A brilliant modern scientist/engineer has been sent back in time to medieval Europe (somewhere between 800 and 1100 CE). It would be very advantageous for him to have many friends among the various Catholic kings and dukes of the era. He has excellent scientific, engineering, and medical knowledge. What services could he provide to his feudal patrons to gather wealth and allies?

The most obvious service would be medical treatment. Proper sanitation with soap and alcohol would be a first step. Medical knowledge quickly cures scurvy and several other ailments. Many terrible medieval afflictions such as leprosy and bubonic plague can be cured with antibiotics, which would take a bit more effort to produce.

Constraints & Details

  • His time machine is broken, and cannot be repaired.
  • He may have a few items intentionally brought back to the past (guns, a laptop, modern medicine, modern seeds, technical manuals), or alternatively a lab accident may have sent him back with just broken lab equipment and salvaged parts from the time machine.
  • If your ideas require specific technology to have been brought back, be sure to mention it.
  • Difficulties related to knowing Latin, the local language, court etiquette, and so on are outside the scope of this question. Assume the time traveler can get by.
  • Time travel paradoxes are not a concern. Assume a time travel model that resolves or avoids paradoxes, or a time traveler who doesn't care about changing the future.
  • If possible, the time traveler would prefer to keep technology for himself rather than giving it away. For example: a king would love to be given a modern gun, but the time traveler keeps them for himself and his inner circle.
  • Accumulating influence within the court of a king can be as important as impressing the king himself. Favors for the king's councilors could have a huge impact, especially councilors who perform the day to day work of running the realm or are the true power behind the throne.

I've read a few novels that cover this topic. In Connecticut Yankee the protagonist leverages his engineering knowledge (explosives, metalworking, firearms, electricity) to outdo Merlin and become the king's top adviser. In Lest Darkness Fall the protagonist builds a distillery and a printing press, and introduces modern accounting and mathematical knowledge.

I'm primarily interested in short term services that the time traveler could provide to his feudal patrons, on the order of weeks or months. Successfully curing the king's daughter of terrible illness is a perfect example of the kind of service I'm looking for. It doesn't transfer tech, only takes a few weeks, and might not anger the Pope.

The time traveler may wish to reserve explosives, firearms, and electricity for himself...

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    $\begingroup$ He can show him Star Wars on his laptop. $\endgroup$ Commented Dec 9, 2015 at 1:27
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    $\begingroup$ Someone had an architect travel to medieval times, and the services were personal comfort. worldbuilding.stackexchange.com/questions/27665/… $\endgroup$
    – Mikey
    Commented Dec 9, 2015 at 1:44
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    $\begingroup$ Possible duplicate of What could an average modern human achieve in medieval times? $\endgroup$ Commented Dec 10, 2015 at 22:33
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    $\begingroup$ @MartinSchröder Not a duplicate. The referenced question concerns an average person traveling unintentionally through time. My question concerns someone with exceptional knowledge and engineering skills who traveled intentionally. My question is also more focused on things that would be useful to existing rulers, rather than things the time traveler could build for himself. $\endgroup$
    – Ben
    Commented Dec 10, 2015 at 23:20

8 Answers 8


Setting yourself up as a medical expert would not necessarily be your best choice.

You will be in opposition to the dominant medical experts of the day. If you ever fail to produce the desired outcome, your unconventional practice will certainly be used against you. Some kings were known to behead failed doctors, etc. even close friends and family. If you are successful, your enemies may accuse you of sorcery.

Improvements such as soap and sanitation would not necessarily be popular as the value would not be a priori considered valuable. So getting a king to implement this and recognize this as successful may be very difficult. A crude optical microscope and basic germ theory may again just be witchcraft.

As a practicing physician your best bet may be to use the technology of the time, i.e., bloodlettings, etc. but supplement them with curatives of your own design that you don't bother to mention. Losing the occasional patient will still be be par for the course, but if you don't upset others by your unconventional medical practice, your better than average success rate will serve you and your patients well. Maybe by the time you get appointed chief court physician you can try some of your wacky ideas of sanitation, etc.

