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The GURPS roleplaying supplement New Sun, based on the Book of the New Sun novels by Gene Wolfe, describes a particular sword as having a hollow channel in its core containing mercury. According to the material, this causes the blade's weight to "shift backward when raised and rush forward when swung" (words in quotations are the entry's exact phrasing).

I assume this is supposed to give forward strikes more force, but I honestly wonder- would such a design contribute to or take away from a sword's effectiveness?

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – HDE 226868 Jul 27 '16 at 20:53
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    $\begingroup$ Check out Skallagrim's sword reviews on YouTube. He's a HEMA (Historical European Martial Arts) enthusiast, and explains a lot about what makes a sword effective, what fighting styles work best with each design, etc. $\endgroup$ – AndreiROM Jul 28 '16 at 14:50
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    $\begingroup$ the real effect is that you have added mercury with nearly twice the density of the steel, it is like adding a lead core. it makes the weapon heavier so mostly it makes it less maneuverable largely negating the benefit of a sword over say an axe. $\endgroup$ – John Mar 18 '17 at 2:50
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    $\begingroup$ It may help you to know that the purpose of this sword (Terminus Est) is to decapitate people. The protagonist of the books was an executioner. It was not intended for combat but for a quick, merciful stroke during an execution. It has a dull end, so not much good for thrusting. It's more like a cleaver or axe in use. $\endgroup$ – Brythan Mar 18 '17 at 4:03

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You'd get "dead blow thrusts", while normal swinging (due to centrifugal force) almost always pushes the mercury to the tip of the sword. You'd essentially be swinging around a sword with a heavier tip and mostly hollow blade, except while thrusting, where you get the "dead blow" effect.

Until you realize what a thrust really is. Having the hollow blade + mercury in the middle is useless, because a dead blow thrust is useless. A thrust is a singular motion - by the time the dead blow action kicks in, you've already fully penetrated your target to your swords maximum potential, especially since the little bit of dead blow weight from the mercury pales in comparison to the amount of force you're pushing into the sword when you thrust.

This: ...causes the blade's weight to "shift backward when raised and rush forward when swung" does not happen.

So really what you have is a poorly designed, weakened sword.

In this case, your assumption that this is supposed to give forward strikes more force is flawed.

Such a design, however, does take away from the sword's effectiveness (if you're looking at the effectiveness from a "how long will this thing last/how likely will it break" standpoint.

Actual effectiveness in combat will depend on the training of the user.

Clarification: A "Dead blow" is when a part of the force of a strike hits the target after the weapon has landed. For example, a "dead blow" hammer is often hollow on the inside and filled with sand (or something like that) - when you swing the hammer, the hammer first connects with the item you're trying to hit, but the force from the sand doesn't connect until a split second later; when the sand travels from the back of the hammer to the front due to the hammer stopping. The point of a dead blow is often to distribute the energy of a strike over a longer period of time, and to minimize rebounds.

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    $\begingroup$ You are right it would be contra productive for thrusting, since the thrust would have less power and pulling back would cost extra work. - But you seem to ignore classical overhead downwards slashes, which feature prominently in several sword-fighting schools. - Even if the sword would only provide a benefit for downward slashes, it could be a viable weapon in these schools. $\endgroup$ – Falco Jul 27 '16 at 9:58
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    $\begingroup$ Given the source, it's probably an executioner's blade rather than something specifically designed for combat. Book of the New Sun is kind of odd. We might as well compare it to a golf club as a thrusting weapon, in terms of effectiveness for what it's designed to do ;-) $\endgroup$ – Steve Jessop Jul 27 '16 at 12:37
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    $\begingroup$ @Falco If you're fighting with a style that only uses downward chopping blows from a weapon with its weight focused at the tip, you don't want a mercury-filled sword. You want an ax. $\endgroup$ – ckersch Jul 27 '16 at 14:42
  • $\begingroup$ @ckersch The implication of the design is that you gain some benefit from not having the weight focused at the tip at the start of the swing, and having it shift in time for the blow to connect. I've seen toy baseball bats that are filled with water that seem to use a similar principle. $\endgroup$ – Random832 Jul 27 '16 at 16:05
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    $\begingroup$ @Random832 Except that you don't hold your sword like a baseball bat - A bat is swung once, then reset to the starting position before each swing, allowing the liquid to deposit at the bottom. In a sword fight, it's not only unlikely that you'll ever get the chance to reset your sword to a position where the liquid deposits at the bottom, you're more likely to keep your sword moving (which means that the centrifugal force will always keep the liquid at the tip of the weapon). $\endgroup$ – Aify Jul 28 '16 at 2:17
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Angular momentum would be conserved.

While swinging, the centripetal force would cause the liquid mercury to flow in the channel and move out along the blade. This would increase the mass at the end of the lever arm increasing the force of the blow. The increased force is not free energy, it would slow the rotation of the swing. This increased momentum would also make the super swings more difficult to control.

