"If you have the opportunity, you can go and look for the history books, and then list the time when the drought occurred in the Central Plains and the time when the northern barbarians invaded south."

"In this way, you can intuitively see that whenever there is a drought in the Central Plains Dynasty, it is the time when the probability of the barbarians invading the south is the highest."

"Even if you don't go south that year, the probability of going south the next year will greatly increase."

"Why?" Li Shimin did not doubt the truth of Qin Tian's statement.

This kind of thing can be judged by only looking through the history books when you go back, and there is no need to lie here.

Qin Tian nodded slightly: "If you can accept this statement, then it will be easy to explain next." ”

"In the same way, you can also observe that once a flood disaster occurs in the Central Plains, the probability of barbarian invasion will be greatly reduced."

"What do you think of with two seals?"

Li Shimin was thoughtful.

Qin Tian revealed the answer: "Although the grassland inhabited by the barbarians is far away from the Central Plains, in fact, even if they are thousands of miles away, on the same continent, the climate of the two places is closely related. ”

Li Shimin nodded, as if he understood what Qin Tian was trying to express.

"Why is this the case?" Sun Simiao hadn't reacted yet.

"It's very simple, because of the floods in the Central Plains, for the grassland people, it is the season of fertile water and grass, and they can eat well and eat well, why do they have to go south to plunder?"

"On the contrary, if the Central Plains region lacks the moisture of rain, the northern grasslands will naturally not be mentioned, the vegetation will dry up, the grasslands will decrease, and people will starve to death.

"Have you ever wondered why most of the north is grassland, while the south is rare and more forested?"

"Why?"

None of the people present had ever thought about it, and in their cognition, it all seemed to be formed naturally, where there was a why.

Qin Tian smiled without saying a word, dipped his fingers in some tea, and casually drew a figure on the table.

"Can you see what this is?"

"This ......"

Qin Tian's drawing was actually a bit abstract, because Li Shimin had never seen a high-precision map, and Qin Tian's drawing was too accurate, so he didn't dare to confirm it.

"This is Chang'an."

Qin Tian tapped lightly on the map.

With the comparison, everyone present suddenly understood.

This is the map of Datang!

"Brother Qin is really well-informed, and he can even draw this kind of thing casually."

Li Shimin seemed to be testing Qin Tian.

Qin Tian didn't pay attention to it, but instead drew a curved curve from west to east on the map.

"So, did you find anything?"

Li Shimin frowned, he vaguely sensed something, but he couldn't grasp the fleeting inspiration.

"Above the curve, there is the grassland, and below the curve, we live."

With an explanation, Li Shimin suddenly understood.

The place where the curve passes is a clear dividing line, separating the nomads from the people of the Central Plains Dynasty.

The line drawn by Qin Tian is the famous 400 mm isoprecipitation line.

"See what?"

Li Shimin frowned and thought hard.

His mind echoed the terrain in the vast territory of the Tang Kingdom.

"If you look at it this way, the farther north you go, the colder it gets, and the more arid the land becomes!"

Qin Tian glanced at Li Shimin appreciatively.

"So this is a good explanation why there are such different living habits and ways in the north and south."

"Why are the barbarians of the north unwilling to cultivate the land, is it because they are unwilling? Not really. ”

"Because the further north they go, the more difficult the living conditions become, so they don't really have the conditions to farm!"

The amount of water required for farming is actually quite large, so the amount of precipitation on the grasslands is simply not enough to meet the needs of farming.

They can only make a living by grazing their cattle.

"However, due to the emergence of drought-tolerant crops such as corn, sweet potatoes and potatoes, in fact, there are already many people living a half-farming and half-grazing life above this curve."

Qin Tian nodded his finger: "For example, here, I believe Brother Li must have an impression." ”

Li Shimin nodded again and again.

"What do you think is the biggest difference between farming and grazing?"

"One eats meat and milk, and the other eats grain?"

Li Shimin gave a relatively simple answer.

Qin Tian shook his head and said, "It's controllable." ”

"Although they all watch the sky for food, on the grassland, a heavy snowfall may freeze all the cattle and sheep of the herdsmen, and the herdsmen will have no way to survive in the coming year, so they can only go south to plunder."

