obtains or endeavours to obtain information in the zone of operations of a belligerent, with the intention of communicating it to the hostile party.
Hence, soldiers not wearing a disguise who have penetrated into the zone of operations of the hostile fleet for the purpose of obtaining information, may not be considered as spies but are to be treated as prisoners of war. Similarly, soldiers or civilians, carrying out their mission openly, entrusted with the delivery of dispatches, or engaged in transmitting and receiving dispatches by wireless telegraphy, are not to be considered as spies. To this class belong likewise persons sent in air-ships or in hydro-aeroplanes to act as scouts in the zone of operations of the enemy fleet or to maintain communications.