This variability of rotational speed and higher than expected force could be useful at initially confusing an adversary, but it would likely also confuse the wielder unless they had trained and adapted their fighting style for it.

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  • $\begingroup$ Might be good against Dune "Holtzman effect" shields, against which you want to go slow at the end of a swing. :) $\endgroup$ – TextGeek Jul 28 '16 at 14:58
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    $\begingroup$ What "centripetal force"?  Do you mean centrifugal force? $\endgroup$ – Peregrine Rook Aug 3 '16 at 4:38
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...would such a design contribute to or take away from a sword's effectiveness?

I would not worry about the weight effects of such a blade too much.

Making a blade hollow and filled with mercury, especially when you make the blade big / hollow enough for significant amounts of mercury, will give you a sword that is much more likely to break than solid designs.

You might feel a certain effect swinging or thrusting the blade. But as soon as your blade encounters another blade in a clash, you will be standing there, sprinkled in mercury and holding a broken sword hilt. While your opponent laughs at you, and cuts you down.

Which makes any effects that mercury might have while the blade is intact pretty secondary.

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    $\begingroup$ I'm going to go out on a limb and say that splashing your opponent with mercury isn't such a bad idea after all if you don't also splash yourself. $\endgroup$ – John Dvorak Jul 27 '16 at 12:47
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    $\begingroup$ @JanDvorak: In a melee, at the cost of your main weapon? I'd not count on the toxicity of mercury to kill that opponent at all... and if that's your plan, there are more effective ways of delivery than inside your swordblade. ;-) $\endgroup$ – DevSolar Jul 27 '16 at 12:50
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would such a design contribute to or take away from a sword's effectiveness?

This would contribute to that exact sword's effectiveness.

That sword is an executioner's sword - a tool specialised for job that is heavily different from swordfighting (like, they don't even have a sharp point for thrusting).

Design is supposed to make this particular sword perform better at axe's job of chopping off immobile kneeling prisoners' heads in one blow.

There were hollow batons filled for about 1/3 with birdshots in '90s in Russia and I've also heard of table tennis rackets with empty handles and a moving weight inside them but I am not sure that this were done because of actual practical benefit or not.

Quick test shows that moving weight kinda works - the blow that feels equally hard feels easier to make so there could be some practical benefit. However, weight's position is messed up after contact with the target - or after other sharp change in movement - so it does not look like easily controllable effect in a fight.

That being said, for all intents and purposes other than chopping off immobile kneeling prisoners heads off in a single blow, it would be pretty lousy sword. From fighting standpoint, it's harder to control, it has a structural weakness in important point and it is way harder to replace - all that in exchange for a better performance at a blow you won't ever get time to prepare or deliver.

I haven't read the book though. If in that world that sword performs good in a swordfight then it should be attributed to the same thing that makes dragons exist and magic work.

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    $\begingroup$ Great first answer, and welcome to Worldbuilding! $\endgroup$ – fi12 Jul 27 '16 at 13:40
  • $\begingroup$ But would it be more effective than a falchion? (I’m assuming there’s a reason they want a more sword-like implement than an axe.) If you’re exclusively using it for chopping, why not balance it like a chopping weapon? $\endgroup$ – Robert Fisher Jul 27 '16 at 14:53
  • $\begingroup$ The "Rule of Cool" applies here. The author is looking for something quite unique (only the Executioners guild makes and uses this sort of weapon), and from what I remember of the book, the Executioner is readily identified (for good or ill) by the carriage of this weapon during his adventures. $\endgroup$ – Thucydides Jul 27 '16 at 17:07
  • $\begingroup$ I'm glad someone pointed out that the sword in the books, Terminus Est was for executions and not for fighting. Its blade ended in a blunt square, useless for thrusting. That said, Severian defends himself with it in a few instances, and the sword is definitely made of superior materials and is very difficult to break. As for a badge of office, the Torturers go shirtless with a cloak of fuligin (think vantablack) and a mask. No other identification needed. The sword is actually unique and Severian is the only Torturer who ever carries it. $\endgroup$ – Juan Tomas Jul 28 '16 at 13:42
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I searched, though I couldn't find much...

It was all about having a moving centre of gravity. Generally in a sword you want the centre of balance to be close to the handle, usually around your guard. This way, it is easy to swing the sword around to attack or defend. However, when making a slashing attack, you want your blade to have a lot of weight to bring to bear on your opponents sword/armour/person therefore increasing the amount of momentum you can deliver to break something or to carry through them and make a deeper wound. The problem is that a heavy blade that is agile requires that much weight again in the pommel to balance the whole thing out. Heavier swords are tiring to swing during a long battle.

To solve this, the theory went that you would create a hollow channel filled partially with a movable mass. In this case mercury, which was dense and a liquid to easily move along this channel. When your were on guard with the sword upright, the centre of balance would rest closer to your grip and you would have an agile sword. Then when you swung, the weight of the mercury would concentrate closer towards the blade and give you a more powerful strike, all without having to add extra weight to the sword as a whole.