"But as for farming, even if there are various natural disasters, there are still a few cases where there is no harvest."

"Poor harvests may be frequent, but a bad harvest is basically impossible."

"So......" Qin Tian paused: "Compared with the herding people, the life of the farming people is more secure. ”

"And how many people can be fed by the same land, five acres of land? But what can you do with a five-acre pasture? ”

The Tang Dynasty itself has a high degree of Hu, and the people often raise horses, cattle and sheep, Li Shimin naturally knows how big the gap between the two is.

"So in the end, the difference between the north and the south is just a difference in how much rainfall!"

"It rains a lot in the south, which allows plants to grow, so there are many forests and there are plenty of conditions for farming."

"But there is little rain in the north, so only pasture, which consumes less water, can grow, and even tall trees can't grow, let alone farming!"

Li Shimin suddenly felt very novel, it was the first time he had seen someone analyze a problem like this.

It's a completely new angle that no one has ever thought of before.

"The weather will change, and there will naturally be disaster years in good years." Qin Tian said lightly.

"Imagine what would happen if the weather was persistent dry and the amount of precipitation was decreasing?"

Qin Tian drew a line again with his hand.

"This line separates the steppe from the Gobi desert."

Then, Qin Tian drew an arrow below the curve at the beginning.

"The closer the place where you can farm, the less pasture you can graze, the closer you get to the south......"

"The result is a barbarian invasion south!" Li Shimin clapped his palms!

It may seem out of tune to analyze national events from this perspective, but when you think about it, it is very reasonable!

The logic that Qin Tian said was linked one by one, closely connected before and after, and there was no flaw at all!

"But if it is the other way around, the south will have bumper harvests year after year, and the wind and rain will be smooth......"

Qin Tian drew an upward arrow, and a smile appeared at the corner of his mouth.

"At this time, it is time for the Central Plains Dynasty to expand northward."

"It was never the will of the Khan that forced the barbarians to invade south, but that everyone could see the rain!"

Qin Tian gave his own conclusion.

"If there is abundant rain, what if the barbarians can grow? How many people can they feed in one acre of pasture? How many people can be supported by one acre of land in the Central Plains? ”

"When the precipitation is abundant, the barbarians will never be the opponent of the Central Plains, because the Central Plains will only develop faster and stronger than them!"

"The way they are harvested in agriculture dooms them to grow more efficiently than the nomads!"

"But on the other hand, once it enters the dry season, the combat effectiveness of the Central Plains Dynasty will be greatly reduced."

"That's not right, is it?" Li Shimin subconsciously retorted.

"The Central Plains Dynasty was stronger in the harvest era, so logically speaking, the dry season should be exactly the same!"

Everyone is not happy, why are the nomads stronger in times of drought?

Qin Tian shook his head: "The drought years mentioned here are not these years or those years, but in a cycle of ten or even a hundred years. ”

"The five elements of the yin and yang cycle rotate, and the abundance of rain is actually cyclical."

The earth's climate change will naturally not be due to some yin and yang cycle, but this method is easier for these people to accept, so Qin Tiancai said so.

"During this period, there is an occasional major drought when there is abundant rain, or when the wind and rain are smooth for several years during the great drought, these are very normal fluctuations."

"Looking at history, don't be limited to the one acre and three points of land in front of you, but take a long-term view."

"What would happen if the next 100 years were to be a time when precipitation was rising?"

This conclusion has already been said before, so naturally there is no need to say it again.

But what if it's the other way around? Why is it that in times of drought, the cultivating peoples are often no match for the nomads? ”

Qin Tian reminded: "The answer to this question can also answer why nomads have always been the confidants of the Central Plains Dynasty. ”

"This ......"

This is completely a blind spot for everyone in the room, who has never even thought about it before.

Therefore, everyone present, including Li Shimin, fell into deep thought.

Qin Tian raised his eyes and looked at the sky.

"It's not too early, so let's stop it for today."

"No, how can it end here!" Li Shimin subconsciously retorted.