The scientific method was not exactly understood at this time. But advantages in science and engineering were generally valued by the educated, which certainly included the royalty and the clergy. You would not share the scientific understanding needed to understand new technologies, but your patron would certainly expect you to share the methods of production.

Just going into production of gunpowder would not make your patron generally satisfied. You would also need to produce cannons that could use the gunpowder (probably bronze unless you wanted to invent steel-making too). And even if you produced cannons and gunpowder, your patron would want to be able to produce them himself so as to not be 100% dependent upon you.

If you don't want to reveal tech, you pretty much cannot reveal it in small doses. Smart observers will pick up on your unconventional ideas that force you into areas you would rather not pursue.

I am a big fan of alternate histories. Lord Conrad Stargard of the Cross-Time Engineers series is essentially in the position of your character without the medical training. He ingratiates himself to the king by full scale engineering - so that he can repel the Mongol invasion of 1240 he is expecting in 10 years. Conrad has little choice - either prepare full scale to repel the Mongols or flee to another area not to be invaded. Since he is a Polish patriot he makes his stand. One thing I like about the series is that it takes time for Conrad to be accepted by the various layers of royalty and for them to put their trust in this outsider. Series had serious weaknesses (later books were pretty bad) and his sexist / racist views are pretty grating. His luck and assistance from the future can also be annoying, but the way be works with the royals is just what an engineer would do.

One way in which Conrad was extra lucky is that he just happened to be carrying a large variety of modern, non-hybrid seeds when he got sent back in time (part of trying to impress a girl). These seeds had a huge advantage compared to historical yields where you often had to plant 20% of your crop for next year's harvest. If you are actually planning your trip, such seeds would also be to your advantage, assuming you can dodge being labeled as a devil-worshiper.

Sorry, have to share one other issue with medicine. If your medicine is in the form of drugs, you may have a problem - route of administration. Injections are clearly not going to be available. Oral administration may be complicated by the corrosive stomach acid. Many medicines require special coating, etc. to get past the stomach, coatings that will not be available. Fortunately, enemas are often a useful route of administration, and as luck would have it, common practice in medieval medicine though as you might expect not as sanitary as you would like.

There are several other technologies that other people have mentioned, though you wanted to avoid sharing technology. The invention of distilled alcohol might be one you would be willing to share. It was already invented by the Muslims, but had not spread to Europe by 1000 AD. So, you could invent it in Europe and realizing the commercial value set up a distillery. Start with fortified wines, graduate to ports and sherries, and eventually hard liquor and you have customers coming to you for years looking for something new to drink. Royalty would come looking for you as a purveyor of fine spirits.

There may be other technologies that you would be willing to share. Not sure why you are not willing. But eyeglasses. mechanical clocks, the spinning wheel, the stern mounted rudder, paper, printing press, the wheelbarrow and others were all waiting to be discovered or introduced to Europe in 1000 AD. Perhaps some of these are things you would be willing to allow outside of your control.

If you really want to control kings, there is the tried and true method. Put them in your debt literally - i.e., become a banker known for generous loans to kings. You will want to become rich very quickly, but given your inventive nature this should not be too hard.

You have a pretty fundamental problem, trying to do this all in a few weeks or months. Getting on the good side with local royalty, etc. is likely impossible in a short time-frame. Your proposed saving the life of the King's daughter is probably unrealistic, the king will not allow you access to his daughter. It is unlikely you would even be allowed to talk to the King. His guard, counselors, etc. are designed as a filter to restrict access. To get access quickly you have to ingratiate yourself to someone else that has access to the king, but they them-self are not particularly hard to access. A member of the guard or household staff might have access (possible indirectly) to the king, and a miraculous cure of one of their family might grant you the inside track.

Such an approach does not give you the inside track to a number of royalty and clergy though. If you manage to get access to a single king in this way, he may treat you are a proprietary resource and not allow you access to anything outside his royal court. To guarantee access to a number of members of royalty you need broader influence that would make it harder for the king to control you. Of course, this argues directly against gaining access in such a short time-frame.

