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§ 12 designed that bullets will pass through without doing more than local injury and without serious effect on the strength or flying power of the machine as a whole; in certain cases components will require to be duplicated in order to realise this intention. It is important to understand clearly that any intermediate course is fatal. Either the bullet must be definitely resisted and stopped, or it must be let through with the least possible resistance; it is for the designer to decide in respect of each component which policy he will adopt. The thickness of the armour required will depend very much upon the minimum altitude at which, in the presence of the enemy, it is desired to fly; also upon the particular type of rifle and ammunition brought to bear. There is a great deal of difference in penetrative power, for example, between the round-nosed and pointed bullets used in an otherwise identical cartridge.

If it were not for the consideration of the weight of armour, there is no doubt that an altitude of about 1000 ft, would be found very well suited to most of the ordinary tactical duties of the aeroplane. At such an altitude, however, the thickness of steel plate necessary becomes too serious an item for the present-day machine, even allowing for the very excellent and highly efiicient bullet-proof treated steel which is now available; at the altitude in question the minimum thickness that will stop a 0.303 Mark VI, round-nose bullet is 3 mm. (⅛ in.), but if attacked by the modern pointed-nose Mauser, nothing short of 5 mm, or 6 mm, is of avail. If we compromise somewhat in the matter of altitude and prescribe 2000 ft, as the minimum height for which protection is to be given, the figures become 2 mm. (about 14 S.W. gauge) for the 0.303 round-nosed bullet, and for the pointed Mauser 3 mm, or slightly over; at present it is not expected that it will pay to armour a machine for the