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The Laws of War on Land. Oxford, 9 September 1880.
Part II : Application of general principles #I. Hostilities #B. Rules of conduct with regard to things #(a) Means of injuring -- Bombardment - Art. 34.
Art. 34. In case of bombardment all necessary steps must be taken to spare, if it can be done, buildings dedicated to religion, art, science and charitable purposes, hospitals and places where the sick and wounded are gathered on the condition that they are not being utilized at the time, directly or indirectly, for defense.
It is the duty of the besieged to indicate the presence of such buildings by visible signs notified to the assailant beforehand.