As a part of his discussion of the duties and activities of the king’s henchmen, the author describes the military methods of the age, arms and armour, military engines and devices used in offensive and defensive warfare, and other necessary equipment.[80] He also discusses the ethics of the military profession to some extent. This part of the work has been made the subject of a detailed study by Captain Otto Blom of the Danish artillery, who has tried to fix a date for the composition of the _King’s Mirror_ on the basis of these materials.[81] It is not likely, however, that the work describes the military art of the North; such an elaborate system of equipment and such a variety of military engines and devices the Norwegians probably never knew at any time in the middle ages. It is the military art of Europe which the author describes, especially the war machinery of the crusades. One should not be surprised to find that he had knowledge of the devices which were employed by the Christian hosts in their warfare against the infidel in the Orient. The crusades attracted the Norwegian warriors and they took a part in them almost from the beginning. The fifth crusade began in 1217, the year of Hakon IV’s accession to the kingship. Several Norwegian chiefs with their followers joined this movement, some marching by land through Germany and Hungary, while others took the sea route. One is tempted to believe that the author was himself a crusader, but it is also possible that he got his information as to the military art of the south and east from warriors who returned from those lands. Footnote 80: Cc. xxxviii-xxxix. Footnote 81: See below, pp. 62–63. From the subject of proper behavior and good breeding the author passes to a discussion of evil conduct and its effect on the welfare of the kingdom. Many causes, he tells us, may combine to bring calamities upon a land, and if the evils continue any length of time, the realm will be ruined.[82] There may come dearth upon the fields and the fishing grounds near the shores; plagues may carry away cattle, and the huntsman may find a scarcity of game; but worst of all is the dearth which sometimes comes upon the intellects and the moral nature of men. As a prolific source of calamities of the last sort, the author mentions the institution of joint kingship, the evils of which he discusses at some length. His chapter on this subject is an epitome of Norwegian history in the twelfth century when joint kingship was the rule. Footnote 82: C. xxxv.