This popped into my head, and it would be of great value to kings and such-like. You just "invent" some pseudo-modern cryptography. You need to choose something much better than they are used to, but that can still be encrypted and decrypted by hand assuming you know the key, but without a computer you will not be able to decrypt without the key. You want something stable in the sense that errors do not propagate so that a mistake does not garble the rest of the message since encryption by hand is error prone. Cryptography was not even a subject of significant study at this time, so some kings would have to have the value of this explained to them, though they are likely familiar with the Caesar Cypher. Kings will understand the value very well once it is explained to them.

  • $\begingroup$ +1, good points all around,... even those which contradict my assertions. $\endgroup$ Commented Dec 9, 2015 at 2:05
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    $\begingroup$ Interesting thoughts about medicine, especially the idea of concealing real medical treatment with the medical concepts of the day. I haven't read the book you mentioned, but the process of becoming accepted by nobility and royalty is exactly the problem I'm trying to come up with a variety of ways to solve. $\endgroup$
    – Ben
    Commented Dec 9, 2015 at 2:24
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    $\begingroup$ Distilled spirits is a good idea that I was already considering. In the vein of alcohol and medical drugs... the time traveler may have prepared for his trip and brought back tobacco and hemp seeds. Becoming the royal drug dealer might put the time traveler in the position of a Rasputin-like adviser/mystic. $\endgroup$
    – Ben
    Commented Dec 9, 2015 at 17:18
  • $\begingroup$ It's true, the time traveler could not simply walk into the court of a king. Think of the services as an attempt to curry favor within the court... not merely with the person of the king himself. The king's steward might be impressed by agricultural and bookkeeping technology, his field marshal by detailed topological maps, and his castellan by improved fortification designs. Any of these men could give the time traveler influence within the court (or perceive him as a threat to their position). $\endgroup$
    – Ben
    Commented Dec 9, 2015 at 17:31
  • $\begingroup$ I guess the short term time frame is an attempt to avoid being pinned down by a single king. The time traveler could attempt to cultivate fame as a miracle worker who heals leprosy among the common folk, with the risk of drawing the attention of the Catholic church (for good or ill). $\endgroup$
    – Ben
    Commented Dec 9, 2015 at 17:42

Improve the plow and related technologies. Everybody needs to eat.

By the time you're talking about, western Europe already had the heavy plow, but plowing was hard, slow work. Plow technology didn't change much for several centuries, after which improvements to lighten the plow and improve its metallurgy were developed. If you arrive toward the beginning of your time range, you can also increase agricultural efficiency by introducing the horse collar (h/t Deolater). And you can further improve agriculture (no matter when you arrive) by introducing better irrigation techniques.

A time traveller who brings those designs (and metallurgy information) with him could offer a medieval king more, better, faster food production for his people. Even a king who doesn't care much about feeding his people (hey, he's got his) should still be interested in the time savings that allows those people to do additional work. Medieval farming was not a path to wealth, so those people aren't likely to just sit around doing nothing the rest of the time. More military levies, anybody?

I focused on agriculture here because it's at the core of food production during this time period. As noted in comments, other food-related improvements are also possible, including the beehive (h/t Draco18s) and bringing back modern or more-varied seeds.

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    $\begingroup$ Just to tag onto this: you could improve the beehive, too. The modern box hive is only 150 years old, give or take. Even not knowing the precise details of how they work, the mental image of them is pervasive enough that a time traveler would be able to figure out how to make one. Prior to 1850 the only way to harvest honey also involved the complete destruction of the hive and loss of the queen. $\endgroup$ Commented Dec 9, 2015 at 16:13
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    $\begingroup$ @Draco18s An improved apiary is a good idea. Along with improved plow designs, are there any other agricultural technologies that might impress King Louis the Pious? The time traveler could have intentionally brought back modern seeds not available in medieval times: potatoes, tomatoes, corn, tobacco, and modern cultivars of cereals, flowers, or fiber crops (hemp). $\endgroup$
    – Ben
    Commented Dec 9, 2015 at 17:10
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    $\begingroup$ If you're going to go for seeds, bring pineapple. Before widespread cultivation began, pineapple was a food for the extremely wealthy. Speaking of things that are easier to transport (e.g. knowledge only)...I am not sure. $\endgroup$ Commented Dec 9, 2015 at 17:19
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    $\begingroup$ @Draco18s I don't think pineapples grow well in Europe, especially not England or northern Germany. $\endgroup$
    – Ben
    Commented Dec 9, 2015 at 17:21
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    $\begingroup$ Depending on exactly when you arrive, the horse collar ( en.wikipedia.org/wiki/Horse_collar) could be a valuable contribution. If you're before AD 900 or so, you will be the only person in Europe with this trick. Per Wikipedia, "horses [with collar] work roughly 50 percent faster than oxen" -- a massive improvement. $\endgroup$
    – Deolater
    Commented Dec 9, 2015 at 19:37

