How to cross a gorge with deep valley plagued by corrosive effluvium in medieval times?

Question

There is a beautiful deep canyon called "Gorge Yor Ei Ou" by the local, it has a deep cleft filled with deadly and corrosive effluvium that doesn't seems to go away. The shortest distance between the 1km tall cliffs is approximately 1km, two villages on either sides wish to do more than simply verbal communication such as yelling across...

I am wondering how can they safely cross this treacherous gorge that seems to stretched for thousands of kilometers in the late 1500AD?

What innovative technology can they use to facilitate the movement of people and working animals such as horses, donkey or even mule without burning a hole in every wallets? The effluvia has claimed many lives in the past and the authority have to cordon off and put in place wooden fences.

Extra details: the pH level is 1.0 and effects only organic materials and forms a gaseous river about 10m high in along the valley

Answer 1

This is your critical condition.

without burning a hole in every wallets?

Between two villages in 1500? You don't.

They might as well be on different continents with the conditions given. An ordinary river a fraction of the size was a major obstacle, with enormous upkeep costs for any bridges, usually funded by tolls. Insufficient traffic means insufficient income from tolls to maintain the bridge, two villages aren't going to be moving enough money to pay for this upkeep, never mind construction. Even major cities didn't build crossings on this scale, the first crossing of the Firth of Forth was at Stirling, 30 miles further up river from Edinburgh.

Consider the long list of British place names with 'ford' or 'bridge' in their names as an indicator of the significance of any sort of easy crossing of such an obstacle.

Your villages will have to live with shouting at each other for another few hundred years.

Answer 2

As an alternative to megascale engineering in the medieval period, perhaps you could instead consider rocket mail?

In the real world, gunpowder rocketry was had a fairly unexciting range until the ~18th century when Mysorean rockets were developed with metal casings that enabled ranges as far as 2km. What the real world lacks is the kind of hugely inconvenient impassable chasm that you've proposed, and hence there's a far greater pressure on people to come up with clever ideas for getting over it.

The Mysorean design was for a man-portable piece of artillery:

but there's nothing to stop the nascent pyrotechnicians of your world from building something a bit more substantial.

I've just learned about Lagâri Hasan Çelebi who might have been the first person to fly in a rocket, who took flight in the 17th century (so a bit ahead of the invention of hot air balloons in Europe, though maybe behind China). I don't have any more information on his work as yet, but it goes to show that powered flight was surprisingly advanced even back then.

Given that kites large enough to carry a man since the 13th century, and Leonardo da Vinci was thinking about hang gliders in the late 15th century, it may be possible that your people will develop rocket gliders and ultimately powered flight long before we did.

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Now, I do wonder about the practicality of firing a line of silk across the chasm and setting up a pulley system or even a Tyrolean, but I'm not sure if medieval engineering would be quite up to that.

Still, the possibility exists...

Answer 3

By plopping metal frames in, and building a bridge over that.

Fistly You're not getting over a 1km gap without touching the bottom. Were this a world with modern materials; <850m you could cross by using a massive trebuchet to launch a bundle of rope, securing it at both ends, and building a bridge from that. However that's with modern trebuchet tech too, medieval may struggle with half that.

However it's only 10m deep, you could easily build on that, unfortunately wood is organic and will be eaten, and if you tip gravel or something in accross the entire span you've built a dam and the level will rise. You need to build a hollow structure out of metal from above.

Firstly you'll need to build a road down to the bottom of the valley, stopping just above the nasty fluid. This is a massive task on it's own.

Assuming iron is available; Have your blacksmiths build iron cubes with diagonal braces. Say 11m long per side. The beams dont need to be particularly thick as the triangle shape from the diagonals will give strength, but it will still be very heavy and require teams on tread-cranes to assemble and lower it into the fluid. If iron isnt available copper or bronze should work... you said it only effects organics so any metal is fair game.

Estimating about ~100-150kg per beam, making ~2 tonnes per cube, and you'd need 85-90 to cross the fluid of doom depending on how close you can squeeze them together with your crane skills. That's 180 tonnes of metal. Not a small amount, but to put that in perspective the Eiffel Tower has 7300 tonnes of iron.

Once you have a cube in the flow, build a road surface out of wood or whatever works for you - its 1m above the dissolving liquid so shouldn't dissolve wood, but if it splashes then you may want to do the whole thing out of iron.

Once the road is built over a cube, you can move your tread-crane onto that to lower the next iron cube section in.

Repeat until your on the other side, where hopefully the other village has been busy cutting a road down to meet you.

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Other ideas:

• Rather than spend 300 years carving out the valley, sit back for 200 years and wait for hot air balloons to be invented.
• Dig under it. This will probably take long enough that you'll start with shovels and finish with modern TBMs.

