The principle of a hot air balloon is very simple, and there are only two difficulties in making it: one is the combustion device and the other is the airbag.

The first point is that without pressure vessels providing gaseous fuel, the efficiency of heating the air is not only low, but also due to fuel restrictions. This is also the reason why the principle of hot air balloons was discovered very early in history, but the first to be put into use in actual combat was the hydrogen balloon. So Roland simply gave up the combustion device and asked Anna to ignite and heat it.

The difficulty of airbags lies in their sealing properties. However, for Roland, who has learned from previous experience, this problem is easy to solve. The outermost layer of the airbag is wear-resistant canvas, lined with cow intestinal membrane in the middle, and the inner layer is light gauze, so that it can prevent air leakage when sewn together, and at the same time, it is not afraid of birds pecking.

Roland opened the bottom of the airbag and asked Anna to spray black fire with a temperature close to the ordinary flame. The airbag slowly swelled up, looking like a wrinkled winter melon. Considering that the hot air balloon needed to be carried by two people to take off, the widest part of the airbag was five meters in diameter, and the intestinal membrane of twelve cows was used, and the sewing work was nearly a week. If it weren't for the lord who was in charge of a single place, it wouldn't be able to make such a big toy.

"Is it flying with the buoyancy provided by the heat?" Anna asked. "I remember the book mentioned that hot air always rises."

"Yes, hot air rises and cold air sinks are common natural phenomena. Explanation by particle theory is that the movement of hot air balls intensifies, and the ability to spread to the surroundings is strong, and the density will become smaller after the volume expands. For air without changes around it, it is equivalent to becoming lighter, so it will rise."

After Roland finished explaining the principle, he found that except for Anna, the other three were confused, which made him secretly sigh at the importance of talent.

When the airbag completely expanded and floated upwards, the traction rope was slowly straightened, and Roland was the first to flip into the hanging basket, and then took over Anna who was crawling.

"I'm still a little worried," Nightingale said uneasy, "why don't you let me go up and give it a try first."

"Believe me, nothing will happen," Roland smiled easily, "and even if an accident occurs, there will be lightning."

"I will catch them, don't worry." Lightning patted his chest.

"And I Gu!" Maisie slapped her chest like lightning.

With a shaking, the hot air balloon led the hanging basket off the ground and gradually climbed upwards. Not long after, the two of them passed the top of the castle, and the whole picture of the town was presented to them one by one.

For Roland, this scene is not unfamiliar. After all, he has seen skyscrapers, but in Anna's eyes, it is a new experience that is completely different from the previous perspective. She lay on the edge of the hanging basket and looked out, holding Roland's arm firmly with one hand, looking excited and nervous.

This is the first time Roland saw her showing such a look. Perhaps someone whose feet have never been away from the ground will feel a slight fear of heights when they fly into the sky for the first time.

After a while, the hemp rope used to tie the hot air balloon under the hanging basket was pulled to its limit and the rise stopped. The hemp rope was about fifty meters long, which was the height of the fifteen or sixteen floors in later generations. Roland asked Anna to reduce the firepower, and the hot air balloon finally hovered in the air.

At the beginning, Lightning was still standing by the hanging basket. When he saw the basket stabilized, he felt relieved and played a chase game with Maisie around the hot air balloon.

Looking down from here, you can clearly see the castle roof, the town where construction is everywhere, the Chishui River flowing from west to east, and the green farmland by the river.

“How does it feel?”

Roland didn't ask with a smile until Anna shrank back her head.

"Thank you for your gift," she said excitedly, "so I can fly so high."

"It can be higher," Roland sat down against the side wall of the rattan hanging basket. "If the hemp rope below is longer, it will be fine to fly ten times the distance, but the higher the airflow, the more complicated it will be, the safety will be difficult to guarantee. Moreover, this is just a primary aircraft. When a piston aircraft is built in the future, ordinary people can fly faster than birds and taller than birds. Then..." He looked up at the blue sky, "People can even fly out of this world and go to the boundless space."

"..." Anna held her breath, her eyes shone brightly, something was about to come out.

"I can't guarantee that it will fly out of the world," Roland was amused by her appearance. "But building a piston machine that allows ordinary people to fly like birds, should still be able to do it in their lifetime." In fact, with Anna, there is no bottleneck in processing ability, and the current shortcomings exist in material performance. Low-quality pig iron can make do with steam engines, cooked iron can make cannons, and steel-made rotor fireworks smelted with black fire is not a big problem, but for internal combustion engines, they probably need high-quality cast iron, cast steel or aluminum. "The Border Town can be like what it is now, all this is your credit, Miss Anna."

After hearing this, Anna was stunned. After a while, she leaned over and said softly, "My mother died in a fire, but not only did I not be buried in the smoke and fire, I became a witch. For a long time, I thought it was my awakening that caused the fire, and I also extremely disgusted with myself as a witch. When I was locked in the prison, I thought that this might lead to liberation, but you rescued me from the prison and taught me how to use my abilities... I didn't expect that flames could have so many uses besides burning and pain." Anna paused, "I should feel extremely satisfied to meet you, but now I find that my thoughts have changed. Sometimes I feel so depressed and uncomfortable in my heart, hoping to get more."

She climbed up to Roland's shoulder, "Even if that's the case, do you still want to hire me?"

Her lake blue eyes sparkled, and her breathing was blowing on her face, and she felt a little itchy. Through her thin clothes, he could feel the other person's soft body and thumping heartbeat. Despite this, she did not avoid her eyes and looked directly at the nightingale. She was always full of enthusiasm and initiative.

“There is… outside”

Before Roland could finish speaking, Anna had already sealed his lips.

After parting, he took a breath, "I want to hire you, Miss Anna."

"Um."

This time, he took the initiative to lower his head and stood close to her.

...

"Huh, wait," Maisie chased lightning for a while, and suddenly stopped in the air and looked at the empty hanging basket, "They are gone!"

"Ah?" Lightning turned his head and looked, "They just sat down."

"Will you not see the scenery outside?"

"The scenery can be seen at any time, but there are not many opportunities like this."

"Opportunity?" Maisie shook her wings and landed on the shoulders of Lightning. "Don't you understand Gu very well, should you go see them Gu?"

"That won't work," Lightning hugged the dove, "This is a sacred ceremony that cannot be interrupted."

"Gu?"
Chapter completed!