If your traveller taught the Kings' advisors about germ theory, waste-management and the importance of controlling vermin, it could greatly reduce the threat of the various plagues which vexed that age. Taking that idea a step further, convince the King to set up a probationary quarantine policy for immigrants. Finally, teach them how and why they should boil water. A kingdom that knows and obeys these practices will survive and prosper while others sicken and die.

On a different tract, your time-traveller should have some modern musical training and a flute or acoustic guitar. With such tools, some talent and some knowledge of folk tunes and ballads, your traveller can keep themselves fed, sheltered and safe, regardless of the company he keeps. Such talent is also a wonderful way to get invited to court in the first place.

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    $\begingroup$ The sanitation stuff is much longer term than I was thinking, and Gary Walker makes some compelling points about it. Music on the other hand is an excellent idea, especially since many STEM folks are also skilled musicians. Taking credit for the works of Mozart and Beethoven might be unethical, but it would probably make you famous. If you know anything about medieval instruments and how musical notation has changed since then, would you be interested in expanding on that? $\endgroup$
    – Ben
    Commented Dec 9, 2015 at 2:19
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    $\begingroup$ I've looked at historical musical notation before. At 1000 AD it was very primitive. At best, you might have some relative pitch, Rhythm was usually totally missing. The idea for musical notation was if you knew the music, but needed a little reminder of how it went, the notation was sufficient. Modern musical notation was $\endgroup$ Commented Dec 9, 2015 at 2:55
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    $\begingroup$ "Taking credit for the works of Mozart and Beethoven might be unethical, but it would probably make you famous. " If you really wanted to be ethical, you could say something like "I have heard that these pieces were written by someone named Wolfgang and someone named Ludwig." $\endgroup$
    – user24353
    Commented Dec 23, 2015 at 10:11
  • $\begingroup$ I'm not sure it would be easy to convince people about hygiene benefits... even today, with all the knowledge we have it's hard to convince people about the importance of washing hands, waste management etc. $\endgroup$
    – Bebs
    Commented Jul 16, 2020 at 12:03

If you had access to writing/drawing materials and were a reasonable artist, you could demonstrate perspective drawing. You may also be able to help architects and engineers by showing them the principles of isometric projection.

You could also entertain them with modern board and card games that you could fabricate. You might need to adjust rules a bit, you couldn't insist on precise spellings in scrabble and monopoly would be difficult to grasp, without tweaks, in a feudal economy.

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    $\begingroup$ I don't think board games would be successful, but modern chess and go would probably be popular. Siege and fortification engineering seem like good fits for a brilliant scientist time traveler, especially if he's mechanically inclined. $\endgroup$
    – Ben
    Commented Dec 9, 2015 at 17:36
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    $\begingroup$ @Ben board games go back pretty far -- hnaftafl (Viking-age), nine man's morris, and dice games at least. Whether you could get the royal court interested in a round of Settlers of Catan is a different question, though. $\endgroup$ Commented Dec 9, 2015 at 22:04
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    $\begingroup$ Games aren't really the height of what a brilliant scientist could bring. They're more what I could achieve, but I like the idea of feudal lords winning and losing their fortunes at Texas hold'em. $\endgroup$ Commented Dec 10, 2015 at 12:00
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    $\begingroup$ Or bringing a few sets of classic playing cards and teaching people poker or blackjack or beggar thy neighbor style games. $\endgroup$ Commented Dec 30, 2018 at 10:46

Let's assume this is the early 800's and Charlemagne is in power.

