Even if the aircraft ventured to descend as low as 5,000 feet it would be faced with another adverse influence. The discharge of the heavy battleship guns would bring about such an agitation of the air above as to imperil the delicate equilibrium of an airship. Nor must one overlook the circumstance that in such an engagement the Zeppelins would become the prey of hostile aeroplanes. The latter, being swifter and nimbler, would harry the cumbersome and slow-moving dirigible in the manner of a dog baiting a bear to such a degree that the dirigible would be compelled to sheer off to secure its own safety. Desperate bravery and grim determination may be magnificent physical attributes, ut they would have to be superhuman to face the stinging recurrent attacks of mosquito-aeroplanes. The limitations of the Zeppelin, and in fact of all dirigible aircraft, were emphasised upon the occasion of the British aerial raid upon Cuxhaven. Two Zeppelins bravely put out to overwhelm the cruisers and torpedo boats which accompanied and supported the British sea-planes, but when confronted with well-placed firing from the guns of the vessels below they quickly decided that discretion was the better part of valour and drew off. In naval operations the aeroplane is a far more formidable foe, although here again there are many limitations. The first and most serious is the severely limited radius of action. The aeroplane motor is a hungry engine, while the fuel capacity of the tank is restricted. The German military authorities speedily realised the significance of this factor and its bearing upon useful operations, and forth with carried out elaborate endurance tests. In numerable flights were made with the express purpose of determining how long a machine could remain in the air upon a single fuel supply. The results of these flights were collated and the achievements of each machine in this direction carefully analysed, a mean average drawn up, and then pigeon-holed. The results were kept secret, only the more sensational records being published to the world. As the policy of standardisation in the construction of aeroplanes was adopted the radius of action of each type became established. It is true that variations of this factor even among vessels exactly similar in every respect are inevitable, but it was possible to establish a reliable mean average for general guidance.