duties ill question more heavily; thus we may take 2000 ft, as representing the lower altitude limit of ordinary military flying. Anything less than this will be referred to in the present series of articles as low-altitude flying. On this question of armour it cannot be too strongly insisted that anything less than the thickness necessary definitely to stop the projectile is worse than useless; a "mushroomed" bullet, possibly accompanied by a few detached fragments of steel, is infinitely more disagreeable and dangerous than a bullet which has not been upset.
An aeroplane armoured in all its vitals with 3 mm, steel, and otherwise designed on the lines indicated, flying at not less than 2000 ft, altitude, will be extremely difficult to bring down; so much so, that unless its exposed structural members be literally riddled and shattered by rifle and machine-gun, or unless a gun of larger calibre be brought to bear, it will be virtually impossible to effect its capture by gun-fire alone.
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