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So in my fantasy world I have a river that is in the shape of a perfect circle if viewed from above. Lets say the diameter of the interior is about 10 km, and the width would be wide at about 200 meters. So a small city could exist in and around the river. The river is magically powered such that it has a constant current that flows through it at a consistent strength. It also magically feeds itself, such that water taken from the river would be normal water, but the river would never run dry.

The river can be utilized for many things, such as powering machines with waterwheels. But that can be done on any river.

I am looking for ingenious, special uses for a river that feeds back into itself. For instance, in this river you could drop something into it, and it would eventually come back to you - something that is impossible in a normal river.

Since this is a fantasy world with magical elements, some magic exists, but it's not a heavy magical world, as such this river is a very special location and I want it to have a very special touch.

The societies in the world may have basic industrial elements like gears, waterwheels, mills, etc. But not steam-power or electricity. How could this river be utilized in a clever and special way that a normal river can not be utilized in?

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    $\begingroup$ You basically have an infinite source of water. With a pump and some patience you could flood the world. $\endgroup$ – Lu22 Jan 4 '17 at 12:00
  • $\begingroup$ I suppose that is True. But let's also assume there would be other fantastical elements in this world too, which would eventually balance each other in some way. $\endgroup$ – Inbar Rose Jan 4 '17 at 12:01
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    $\begingroup$ You have a source of infinite energy and infinite power, also known as a perpetuum mobile of the first kind. Hint: divert some water in canal and power a mill or a factory; repeat ad infinitum because the magic keeps constant the amount of water in the river. $\endgroup$ – AlexP Jan 4 '17 at 13:23
  • $\begingroup$ @AlexP That seems like the start of a good answer, please elaborate :) $\endgroup$ – Inbar Rose Jan 4 '17 at 13:28
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    $\begingroup$ I'd be worried about the quality of the water - "you could drop something into it, and it would eventually come back to you" implies that there isn't a (magic) purifier built into the river, so all sorts of pollutants will accumulate in the water and the sediment... $\endgroup$ – Michael Schumacher Jan 4 '17 at 14:51
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Power heavy industries

Water wheels were used first and always primarily for milling grain. millrace channels could take this water throughout the city to power water-wheels all over. In a city environment, they could be used for throwing silk, fulling wool, scutching linen, spinning yarn, driving weaving shuttles, in trip hammers for anything that needs to be pounded, particularly iron and metal, de-husking rice, crushing wood/bamboo pulp for papermaking, rolling metal sheets (like gold leaf), spinning grindstones, and driving air into a blast furnace. Surely there are other things I missed.

Dispose of garbage

If you cut a channel leading out of the magic river, and direct that channel to somewhere else that you don't care about (a desert, the ocean), you can dump all your trash into the channel and get rid of it. The magic river will replenish itself

Irrigation

Suppose you live in a seasonally dry climate that is warm year round (like the Mediterranean, or the African savannah, or the plains of North India). Alternately, this city could be in a hot desert like the Sahara. If there were an unending source of water and warm climate all year round, you could grow crops all year round. And not just any crops, extremely productive ones like paddy rice. An unending water source could allow you to flood fields and produce two or even three crops of rice per year. You could even grow fish, shrimp, or crawdads in the paddies to really up the calorie density. Needless to say, you could sustain a very large population with such food productivity in a small area, even with medieval technology.

Transportation

Everything too heavy for one person to carry would travel by barge in this city. Canals leading off of the main river could be built on any flat land near the city. I imagine the city itself would be crammed up onto and over the banks of the river, just as Venice is built over its canals. Anyone who wanted to move, say raw iron bars from the foundry to the smith's would just toss them on a barge and pole them through the channels to their destination. This would significantly reduce the amount of pack animals in the city, which would allow high population density with much less manure in the street.

