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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. 36.
Art. 36. Extenuation of the principle of capture. When a public or private vessel belonging to one of the belligerent Powers is, at the commencement of hostilities, in an enemy port, it is allowed to depart freely, immediately or after a reasonable number of days of grace, and to proceed, after having been furnished with a passport, to its port of destination, or to any other port indicated.
The same rule should apply in the case of a ship which has left its last port of departure before the commencement of the war and entered an enemy port while still ignorant of hostilities.