Answer 4

Build a mole.

Though I'm more accustomed to moles that burrow under the ground, in ancient times they could cross over the sea. Alexander of Macedonia famously built such a structure, about a kilometer long, during the siege of the island city of Tyre. (Tyre is now a city on the seacoast of Lebanon)

Some might protest that the mole has to be 1 km tall, but this is not so! You need merely dig a mine shaft down to nearly the level of the gorge floor (ideally with a gentle slope to encourage light traffic, and hoists to facilitate the lowering of large wagons that can't handle the grade). You can still dump in the building materials from above if it's convenient - try not to do it during the workmen's shift. :). I assume that fumes are not excessive if villages can exist nearby, but if they are a problem you can try building a raised road bed above the main part of the mole as the work progresses.

If your corrosive substance is so terrible that it devours rock and sand, we may have to resort to flying machines - perhaps a trebuchet and some of those airbags from the Mars landers?

Answer 5

By Building Most of an Earthen Dam with Stone Bridges over the Gaps

While still absurdly expensive, the cheapest thing available to ford this gaseous river is dirt and stone.

The cliff walls are 1km high and the river is only 10m deep. You're not going to be able to build a 1km high structure, so you're going to have to dig some kind of slope into the walls just to get closer to the ground. All of that digging is going to displace dirt.

Take your dirt and build a dam. Of course, building a dam will cause the level of the gaseous river to rise, but the rise will be significantly limited if you leave big holes in your dam.

Every 100m or so as you're building you dam, you stop dropping dirt and lower stone blocks that can form the basis for a bridge. Each of these gaps will let more of the gaseous river pass through.

Fluid dynamics isn't my thing, so I'm going to grossly overestimate how much earth you'd have to move:

• The dam needs to be 1km long and 5m wide (rough estimate based on the width of the great Wall of China which could support two chariots traveling in opposite directions in some places)
• If you put a 10m gap in your dam for every 100m of dam, then you'd need the dam to be 100m high for the same cross-sectional area of gas to flow through these openings. That's a vast oversimplification of fluid dynamics, and I don't honestly know if it would be more or less than that.

That means we're talking roughly 500,000 cubic meters of earth and stone to get moved into the valley, and I would probably take that as a minimum because that's assuming a uniform width of the 100m walls which would not be very likely or stable. For ease of engineering in a corrosive environment, you'd want the earth to be piled in at least a 60° angle so that it meets the minimum angle of repose of piled earth. Such a dam would have a base about 125m wide, and would thus have a volume of closer to 7 million cubic meters.

For reference, the Great Pyramid at Giza is roughly 2.5 million cubic meters, so this incredible feat of engineering is roughly equivalent to building 3 of them.

Also for reference, the walled City of York used roughly 25,000 cubic meters of stone in the construction of its 3km circumference walls, so this is like building walls around 300 major cities.

Edit: I found This Article that described the Great Pyramid as needing 52 million man days. Even with more modern working techniques than the ancient Egyptians, I'd expect similar man-hour requirements due to the special constraints of a corrosive gas on the floor of a giant chasm.

What if I change the parameters?

The way I see it, your conditions are too extreme for any medieval crossing. Constantine's Bridge over the Danube River was close to double the length, but it was only 10 meters above the water, used a great deal of wood, and only remained in use for 40 years.

If you wanted to reduce the construction requirements down to only the scale of a single wonder of the world (i.e. 2.5 million cubic meters of earth), you could reduce the width of the chasm to closer to 350m instead of a full 1 km.

Alternately, you could reduce the flow of the miasma that needs to be crossed. If it were only 5m deep (still deep enough to prevent most traditional techniques), then the height and width of the dam are also both halved, reducing the amount of work to only 1.75 million cubic meters.

Cut the flow down to only 2.5 m deep (still taller than a human and nearly impossible to cross by other means), and the construction is back down to 500,000 cubic meters of earth and stone which is only 20 walled Cities of York and much more reasonable for a major trade hub (less so for two insignificant villages).

Answer 6

Wear a medieval « gas mask ».

First, craft an hermetic mask. Then, link this mask to a wearable device by pork casings. This device will be on your back and filled with a very basic solution. Think of a giant hookah but without the coal. Inspire and the corrosive gas will pass throught the basic solution leading to something « breathable ».

Then, wear a full leather costume and apply some very sticky fat all other. This fat need to be waterproof ; think of something like Vaseline for example.

Finally, use a winch go to bottom of the canyon with a thick boat and pass to other side where another winch will lift you up.

PS: Don’t forget to quickly remove the fat once on the other side.

PPS: If the gas is corrosive enough to neutralised your basic solution in a matter of minute, build a much bigger tank directly on the boat.