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  • $\begingroup$ This is a really good answer, but everything here can be done with a regular river. I am no expert, but I would say most rivers have been around consistently for thousands of years, and humans have done everything you have mentioned above with them. I am looking for a specific thing that can only be done with this magical river, something extremely special, not just that it's really good at doing. $\endgroup$ – Inbar Rose Jan 4 '17 at 14:08
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    $\begingroup$ @InbarRose The heavy industry in medieval times was in the country-side where steep streams provide enough power. With a magical river with magical flow, there would be enough power in the relatively flat land of the city to drive water wheels. The garbage and irrigation is hard to do with a regular river since you will run out of water. The canals can't be done with a regular river, since regular rivers have variable flow rates and would flood the city in the wet season (thats why Venice uses the ocean, not a river). I argue that you do need a magic river to do these things. $\endgroup$ – kingledion Jan 4 '17 at 14:28
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    $\begingroup$ Another reason it can't be done with a regular river is that a regular river is one-way. This one is, too, BUT every place is downstream from every other place. $\endgroup$ – WGroleau Jan 5 '17 at 4:35
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Use the circular river to create a helical updraft, for gliders to use to gain altitude.
This is fanciful, but so is a circular river: Whenever needed, put out your fleet of reverse-sailboats on the circular river. Enough reverse-sailboats, regularly spaced organize the urban heat island effect of your city into a stable, safe to ascend updraft. Now you can have gliders ascend in a smooth helix, before departing to glide about your realm's business. I suppose outgoing mail will always be faster than incoming.

Some detail (as requested):

First off, I have assumed that your city folks use energy, probably fire, for warmth, cooking and light. The more energy your city uses, the better this would work. Concentrated energy use in a city causes real effects, including shifting local rainfall patterns. For details see:
https://en.wikipedia.org/wiki/Urban_heat_island

In a gravity field, hot air rises; the mechanism is called convection:
https://en.wikipedia.org/wiki/Convection

Cities can create significant updrafts via thermal island effect. (The Time-Life Science Library book weather has a nice photo of Paris under a rainstorm caused by its own heat island. Can't find my copy, sorry.) The cloud was literally just over the city proper.

Air currents, like our urban thermal updraft tend to organize into vorticies, such as dust devils, tornados or hurricanes. As
https://en.wikipedia.org/wiki/Whirlwind
states: "A whirlwind is a weather phenomenon in which a vortex of wind (a vertically oriented rotating column of air) forms due to instabilities and turbulence created by heating and flow (current) gradients."

For safe, efficient use raising gliders, we'd want the updraft to organize into a stable vortex. Nature will tend to do this when conditions are right (no greater storm overpowering/disorganizing our updraft or blowing it sideways too much), but we can ensure it, by using a fleet of "reverse sailboats" (or perhaps 'wind boats') to stir the air in a circular fashion, centered nicely around the heat-island-induced updraft.

The function of these 'wind boats' is different that regular boats: These use the water current to push the air around in a big circle (instead of using air currents to push the boat.) These boats don't need to be streamlined; they'll work better if they have none. They do need to be buoyant and hold up strong masts bearing sails. Put out the fleet of wind boats on fairly sunny, low-wind days (good conditions for vortex formation) and alert the Glider Corps!

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  • $\begingroup$ What an interesting idea, could you expand on it? I would like to know more about the mechanics of this and how it could be implemented in a traditional fantasy world $\endgroup$ – Inbar Rose Jan 5 '17 at 9:47
  • $\begingroup$ @Inbar Rose, I've added some detail, as you asked. This is qualitatively sane, but I haven't run any numbers, nor am I a meteorologist. Probably needs a big, fast river to be feasible in reality, but in a fantasy, it's probably no more outlandish than, say, dragons. $\endgroup$ – Catalyst Jan 5 '17 at 11:56
  • $\begingroup$ Great detail! Okay, now the boats make more sense, you are using the water to push the boats which pushes the air. Interesting. And truly "special". The Glider idea is very fanciful for a fantasy world, very nice idea. Thank you. $\endgroup$ – Inbar Rose Jan 5 '17 at 12:17
  • $\begingroup$ This sounds like a math challenge to see if this actually works. $\endgroup$ – kingledion Jan 5 '17 at 13:02
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The transportation is only thing that utilises the speciality of a circular river. Waterwheels do not benefit of circularity. The circularity does not provide anything for the infinity of a water supply. I assume that the water is not edible. Around any river the water supply is basically limitless for a medieval technology, so it is nothing special.

Travelling takes time and thus the cities have their limits for growing. Circulating river gives you transit that the medieval cities lacked => super city (10km diameter is a big deal if you have to do it by foot). You could possibly have almost a floating city on that river. Shops that can locate them selves on a different places on different times and such...

