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Manual of the Laws of Naval War. Oxford, 9 August 1913.
Section IV : On the rights and duties of the belligerent with regard to enemy property - Art. 38.
Art. 38. Enemy vessels, public or private, which left their last port of departure before the commencement of the war and which are encountered on the high seas while still ignorant of the outbreak of hostilities, cannot be captured. They are only liable to detention on the understanding that they shall be restored after the war without compensation, or requisitioned, or even destroyed, on payment of compensation, but in such case provision must be made for the safety of the passengers on board as well as for the security of the ship's papers.
But, where these vessels shall be encountered at sea before the expiration of a sufficient period to be granted by the belligerent, seizure is not permissible. Vessels thus encountered are free to proceed to their port of destination or to any other port indicated.
After touching at a port in their own country or at a neutral port, these vessels are subject to capture.