For Yin Yangcheng and Duyang City, which are only a dozen miles long, the width of the bottom of the city wall is only five or six meters, and the city wall can indeed be dug down by directly digging the corner of the wall from the ground, but for the city wall of the city wall of more than 30 miles in circumference, the width of the city wall is mostly more than 12 meters, just by digging the foot of the wall from the ground, without hollowing out the foundation of the city wall, it is difficult to dig up the local city wall, and it is necessary to pay great casualties.

Looking at the scorched earth outside the city that had been cut to the ground, Bai Liyan asked:

"Brother Sikong, no other abnormalities have been found?"

"Yes, yes, but the end will not seem to be abnormal."

"Oh, tell me, what's so unusual."

"The rebels were mainly concentrated digging corners within thirty paces on either side of the gate, and about five paces apart about ten paces away, both on the left and right sides of the gate."

"Shhh...... Take me to see these things! ”

Digging the foot of the wall usually concentrates on digging the weak points of the city wall, either to excavate a few weak points, or to excavate one or two places, it is impossible to dig two places to separate the distance and dig two places, and then separate a section in the middle, which can not improve efficiency, but also may be half the effort.

When I came to the thirty-two steps east of the south gate, I found that after the rebels dug thirty steps to the east, they inexplicably jumped five steps and continued to dig ten steps east.

"It's weird, what kind of reputation are the rebels doing!"

"General, is there any fraud in it?"

"There must be fraud, the problem is that even if we know that there is fraud, it's useless if we can't see it."

As the saying goes, 'when things go wrong, there must be demons', and it is obviously not normal for rebels to jump five steps away without digging. Just as he was thinking about it, he suddenly heard that Sikong Nan stopped drinking:

"General be careful!"

As soon as the words fell, several arrows pierced Bailiyan head-on, Sikong Nan and several others were quick with their eyes, and the wooden shield on three sides instantly blocked the arrows that were fired, and Bailiyan reacted like lightning and immediately lay down on the ground, rolling backwards and quickly hiding in the bunker.

"General, it's okay!" South Sikong Road.

"It's fine."

Bailiyan stood up, followed the direction of the arrow, and saw that the car thirty steps away was facing the section that had not been excavated under the city wall, and it seemed that the location of the car existed as a reference.

"Brother Sikong!"

"The end will be."

"Go, go to the west side of the south gate thirty steps, and see if there is a temporary car in front of the west side thirty steps!"

"What does the general mean?"

"The rebels must have dug a tunnel thirty paces away, go and see!"

"No!"

The corner of the car and the unexcavated wall are located on the same line, and it is obviously not possible to provide reference instructions to the rebels who are digging the wall, but to provide reference instructions to the rebels who are digging the tunnel.

The reason why the rebels did not dig the corner of the wall was not that they could not dig it, but that the dark passage had been dug under the corner of the wall, and once it was dug from the ground, it would be exposed.

When he was young, Sikong Nan confirmed his previous guess, thirty-three steps to the west of the south gate, thirty steps to the front, and sure enough, there was also a car.

"General Wei, how is it now?"

"Go, organize the brothers, from the inside of the south gate to the east and west, thirty-two steps each, dig me one person high in the ground, dig under the city wall, I want to see what the rebels are doing!"

"No!"

At this time, Bailiyan was almost certain that the rebels must have dug a tunnel under the city wall, but why they dug so much is unknown.

Sikong Nan organized a hundred people, divided into the east and west sides of the south gate to dig in turn, digging into the wall about a zhang, a foul smell immediately came to the nose, the soldiers covered their noses and cursed:

"Fuck, it stinks."

"Bastard, what the hell is so disgusting!"

……

There was an extremely disgusting stench in the air, even more disgusting than the smell of decomposing corpses.

"General, I don't know what the rebels have buried in the city walls, the stench is overwhelming. In the event of an epidemic, the consequences are unimaginable. Sikong Nan reminded.

"Let's go get quicklime and alcohol, and all the diggers cover their mouths and noses with cloths, and keep digging for me."

Bailiyan can be sure that the stench emitted from the walls is not the smell of decomposing corpses, but more like the hydrogen sulfide emitted after the fermentation of decaying plants and human and animal waste. Just like a septic tank, fermentation produces a large amount of toxic and harmful hydrogen sulfide gas.

Under the strong order, the soldiers continued to dig into the city wall despite the stench, and at dusk they dug to a point of two feet, and suddenly found a wooden barrel two feet in diameter and about three feet high. Barrels were piled on top of each other, five tiers upwards, and the tunnels on both sides led to the city gates.

This means that in the past half a month, the rebels have successfully diverted Bailiyan's attention by using stone cannon carts and bed crossbows to bombard the city wall for half a month, and secretly dug out a rectangular space with a height of one and a half zhang, a width of seven feet, and a length of nearly twenty-five steps within thirty steps on both sides of the south gate, and piled hundreds of wooden barrels into it.

The barrel was dragged out and opened, and a piece of soldiers were spit out on the spot.

Nothing else, the barrel contained only human feces and rotting straw, and it was only half a barrel.

Seeing this scene, Sikong Nan's face was like ink, and he couldn't help but say:

"General Wei, the black scarf thief has been busy for a long time, just disgusting us?"