However, the sword might be kinda brittle (hollow, and only partially filled. So the mercury cannot fully absorb the force). And anyways, this is only good for one attack. If you're going to defend, this seems difficult, and you'd be put off balance by the shifting mercury.

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  • $\begingroup$ Great answer!!! The technology of the sword is understood as miraculous (science so advanced it appears "magical", like most of the technology in that particular series.) Thus brittleness is not a factor--the alloy used for the sword has extraordinary properties. $\endgroup$ – DukeZhou Jul 19 '17 at 17:17
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A sword has a balance point - its weight center. There seems to be no consensus about at what exact distance from the hilt the balance point should be, but there is a firm consensus that it should be quite closer to the hilt than to the other extremity. Anyway, part of handling a sword is adjusting to its balance point. The sword you describe is unbalanced by definition: its balance point changes with every movement, making it much more difficult for its user to adapt. It could, I guess, be useful as a sword that has only one specific use (such as a sword used exclusively for beheadings of prisoners - see RedSonja's comment) in which the shift of balance would be predictable. But for an actual fight, where it is necessary to make several different movements, it would be a very lousy sword. If you have to parry, for instance, you want your balance point to be between your hand and the point of impact of the enemy sword, otherwise the momentum of the edge will work against your ability to keep hold of the weapon. You definitely don't want to not know where the balance point of your sword is.

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    $\begingroup$ The distance from the hilt is determined by the purpose of the sword. Swords Cut, Pierce, and chop. Each of these are more effective when their center of weight is closer or further away and so looking for a general center of weight for swords isn't really good strategy. $\endgroup$ – Durakken Jul 27 '16 at 13:33
  • $\begingroup$ Solid answer. As a student of a master who practices with exotic weapons and combinations precisely because of the difficulty, I can tell you that is one of the advantages of exotic weapons. (This comes with the caveat that simple, reliable weapons are always preferred in situations with real stakes. The teach also suggested that rather than using one's fists, a good strategy is to "pick up a brick and hit them in the head with it";) $\endgroup$ – DukeZhou Jul 19 '17 at 17:22
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This sounds like a good thing in theory only. There are a number of "perpetuum mobile" designs premised on similar misconceptions. But angular momentum and momentum are not magically increased but preserved, so as the mercury travels through the centre of mass, its part of the angular momentum is taken over by the sword, meaning that the sword will resist swings. When the mercury hits the end of the channel, its moment is consolidated with that of the sword, meaning that if this does not happen at the precise moment where the tip hits a target frontally, the sword will get yanked out of your hand. Generally you will be required to time all of your technique to the handling of the sword and evasive action will have just as hard an effect on your sword grip than what you intended to deliver at your enemy.

It's like fighting with a ball-and-chain flail blindfolded. Except that the chain has more degrees of freedom, so it's more like a spear with a sliding spiked ball on it. How much use is this going to be for swinging?

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Just swing around a 1/4 filled bottle of water and you'll find that it makes for a difficult to control "weapon". Even for an executioner sword, it would feel clumsy and the executioner might overshoot, applying the force of the swing in the wrong place.

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As a sword goes, it would be ineffective. Swordsmen are quite picky about the balance of their sword. Having the balance shift during combat would not be popular. Also, mercury has a nasty habit of amalgamating with other metals and making them weak. You'd have to be choosy with your metallurgy.

That being said, there is something to this story. The Seven Star Staff is one of the weapons of Baguazhang, one of the internal martial arts of China. The Seven Star Staff consists of a length of bamboo with 7 cavities, each of which is partially filled with mercury. It's a staff, not a sword, but it's along the right lines. The theory was that the weapon would strike you, and then the mercury impact would truly hurt you. I'd also argue much of the benefit of it is that the internal state of the staff is hidden unless you're holding it. Given how much the mercury matters, not being able to see where it is in each of the cells could create quite a disadvantage for one opposing the Seven Star Staff. It certainly fits with the styles popular with the internal martial arts of China!

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  • $\begingroup$ Whereas the practitioner of the Seven Star Staff knows precisely where the mercury is, and how it will behaved, based on practice and feeling. Great answer! $\endgroup$ – DukeZhou Jul 19 '17 at 17:25
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Yes it could be useful. I am not sure what the book describes though ... never read it but as far as physics goes you'd get a lot faster initial movement in a sword.

Everyone here have missed the point of swords weight being trade on its length in fencing, extra length is useful but some cases one might use a short sword that can be concealed. A sword that is bound by length has the extra weight option if the wrist can take it. It's not like anyone ever concealing a short sword has a need to confuse its opponent but thrusting a weapon is something that don't really need a lot of momentum and its the slashing that needs it... to be effective.

Although I haven't really held a real sword or done any fencing it might really feel like the sword has a mind of its own ...or a soul... but think of it this way - space shuttle burns most of its fuel just to get to off the ground. And everyone knows that you'd never actually conceal a weapon I mean whats the point of having one if you cant post pics of yourself with it on Instagram.

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