1784 At noon on July 6, people gathered nervously in twos and threes, sometimes looking up at the sky and sometimes blindfolded. What are they doing? It turned out that they were watching a strange sight in the sky: a big "flying fish" floating in the air.

This is the test flight of the French Robert brothers. "Flying Fish" is a big balloon. Before that, the Mongerfield brothers had successfully completed the manned balloon flight, but their balloon had a fatal shortcoming, that is, it could not control the flight direction, and it was quite wonderful as a game or performance. If you want to send it by air, it won't work. Therefore, the Robert brothers came up with the idea that balloons should be equipped with propulsion equipment to enable them to fly directionally. They think that the principle of balloon's ups and downs in the atmosphere is the same as that of fish's swimming in the water, so they make their own balloons in the shape of water drops, with a length of 16 m and a diameter of 10 m, and the volume of the airbag is 940 cubic meters. When filled with hydrogen, they can carry hundreds of kilograms of heavy objects into the sky.

At the beginning of the first test flight, the fish-shaped balloon rose smoothly after the airbag was filled with hydrogen. Seven people in the hanging basket paddled hard on the wooden frame with big oars made of silk, controlling the navigation of the balloon. This huge "flying fish" can turn clumsily in the air. However, before they cheered, the Robert brothers found that something was wrong-the balloon kept rising slowly, and the airbag gradually expanded and was about to explode! It turns out that the higher the balloon rises, the lower the atmospheric pressure, the smaller the atmospheric pressure outside, and the volume of the hydrogen balloon continues to expand. Once the balloon bursts, the fate of these test pilots is unimaginable. In a hurry. Robert the Great grabbed a dagger, took great risks, climbed the hanging rope and forcibly punctured the airbag. With the long "stabbing" sound, hydrogen leaked out and the balloon finally landed safely.

Two months later, the Robert brothers flew into the sky in a balloon with a deflation valve. This time, they flew for seven hours in a row and landed as planned. The balloon that can control the direction made by the Robert brothers is an airship. The airship they designed is very reasonable in shape and accords with aerodynamic principles. In the future, all airships made by people will basically adopt this shape.