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  • $\begingroup$ I suppose the width of the river would be a factor. Something I have yet to determine exactly. (edit: I decided about 200 meters makes sense to me) $\endgroup$ – Inbar Rose Jan 4 '17 at 12:24
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    $\begingroup$ A current that is constant, as stipulated in the question, would 100% affect the use of water wheels. You could have as many as you wanted as large as you wanted and the river would still turn them. That power could be used for anything. $\endgroup$ – Nathaniel Ford Jan 5 '17 at 1:59
  • $\begingroup$ @NathanielFord True, I did not think about what locally constant current would mean. But I kind of assumed that the water knew magically to act normally with the contact. Otherwise that river would be really insane if it would have all the time that constant velocity; it would be really dangerous to use because it could destroy everything that blocked its way. In normal rivers the momentum of water is built on a fairly short distance, so nicely behaving water would have no special use in a circular river. $\endgroup$ – user3644640 Jan 5 '17 at 7:52
  • $\begingroup$ I'm not sure that you'd have to worry about 'destroying everything' - assuming the velocity was set to some reasonable speed then anything that would normally be able to divert the water at that speed would be fine (erosion aside). But you're correct: this is insane. Such a magical river basically would wreck the local physics: unlimited water, unlimited energy, unlimited heat dissipation. $\endgroup$ – Nathaniel Ford Jan 5 '17 at 17:47
  • $\begingroup$ If it would be able to divert. Otherwise it would have no means to maintain its velocity other than to destroy. $\endgroup$ – user3644640 Jan 6 '17 at 15:33
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You can use the vast quantities of water for military purposes. Build a circular dam all around it. Make it like a rampart with mobile parts. If an enemy army come you just have to open some part so it is completely flooded before reaching the city.

The interest is that you can have a very big city without the need to maintain an army: pretty neat if your neighbors are aggressive barbarians.

Alternatively you can terrorize the neighbors by threatening to throw on their city an infinite flow of water.

There could be a problem to close the dam once it is open, but I am pretty sure such a system could have been invented.

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A lot at once

Step 1: Use the water flow to turn water wheels/turbines and thus generate (a lot of) electricity.

Step 2: Use this to heat the water from the river to near the boiling point. Increase the water pressure to "supercharge" and make the water even hotter without it boiling (Chart).

Step 3: Dump the hot water back into the river. The hot water now will lay on the surface of the river and emit quite a lot of heat to the surroundings, heating the whole city and generating a mild, warm, somewhat moist local climate.

As a side effect, you have nearly sterilized water for drinking, bathing and medical purposes. You can also distribute the hot water in pipes to increase the area of effect. Plus, you have very hot water to boil your enemies.

The hot water surface will create an upwards draft of heated air, sucking in the cold air from the city streets, clearing away the miasma (if there is some) - so you also have a good ventilation. And (credits to @Catalyst here) you can also use the upwards draft to start your gliders...

If electricity has not been discovered, you can heat the water by friction. (Let the turning piece of the water wheel/turbine rub with something else (preferably from metal) and pipe the water over the hot friction elements. This is however far less efficient and friction elements will have to be replaced a lot (Except you have some magical adamantium metal as well...). You can also heat the water to steam to drive your steam engines.

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Turn the entire thing into a massive modified prayer wheel.

The following assumes a counter-clockwise direction of water flow.

Picture a large disk with a gear-tooth edge. Set this in a gear-tooth track along the outer edge of the circle submerged the distance of about a quarter or so of the diameter of the disk. Each tooth has a specially made cavity on its "counter-clockwise" side (right side, when the tooth is at the bottom). This cavity catches water and bounces it back out, generating a push on the bottom of the wheel, causing it to turn in a clockwise direction, causing it to roll against the current. The prayer symbols are written along the outer edge, near the gear-teeth.

The water that is touched by the wheel becomes blessed and carries its purifying power into all life forms that drink, bathe and swim in it. As the water is constantly coming back around the circle, it becomes very pure and very blessed.

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Delivery and communication. Letters and packages from any point to any point can be delivered by this river. At various points, a mail boat is grabbed when it appears, incoming for that sector is removed, outgoing added, and the boat released to continue around.

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A perpetual, geographically-confined lantern river festival

Many festivals around the world culminate in sending a flotilla of floating lanterns down a river. It looks nice. Where do they go? No-one knows.

On a circular river, the lanterns would all just keep going round and round. If enough lanterns were released at a single point, the populace could watch as they slowly spread out to form a ring of light around the city. How long would this take? That's one for the mathematicians. Perhaps the process could have some social significance.

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  • $\begingroup$ That's not really industrious, but I love the image. I'm imagining the floating lanterns only released at full dark, perhaps during a fog.... Or different colored lanterns, with religious (or political) import? $\endgroup$ – Catalyst Jan 5 '17 at 14:40

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