If it were not for the consideration of the weight of armour, there is no doubt that an altitude of about 1,000 feet would be found very well suited for most of the ordinary tactical duties of the aëroplane. 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 efficient bullet-proof-treated steel that is now available; at the altitude in question, the minimum thickness that will stop a 0.303 Mark VI. round-nosed 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 2,000 feet as the minimum height for which protection is to be given, the figures become 2 mm. (about 145 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 duties in question more heavily; thus we may take 2,000 feet as representing the lower altitude limit of ordinary military flying.... On this question of armour it cannot be too strongly insisted upon that anything less than the necessary thickness 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 that has not been upset. An aëroplane armoured in all its vitals with 3 mm. steel, and otherwise designed on the lines indicated, flying at not less than 2,000 feet 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. 4. _Rapid despatch carrying to a distance._ Considering the advantages of the swift monoplane for carrying despatches from one commander to another, it would seem that in time it must oust the despatch rider. There is no obstacle to the despatch rider. The difficulties and delays of hills, woods, and rivers melt away before his ever onward course. The despatch rider on horseback may have to face the sudden appearance of the enemy, but if the aëroplane despatch carrier does, he has only to rise up out of his range of fire, and, still undisturbed, he can make his way towards his destination. There must surely already be many instances of the use of the new arm in this way in the present war. It has been reported that the Germans used aëroplanes to send messages to recall German troops stationed in the village of Coutrai to reinforce those at Charleroi. 5. _Distributing handbills to cities._