Shi Lu shook his head and asked, "Is this useful?" ”

Li Tangshan sorted out his thoughts and explained to them: "There is a thick atmosphere on the surface of the earth, due to the action of the earth's gravity, the atmosphere is "sucked" to the earth, although the air is very light, but there is still weight, with weight it produces force, and its effect on objects is pressure. The existence of atmospheric pressure, which can be said to be the result of the gravitational pull of the Earth, was demonstrated by the famous Magdeburg hemispheric experiment. ”

Lu Yique asked: "What is the Magdeburg Hemispheric Experiment?" ”

Li Tangshan continued: "After 1800, in the far west, there was a scientific experiment conducted by Otto von Glick, the mayor of Magdeburg, in Regensburg (now Regensburg, Germany) in the Holy Roman Empire. ”

Li Tangshan told you the story of the Magdeburg hemispheric experiment.

The mayor of Magdeburg, Otto von Glick, heard about Torricelli and that there were many people who did not believe in atmospheric pressure; A few people were also heard laughing at Torricelli. Therefore, although Glick, who was far away from Italy, was in Germany, he was very upset about Torrician.

He hurried to find the glass tube and the mercury, and re-enacted the experiment of Torricari, and concluded that it was accurate; The air is then pumped out of a well-sealed wooden barrel, and the barrel is "bang!" The sound was "crushed" by the atmosphere!

One day, he and his assistant made two hemispheres, 14 inches in diameter, and invited a large group of people to do a "big experiment" on the outskirts of the city.

On May 8 of this year, the city of Magdeburg was sunny and beautiful, and a large number of people gathered around the experimental field, which was very bustling and lively. Some support Glick, hoping that the experiment will be successful; Some assert that the experiment will fail; People are talking, arguing, prophesying; Others ran through the streets and alleys to the experimental site while shouting: "The mayor has put on a circus!" The mayor played the circus———"

Glick and his assistants publicly put a rubber band in the middle of the brass hemispherical shell; Then fill the two hemispherical shells with water and put them together; Then all the water is pumped out to create a vacuum in the ball; Finally, tighten the faucet on the valve to close it. At this time, the surrounding atmosphere pressed the two hemispheres tightly together.

With a wave of his hand, the four grooms brought sixteen tall horses, four on each side of the ball. As soon as Glick gave the order, the four grooms whipped their horses and pulled them back! It's like a "tug-of-war". 4 grooms, 8 big horses, all covered in sweat. However, the copper ball is still intact. Glick had to shake his hand and pause. Then, the left and right teams multiplied. The grooms drank some water, wiped the sweat from their foreheads, and prepared for the second experiment.

Glick waved his hand again, and the experimental field was even more lively.16 big horses, pulling hard, eight grooms were shouting loudly, whipping the horses......

The crowd on the experimental field stretched their necks and watched vigorously, from time to time making "Wow!" Wow! The ringing of the sound.

Suddenly, "Smack! With a loud bang, the copper ball split into two halves. After the experiment, some people still did not understand why the two hemispheres could not be opened, and asked him in a gossipy manner, and he patiently explained in detail: "Normally, we close the two hemispheres closely, and they will be separated without force. This is because there is atmospheric pressure inside and outside the ball; Cancel each other out of balance. It's as if there is no atmosphere. Today, after I pumped it into a vacuum, there is no outward atmospheric pressure in the sphere, only the atmosphere outside the sphere tightly presses the two hemispheres ......".

That is, before pumping, the external pressure of the hemisphere is equal to its internal pressure equal to the atmospheric pressure. But after pumping, the external pressure of the hemisphere is greater than its internal pressure, and the inside of the hemisphere is vacuum. It's as if atmospheric pressure "presses" both hemispheres. So these two hemispheres have to use a lot of force to pull apart.

The first time everyone heard that there was atmospheric pressure, and that they could also use air pressure to measure altitude, they all felt unacceptable, but everyone still believed in Li Tangshan in their hearts because they often brought some incomprehensible remarks to everyone, but it turned out to be correct later.

After two days of preparation, there is a shortage of skilled craftsmen, and the bamboo U-shaped pipe is quickly built in the wild!

Li Tangshan said: "Shi Lu, you check the tightness of the bamboo, and mark it with red cloth strips at the water inlet and water outlet positions, so that you can see if they are in a horizontal position at the far away brothers." ”

Li Tangshan turned to Lu Yique and said, "Lu Yique, is the cloth ruler you prepared ready?" ”

Lu Yi nodded and said, "I will prepare three cloth rulers, each of which is dozens of zhang long." ”

Li Tangshan shouted to Wu Rui in the distance: "Okay! Wu Rui, you can yearn for water in the bamboo pipe! ”

Wu Rui waved the flag and shouted to the construction site: "Brothers, fill it with water!"

After a while, Shi Lu shouted: "The water at the outlet is overflowing, Wu Rui, you stop filling the water over there, Lu Yique, is the water inlet position and the water outlet position a horizontal position?" ”

Standing in the distance, Lu Yique looked at it carefully, nodded and shouted at them: "The two markers are in a horizontal position, you fix the markers." We can measure the next section of height, and we can move a little faster. “

Li Tangshan was overjoyed in his heart and said: "If you continue to do this, you can finally know what is the difference between the two rivers?" , and the error will not be too big in this way. ”

The next day, Cai Lun went to the Juntun District on time to find Li Tangshan, and when Cai Lun knew that Li Tangshan wanted to teach him skills and theories, Cai Lun happily agreed. Cai Lun was very attracted by Li Tangshan's junior high school level physics knowledge of this half-baked master, although Cai Lun knew a lot of vocabulary, Li Tangshan didn't dare to speak too deeply. Cai Lun was still impressed by Li Tangshan's being a top student of MIT and Beihang University.

Shi Lu and his team used the principle of atmospheric pressure provided by Li Tangshan to measure the water level between the two rivers is only between 7 and 9 meters, and the construction of the canal is much less difficult than expected. Li Tangshan was overjoyed and immediately notified Zhao Tuo to call them awaited. They wrote to Qin Shi Huang, and the Qin Emperor agreed with Li Tangshan and Shi Lu's construction direction, and ordered Shi Lu to build the canal according to the plan. Li Tangshan used his free time to teach Cai Lun the basic knowledge of papermaking, and Li Tangshan couldn't figure it out. Li Tangshan also taught him some basic principles of metal smelting technology, but Li Tangshan still didn't dare to talk too deeply, so that Cai Lun could slowly attract and master it.

Li Tangshan took advantage of the time difference in the book to make the construction plan of the main project of the canal, such as the ploughshare, the big balance, the small balance, the south canal, the north canal, the discharge balance, the culvert, the steep gate, the weir dam, the Qin embankment, the bridge, etc. Among them, there are 36 steep gates in the north and south canals (including 4 in the north canal and 32 in the south canal), as well as about 2 kilometers of embankments between the south steep mouth and the old road of the Xiangjiang River, all of which are marked one by one. Li Tangshan and asked Cai Lun to stay to help Shi Lu participate in the construction of the project, on the other hand, he could learn advanced manufacturing technology from Lu Yiqi and them.

However, the heavy rain for several days made Li Tangshan and them worried, but the worrying thing still happened, and the flash flood still washed away the channel dug in the early stage.