Bai Liyan said coldly:

"Hmph, it really lives up to the divine power of the Thunder Heavenly King, and I really underestimated the black turban thief."

Sikong Nan was puzzled:

"What does the general mean?"

"Brother Sikong, you can know why the army of Lu Ji died in battle with Ma Genghe back then."

"Was it smoked to death by dung?"

Everyone couldn't help laughing, but when they saw Bailiyan's face, no one dared to be happy. At this time, Bai Liyan continued:

"In fact, dung can also kill, and the army of Lu Ji is deterred by such dung buckets buried in the ground."

Human and animal manure can not only produce hydrogen sulfide gas, but also can produce biogas with plant decay, biogas and a certain amount of hydrogen sulfide gas, ammonia are explosive gas, of which the biogas explosion point is very low, and ammonia and hydrogen sulfide gas detonation point is high.

Therefore, under normal circumstances, hydrogen sulfide gas and a small amount of ammonia in the open septic tank are not easy to explode even if they encounter an open flame. But if it is semi-closed and there is a small amount of oxygen, a spark will explode violently.

The rebels hid wooden barrels capable of producing biogas and hydrogen sulfide gas into the walls of the South Gate, in order to detonate the walls at a critical moment and blow them down. The reason why the foot of the wall was dug on the east and west sides outside the south gate was to make the city wall fall outside the city at the moment of the explosion, and not to the inside of the city.

If it is dumped into the city, it will inevitably form a high slope outside the city and a low slope inside the city, and the rebels still need to use equipment to kick the wall to attack the city, although the height of the climb is greatly reduced, but it is still impossible to pass quickly.

If it is blasted outward, it will inevitably cause the city wall to collapse outward, and finally form a slope that is high inside the city and low outside the city, which is conducive to the rebels attacking the city wall and attacking the city.

At that time, the Lüji army and the rebels fought at the Mageng River, and the rebels annihilated the Lüji army with absolute strength and without much effort, and after the capture and other intelligence corroboration, it is almost certain that the rebels used some kind of explosive material of unknown circumstances.

Bai Liyan initially suspected that the Black Turban Army might have explored black powder, but since then, there has never been a figure of black powder, and the current productivity is not enough to mine a large amount of nitrate and sulfur, especially less sulfur, to launch an explosion that deters 80,000 people, the dose will not be less.

Now, the means of explosion used by the Black Turban Army should be biogas and hydrogen sulfide gas. The explosion caused by the hypoxic deflagration method is most likely the origin of the name of the "Thunder King" of Thunder.

Of course, the explosion of biogas alone was not enough to severely damage or even annihilate Lü Ji's 80,000 horses, but in the face of such an unprecedented loud noise, it was undoubtedly a huge blow to the morale and morale of the entire army.

At this time, Sikong Nan asked:

"General, what's the explosion?"

"Explosion, ......" [Note 1] Baili Yan suddenly found that not only did he not have the word explosion nowadays, but also did not know about explosion, which was indeed a thorny problem. Thinking of this, he continued, "The so-called explosion, the sound is like thunder, it can be seen that with the light of fire, you can hear a loud noise, and it hurts people's lives, so it is called an explosion."

As I expected, the barrels inside the city walls were enough to blow up the walls in an instant, and the consequences would be unimaginable. ”

"It is so powerful that the barrel will be brought out quickly, so as not to hurt the brethren."

"No, although this method is powerful, it is not without flaws, as long as it is exposed to the wild and the stench is dispersed, there is no longer a threat."

The hazard of biogas lies in the deflagration caused by open flames in confined spaces, and ventilation measures are taken to dilute the explosive gas within the unit density, which can effectively reduce the critical point of biogas explosion concentration. The rebels only filled the barrels with half of the straw and dung, just to have enough space to produce explosive gases, which could cause a huge explosion in an instant if they were exposed to open flames.

The soldiers took out several wooden barrels one after another, and sure enough, they found a cave that longed for the height of one person outside the city, and the ground of the cave was poured with pine oil and oil, all the way to the base of the barrel, and the rebels only needed to light a fire at the end of the entrance to the road outside the city, and the entire south gate could be detonated in an instant.

The two cars on the secret passage are either the rebels to maintain the unobstructed passage and hide the entrance of people's eyes, or to provide an outlet for a fire in the secret passage, to prevent the air from continuing to burn after the air is burned out because the secret passage is too long, and the car is used as a barrier to dig a hole under the car.

Unable to avoid startling the grass and snakes, Bailiyan asked people to fill the wooden barrel back in its original place, temporarily bury it with soil, and then let people make holes in the inner side of the city wall to breathe.

Before dark, on the way to the Guangxin Mansion by car, the strong men collected by the Gongfu were carrying hoes and copper shovels in groups, heading towards the south of the city, and Bailiyan immediately accelerated his speed and rushed to the Gongfu.

If Luo Songting were to build an urn city in the south of the city at this juncture, once the city wall exploded, the consequences would be unimaginable.

[Note 1] Due to the different levels of cognition, concept, and understanding and mastery of natural phenomena, humans and animals have an innate instinct to fear and fear the sound, light, and electricity of unknown phenomena. When people are faced with the fear of the unknown, their mental will is highly susceptible to the blow of unknown external causes.