Answer any questions he has for you

Given the timeframe of weeks to work with, the fastest and most powerful services renderable to a king will be conveying modern, accurate information. Architectural projects take years or decades to pull off. Training metallurgists or blacksmiths can take months (though highly skilled blacksmiths could be taught more quickly.)

Establish Credentials

You'll need to do something to gets you the respect of Charlemagne and his court. If you can't do this then you'll be labeled as a lunatic and imprisoned or executed.

Possible means of proof-of-credentials:

  • Medical - Risky as your treatments may fail. Treatments are also highly dependent on the expertise of the time traveler (as I'm assuming that modern medical supplies didn't make it on through on the trip back in time.)
  • Physics - Simple physics demonstrations. Demonstrations involving simple grade-school level physics principles might do the trick and can be organized quickly.
  • Astronomy - Make predictions about location of planets or predict astronomical events (a la Connecticut Yankee).
  • Engineering - Making a small tool to solve a common vexing problem (or just designing a small tool which is then made by a blacksmith). Designing a new weapon system
  • Weapons - Built a gravity-powered trebuchet. Normally, these wouldn't be invented till 1000AD. The traveler doesn't need to design a Warwolf but even a smaller version would outperform any siege weapon in Charlemagne's arsenal. (This is my preferred method for establishing credentials for a number of reasons: Charlemagne was a warring king with plenty of combat experience. Anything that makes his conquests more successful will greatly increase the time travelers prestige.) (Work on a Floating Arm King Arthur trebuchet in secret which is far more powerful than a traditional gravity powered trebuchet. It is, however, more complicated and error prone.)

Personally, I would avoid giving the impression that I'm a magician, preferring to establish the reputation as an incredibly well-learned man.

Realize that the time traveler is operating in an environment of science based on extreme inferences from old Greek and Roman texts.

Standards in Weights and Measures

Standard weights and measures across Holy Roman Empire will go a long way to encouraging and enabling commerce...and where there's commerce, there's money to be made. The traveler may not have to push through an entire monetary modernization program, just plant the idea in Charlemagne's head along with the benefits that will come from having standards of measure. With most any king, if you can show how a particular monetary policy will make him richer (to achieve his ends) and his country more powerful, he'll probably go for it.

Invent the Corporation/Markets

Unleash capitalism on the world! There will be lots of muddling around with the concepts and getting the rules right but capitalism excels at generating wealth (it also excels at oppressing and exploiting poor people, so maybe introduce ethics too).

Invent the rule of law

  • Basically invent the Magna Carta early. This may be really tricky because it took some really strong arm-twisting to get King John to accept the Magna Carta in England. Perhaps if you make a really good case to Charlemagne and his nobles, showing them how everyone would gain, they might accept it. Maybe.
  • $\begingroup$ Do you have any specific ideas about what actions you could take to establish credentials? Your mention of Charlemagne made me think of maps- modern versions would be enormously superior to medieval counterparts and very valuable to a conqueror (or a merchant!). Economic reforms would probably be longer term... you mention standard weights and corporations, but I would also include decimal numbers, letters of credit and double-entry bookkeeping. $\endgroup$
    – Ben
    Commented Dec 9, 2015 at 16:57
  • $\begingroup$ @Ben I've added in a few methods. Building an operational gravity-powered trebuchet could be done quickly (week or so) with good carpenters/blacksmiths. $\endgroup$
    – Green
    Commented Dec 9, 2015 at 18:29
  • $\begingroup$ Charlemagne actually did reform coinage and associated weight measures in 793 or so. It somewhat lasted till 1971 when Great Britain decimalized its currency. $\endgroup$
    – Crissov
    Commented Dec 9, 2015 at 22:33
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    $\begingroup$ Hot air ballooning would be very useful in both warfare (spotting enemy positions) and map-making as well. Anyone with a vague knowledge of even primary-school science should be able to make one from existing materials in almost any time period VERY quickly. Credentials established. $\endgroup$
    – IchabodE
    Commented Dec 9, 2015 at 22:58
  • $\begingroup$ Hot air balloons were invented in the 1783. So it sounds like a big advance. I wonder though if it would really have the intended effect. I.e., once people see it they think, "I could have done that." Thus, discounting your credentials. Same thing applies to several other inventions mentioned, but the way you phrased it made me think that something new, but too simple to replicate would not have the desired effects. There was no such thing as patent law so your only protection was a trade secret or a guild willing "to convince" non-guild members that using the idea was not recommended. $\endgroup$ Commented Dec 12, 2015 at 6:20

