A dirigible balloon, it has been suggested, is comparatively safe while moving in the air, but is subjected to severe strains when anchored to the ground, if exposed. It must have either safe harbors of refuge or actual shelter buildings--dry docks, so to speak. In an enemy's country a ravine or even a deep railway cut might answer in an emergency, but the greatest reliance would have to be placed on quick return trips from a suitable base. The balloon would be, perhaps, a more effective weapon in defense than in attack. Major Squier regards a flying height of one mile as giving reasonable security against hostile projectiles in the daytime. A lower elevation would be sufficient at night. Given a suitable telephotographic apparatus, all necessary observations could easily be made from this altitude. Even in the enemy's territory, descent to the earth might be possible at night under reasonably favorable conditions. Two sizes of balloon would seem to be indicated: the scouting work described would be done by a small machine having the greatest possible radius of action. Frontiers would be no barrier to it. Sent from England in the night it could hover over a Kiel canal or an island of Heligoland at sunrise, there to observe in most leisurely fashion an enemy's mobilizations. [Illustration: GERMAN GUN FOR SHOOTING AT AEROPLANES (From Brewer's _Art of Aviation_)] At the London meeting of the Institute of Naval Architects, in April, 1911, the opinion was expressed that the only effective way of meeting attack from a flying machine at sea would be by a counter-attack from the same type of craft. The ship designers concluded that the aeroplane would no more limit the sizes of battleships than the torpedo has limited them. For the more serious work of fighting, larger balloons would be needed, with net carrying capacities perhaps upward from one ton. Such a machine could launch explosives and combustibles against the enemy's forts, dry docks, arsenals, magazines, and battleships. It could easily and completely destroy his railroads and bridges; perhaps even his capital itself, including the buildings housing his chief executive and war office staff. Nothing--it would seem--could effectually combat it save air craft of its own kind. The battles of the future may be battles of the air.