But when he looked up, he was surprised to find that the temple fair had long since ended, and the sky had already darkened.

"If you don't go back, it's time for curfew." Qin Tian said with a smile.

Li Shimin was stunned.

But it's just a matter of small talk, why is the day gone?

"No, ...... today" Li Shimin looked at Qin Tian, swallowed half of his words and returned to his stomach.

"No, it's not the time yet, it's not the time to reveal your identity to Qin Tian."

Li Shimin glanced at Wei Chigong.

Wei Chigong couldn't understand these things at all, and there were already all kinds of circles in his eyes.

As a result, he stopped listening to the conversations of several people early on.

I don't understand, my head hurts, and I'm going to grow my brain!

"Well, that's all for today!" Li Shimin stood up and arched his hand.

"Another day, Shi will definitely visit in person, thank Brother Qin for his help, and ask Brother Qin for the answer to this question."

Li Shimin smiled heartily: "I will also bring my answer when the time comes, if the answer is not good, I hope Brother Qin will not laugh at me." ”

Qin Tian smiled, saluted everyone, turned around and left with Mu Chun and Ling Xia.

After leaving, Mu Chun looked at Qin Tian with some surprise, his eyes full of admiration.

"Master, you know a lot!"

Qin Tian smiled slightly: "It's not that I know much, I'm just standing on the shoulders of giants." ”

"What giant? Who? Where did the giants come from! ”

Ling Xia's eyes widened.

Qin Tian smiled dumbly: "Don't think so much, just eat your honest and play with yours!" ”

"Oh!" Ling Xia nodded heavily.

Heartless, she quickly put the matter behind her and ignored it.

On the other side, Wei Chigong asked tentatively: "Sage, do you want to ...... that?"

Li Shimin hesitated for a moment: "No, no, it's not the time yet." ”

"Wait until I test his tone!"

Li Shimin is eager to seek talents, so naturally he will not let go of such a talented person who knows astronomy and geography in the field.

Today's conversation with Qin Tian is better than him reading for ten years!

It may seem that these things are useless at all, but when you know the essence of the matter, many problems will be solved.

It's like Li Shimin never thought about how to solve the problem of nomads once and for all.

But now, there was a vague thought in his mind.

"Let me think about it, why is that......"

This night, Li Shimin did not fall asleep.

According to Qin Tian's theory, since farming is superior to grazing, why is the status of the two reversed in the dry season?

Li Shimin had no sleep all night, and even when he went to court the next day, he was listless and full of dark circles on his face.

Li Shimin thought about it for three days, but he couldn't come up with an answer that he thought was perfect.

Those thoughts were ridiculous even to him himself.

In the end, Li Shimin personally brought Wei Chigong to visit Qin Tian.

……

"I thought you'd wait a few more days before you came." Qin Tian looked at Li Shimin and said with a smile.

"There are some things at home that I need to deal with personally, so I lost some time." Li Shimin's face was a little unnatural.

"This time I came mainly to thank Brother Qin for his help, so I ......"

Qin Tian smiled and shook his head: "That's it, just deceive yourself with this, you can't deceive me." ”

"Come in, since you want to know the answer to that question so much, then I'll give you the answer!"

Li Shimin hurriedly handed over the results of his thinking in the past few days.

Seeing that Qin Tian was a little surprised, Li Shimin hurriedly explained: "This is my idea, please ask Brother Qin to help rectify it." ”

Qin Tian couldn't laugh or cry a little.

Looking at Li Shimin's paper-like strategy, Qin Tian inexplicably had a feeling of collecting homework.

But he still opened it and looked at it again.

"Yes, it's hard to think of that." Qin Tian gave his own comments: "Although some of the content is too shallow, it is not all without merit. ”

Li Shimin's face was a little helpless.

This is a strategy that he meticulously spent three days writing, and it ended up with such an evaluation in front of Qin Tian?

Although he said that these things were not worth mentioning, Li Shimin was still a little proud after writing them.

He even felt that Qin Tian was shocked after seeing this strategy, and then ......

(End of chapter)