Swords first because that is a portable skill that could be established in the hinterlands away from the primary seat of power, and could facilitate an introduction to the king.

Assuming excellent knowledge of metallurgy, Your time traveler could establish his bona fides with making really good swords (or whichever weapon the king of the realm favors) Nothing speaks to power like a better weapon. Good horse shoes, stronger armor, that kind of thing.

The Franklin stove or a sealed box cast iron wood stove for heating the impossibly drafty castle. Appeal to the vanity and comfort of the king to build reputation in a way that avoids accusations of witchcraft. Mr time traveler could build out from there as an adviser, but he would want to stay as far away from court intrigue as possible. Improvements to sanitation and how to dig good wells would be next on the agenda, proposed as ways to improve fortifications against a potential siege.

Broader agricultural improvements would spill out from there, crop rotation, the horse collar, developed around the same time as the trebuchet.

One of the real keys would be to keep a steady stream of weapon systems, kings comfort products, and stuff to benefit society all about in equal measure in order to secure a place and to keep it, along with a strong but quiet reputation so that you are kept around or overlooked when the inevitable, possibly violent, change in power happens. Remember that plenty of medieval rulers met their end on the point of an assassins dagger or to poison rather that in the chaos of battle.

Avoid excercising knowledge that involves anything that the cause and effect cannot be clearly seen and try not to jump too far technologically at a time. Anyone can see how a trebuchet works. A horse collar is one of those "why didn't I think of that" inventions. So is the ball bearing. You can move things along very fast without being suspicious this way.


Clarke's third law: “Any sufficiently advanced technology is indistinguishable from magic.” As a result the scientist probably would be killed for being a magician.


From the book Sapiens, it seems that for the majority of human history, the way technology was viewed was that it was largely unchanging. It was in the realm of possibility that an Roman Legion had the tools to beat most medieval armies. Since the fall of the Roman Empire, technology has been viewed by medieval Europeans as being in a standstill at best, and in decline at worst. Nobody thought that technology was the answer to anything, and its developments came at an incredibly slow rate, due to the lack of formalized education and the absence of financial support in discovering new tech. With religious institutions believing that they hold all the answers (and frequently condemning individuals that claim to know more than them), people stayed in a state of massive ignorance where they believe they knew everything, and everything they don't know isn't important enough to learn.

So the better questions to ask would be the following:

  1. Would the culture even be receptive of future tech, even if the proof of concept works?

  2. How will the technology even be disseminated to the masses? Will it be kept in the hands of the people that the time traveler teaches and then quickly forgotten about?

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    $\begingroup$ While I agree in general with your sentiments, I think that at least from the 13th century onwards the beginnings of the scientific method were starting to change the perception that technology was static. Even within a single person's lifetime in the 13th-14th century, you could watch the expansion of mechanical clocks, reading glasses, cannons or paper, if one were of the educated class. $\endgroup$
    – kingledion
    Commented Jan 11, 2017 at 17:35
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    $\begingroup$ The 13th and 14th century is at least 100-500 years after the time period mentioned in the OP. This is like pre-gunpowder Europe only a couple centuries removed from the fall of the Roman Empire. From the end of the Roman Empire until the discovery of the New World, most of Europe was this derelict backwater patch of squabbling fiefdoms compared to the Ottomon and Chinese empires. Even then, I doubt that those two nations had much interest in scientific progress. Gunpowder existed for 500 years before anybody thought to weaponize it. $\endgroup$
    – Alexexy
    Commented Jan 12, 2017 at 0:04

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