Editing Fourth Bridge

|universe=[[Cosmere]]

The '''''Fourth Bridge''''' is [[Roshar]]'s first airship, which uses hundreds of conjoiner [[fabrial]]s to move through the air. Named after [[Bridge Four]], it was built in the first year of the [[True Desolation]]s by a team of [[ardent]]s and engineers led by [[Navani Kholin]].{{book ref|sa4|3}}

The ''Fourth Bridge'' is a rectangular, wooden box; a hundred feet long, sixty feet wide, and about forty feet high, with three decks within. Fans are built into the back to increase speed, though for now, the difference is minimal. At full speed, the ship travels at roughly five knots.{{book ref|sa4|3}}

The back half of the upper deck is covered by a semi-round roof, while the front half is open-air and surrounded by a railing. [[Bridge Four]]'s original bridge is embedded into the floor; some of the [[Windrunners]] have a habit of kneeling down and touching it before taking off. A command station, composed of several tables, is typically placed in the sheltered section, though in some cases, it might be moved to the front. The furniture is bolted down.{{book ref|sa4|3}}{{file ref|Navani's Notebook - The Fourth Bridge.jpg|text=The Fourth Bridge}}

The lower decks serve as the hold. The bottom one has a broad door that allows large amounts of people to come in and out at once. It functions akin to a drawbridge; it forms a wall and can be lowered and raised by ropes to open and close. While sparsely-furnished, the holds can accomodate several hundred people.{{book ref|sa4|3}} There are numbers painted on their walls, dividing them into sections.{{book ref|sa4|6}}{{file ref|Navani's Notebook - The Fourth Bridge.jpg|text=The Fourth Bridge}}

The airship doesn't so much fly as it is held in the air by [[fabrial]]s. There are hundreds of gemstones forming a lattice of massive conjoiners embedded into the ship. They are connected to a pair of "dummy ships". One of the dummies is hanging off the side of the [[Urithiru]] plateau; the other stands on the [[Shattered Plains]].{{book ref|sa4|15}}{{book ref|sa4|19}} The airship is steered from those two lattices; the ''Fourth Bridge'' communicates its needs to them through [[spanreed]]s. As they cannot be used for long periods of time due to movement, the scribes use them by turning them on and off, creating a code of blinks.{{book ref|sa4|3}}

The conjoiner fabrials are isolated using [[aluminum]] so that only certain planes of movement are transferred between the paired gem halves. The Urithiru structure can only convey vertical motion; the Shattered Plains lattice can only transfer horizontal motion. The former is lowered to raise the airship, while the latter is moved around to make it fly and turn in any direction.{{book ref|sa4|19}}{{book ref|sa4|3}} To allow the ship to continue moving when the space for the lattices runs out, an additional aluminum mechanism is used to reverse the motion while the gemstones are temporarily unhooked.{{book ref|sa4|3}}

The entire structure requires significant manpower to function properly. In Urithiru, hundreds of people operate the winches that raise and lower the vertical lattice; on the Plains, squads of [[chull]]s are used to move the ship horizontally. The ''Fourth Bridge'' requires hundreds of gemstones to be perpetually infused with [[Stormlight]].{{book ref|sa4|3}} Fans have been installed to ease the load on the horizontal fabrials, but as of right now, they are barely functioning. There are also weight concerns, as adding too much mass to the existing structure might cause the gemstones to crack.{{book ref|sa4|6}}

== History ==

{{image|Ship Designs.jpg|side=right|width=250px|Navani's early airship designs}}

=== Development ===

==== Floating Platforms ====

{{quote

|Hey. Is that platform '''flying'''?

|[[Adolin]]{{book ref|sa2|35}}

}}

[[Navani Kholin]] began conducting her first tests a few months after arriving on the [[Shattered Plains]], in the last year of the [[War of Reckoning]]. The earliest experiment was comprised of a tall tower, with its top and the column built as separate pieces, and a paired circular platform with crenellations. After infusing the embedded fabrials with Stormlight, when the pillar was rolled away, the top of the tower remained floating. As the top was pulled down, the platform rose into the air, creating an archer parapet unassailable to anyone standing below. While this was already useful, even back then, there were plans to do more with the technology.{{book ref|sa2|35}}

Over time, Navani and her [[ardent]]s improved the platforms even as they began planning for more ambitious projects. Shortly after the [[True Desolation]] began, Navani started designing proper airships; her initial sketches were more ship-like, with massive sails attached upside-down to the bottom of the hull.{{book ref|sa3|50}} However, proper construction began only after the [[Battle of Thaylen Field]], as Navani threw herself into work following the death of her son, [[Elhokar]].{{book ref|sa4|3}} A few months after the battle, the Alethi built a pair of massive platforms off the sides of the [[Urithiru]] plateau as their early experiments. By then, rumours of what they were planning began to spread covertly among the high-ranking members of the coalition.{{book ref|sa3.5|2}}

==== Aluminum ====

{{quote

|[Huio] left a bit of foiled aluminum in the mechanism, and it is interfering with the conjoined rubies in a '''fascinating''' way.

|[[Rushu]]{{book ref|sa3.5|9}}

}}

As the twin platforms were being constructed, the ship ''[[Wandersail (1174 ship)|Wandersail]]'' set out to explore [[Aimia]]. As a favor to the expedition's leader, [[Rysn Ftori]], Navani sent along [[Rushu]], an ardent specializing in new fabrial designs.{{book ref|sa3.5|2}} Rushu attempted to make Rysn's chair float; however, the fact that the conjoiner fabrials had to repeat the exact motion -- or the reverse of the exact motion -- of each other meant that the applications were severely limited.{{book ref|sa3.5|5}}

