Protect the sky?
During World War One anti-aircraft defenses were mostly constituted by artillery and machine guns pointed at the sky, aided by searchlights and distance detectors borrowed by the Navy. To actually hit a plane was a matter of luck. As noticed by Hugo Gernsback "stopping an aerial raid by means of anti-aircraft guns is a notoriously impossible undertaking".
The solution he proposed was the "lasso shell". Two incendiary and two explosive bombs were attached to the shell via metal wires. Since every artillery shell, once fired, turns on itself, the bombs would have rotated "as a small solar system" - catching any plane in their wake. A miniaturized version was proposed for air-to-air fighting.
The "electric ear", as Popular Mechanics calls it, was actually used. Four megaphones linked to an amplifier and a receiver allowed to very roughly estimate the distance of the airplanes in low visibility conditions, by listening to the engines sound.
An American inventor even proposed to "mine the air" over London, using nets and explosive balloons. But, as the article writer notices, there is a city under the minefield: "Londoners could prefer the bombings". This underlines a frequent mechanism of war popularization: the editors know very well that the invention is impractical and downright absurd, but they publish it anyway, sometimes just to criticize it. Cover and title keep up the pretense of an innovative, even revolutionary, innovation, while the article takes the idea apart. It is a commercial strategy, devised to attract the public attention through the war theme and the rich illustrations; but it is also a didactic strategy: the reader get to know all the scientific reasons why an invention such as this cannot work (in this case the behavior of hydrogen inside a balloon and the reality of aerial warfare)