Night operations, however, have not proved eminently successful. The very darkness which protects the aerial prowler also serves a similar purpose in connection with its prey. But aerial operations under the cover of darkness are guided not so much by the glare of lights from below as betrayal by sound. The difference between villages and cities may be distinguished from aloft, say at 1,500 to 3,000 feet, by the hum which life and movement emit, and this is the best guide to the aerial scout or battleship. The German authorities have made a special study of this peculiar problem, and have conducted innumerable tests upon the darkest nights, when even the sheen of the moon has been unavailable, for the express purpose of training the aerial navigators to discover their position from the different sounds reaching them from below. In other words, the corsair in the skies depends more upon compass and sound than upon compass and vision when operating after dark. The searchlights with which the Zeppelins are equipped are provided merely for illuminating a supposed position. They are not brought into service until the navigator concludes that he has arrived above the desired point: the ray of light which is then projected is merely to assist the crew in the discharge of the missiles of destruction. The Zeppelin, however, owing to its speed, both in the horizontal and vertical planes, is essentially a unit for daylight operations. The other airships which Germany possesses, and which for the most part are of the non-rigid type, are condemned to daylight operations from the character of their design. Owing to their low speeds they may be dismissed as impossible aerial vessels for hazardous work and are not regarded by the German authorities as all-round airships of war. Craft of the air are judged in Germany from the one standard only. This may be a Teutonic failing, but it is quite in keeping with the Teutonic spirit of militarism. Commercialism is a secondary factor. To the German Emperor an airship is much what a new manufacturing process or machine is to the American. Whereas the latter asks, "How much will it save me on the dollar?" to the War Lord of Germany--and an airship notwithstanding its other recommendatory features is judged solely from this standpoint--the question is "What are its military qualifications?" When the semi-rigid airship "V-I" was brought before the notice of the German military department the pressing point concerning its military recommendations arose at once. The inventor had foreseen this issue and was optimistic. Thereupon the authorities asked if the inventor were prepared to justify his claims. The retort was positive. Forthwith the Junkers decided to submit it to the test.