A breakthrough came from [[Huio]], one of the [[Herdazian]] [[Windrunners]] assigned to assist the expedition. An amateur fabrial scholar, Huio disassembled one of Rushu's spanreeds and began experimenting on the gemstone with [[aluminum]]. After he put the spanreed back together, it began to behave oddly, transferring the movement of its conjoined twin in an odd, jittery way. Realizing this would be a problem that couldn't be hidden, Huio and the other Windrunner on board, [[Lopen]], brought the broken spanreed to Rushu.{{book ref|sa3.5|7}} Upon examining it and experimenting further on Rysn's chair, Rushu realized that Huio accidentally used aluminum to isolate the spanreed's planes of motion, so that only movement in specific directions was transferred between the two. This allowed for the creation of a floating chair that could move freely by being pushed around.{{book ref|sa3.5|9}}

Rushu later sent the information about Huio's discovery to Navani, who ordered further experiments. As a result, eventually, [[Falilar]], one of her ardents, made a breakthrough that allowed for an airship to be constructed.{{book ref|sa4|19}} A new design, based off the chasm-spanning bridges from the [[War of Reckoning]], was built and tested on the [[Shattered Plains]]. After months of deliberation, it was named the ''Fourth Bridge'', after the people who, over a year prior, had saved [[House Kholin]] from destruction.{{book ref|sa4|3}}

=== Evacuation of Hearthstone ===

{{image|Fourth Bridge by Esther Shrader.jpg|side=right|width=250px|[[Navani]] and [[Dalinar]] aboard the airship}}

About a year after the Battle of Thaylen Field, the ''Fourth Bridge'' began conducting endurance tests. For several weeks, the ship flew above the [[Steamwater Ocean]], hiding from [[highstorm]]s in coves and [[lait]]s. At that time, it was staffed only by a small number of engineers and sailors to maintain it. It was then that highmarshall [[Kaladin]] suggested the first field deployment: rescuing all the citizens of his occupied home town, [[Hearthstone]]. [[Dalinar]] agreed, and the ship changed course, with Windrunners bringing in other [[Knights Radiant]] and the command staff, including Dalinar himself and [[Navani]], about a day before it reached the town. [[Kmakl]], the consort of the [[Thaylen]] Queen [[Fen Rnamdi]], was likewise present.{{book ref|sa4|3}}

The arrival did not go smoothly, as Kaladin's approach to Hearthstone was discovered, leading to a squad of [[Fused]] being sent their way. However, the ship still successfully made landing, delivering the Radiants and picking up the citizens of Hearthstone and other refugees. The first among them was the Herdazian general [[Dieno enne Calah]], who asked that the ship be used to pick up the remainder of his resistance forces, which Dalinar agreed to.{{book ref|sa4|3}}

As the evacuation progressed, the battle began to rage in the skies around the ship. The Fused were fascinated by the ''Fourth Bridge'', trying to approach and take a look at its mechanisms;{{book ref|sa4|5}} [[Zandiel]] was able to gain a good enough look at the ship that he made some sketches of the craft.{{book ref|sa4|14}} Nonetheless, the Fused attack was pushed back and the ''Fourth Bridge'' successfully returned to the air, then began the long journey back to the Shattered Plains.{{book ref|sa4|9}}

=== The Fused Response ===

{{quote

|'''We''' could not have crafted the flying device they have.

|A [[Fused]] of the Nine{{book ref|sa4|14}}

}}

In [[Kholinar]], the airship caused some concern among the Nine who ruled the Fused, leading into a discussion about humanity's newfound technical prowess. It was this conversation that allowed [[Raboniel]] to suggest the plan to invade [[Urithiru]].{{book ref|sa4|14}} Meanwhile, the ''Fourth Bridge'' arrived on the Shattered Plains twenty days after it departed. It landed there, letting the refugees off board.{{book ref|sa4|18}}

It's unclear what happened with the airship afterwards. [[Falilar]] and Navani both suggested a number of issues to resolve, including fixing the fans and checking whether the weight didn't crack the gemstones; as such, it probably remained on the Plains.{{book ref|sa4|9}} In Urithiru, Navani began workshopping ideas for new designs, though her work was interrupted by the [[Voidbringer]] invasion.{{book ref|sa4|42}} With conjoiner fabrials nonfunctional after Raboniel turned the tower's defenses on the Radiants, the airship most likely remained grounded for the entire period of occupation, as the vertical movement lattice was mounted in Urithiru.{{book ref|sa4|39}}

== Trivia ==

* The ship's speed -- about five knots -- is equivalent to about 9.2 km/h, or 5.8 mph.

* Brandon joked that "how can I make airships with this" was the third or fourth question he asked himself when creating [[Roshar]]'s magic system.{{wob ref|14085}}

* Up until very late in the publishing process for ''[[Rhythm of War]]'', the discovery of the interactions between aluminum and fabrials, which made the ''Fourth Bridge'' possible, was attributed to [[Azish]] scholars; the version of chapter 3 released publicly on Tor.com still has this explanation.{{url ref|url=https://www.tor.com/2020/07/28/read-rhythm-of-war-by-brandon-sanderson-chapters-two-and-three/|text=''Rhythm of War'': Chapters 2 and 3|site=Tor.com}} Brandon disliked it, and when writing ''[[Dawnshard]]'', he changed it so that [[Huio]] and [[Rushu]] were behind the discovery.{{wob ref|14403}} The change narrowly made it into book before release.

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