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The Romans had pumps, aqueducts, water wheels (which were used to grind flour, saw wood & stone etc), pipes (they are known to have used stone, clay, lead & copper for pipes) and valves.

All the ingredients in fact for a pneumatic tube system for delivery of small packages.

So, would an alternate past where the Romans had actually put all these things together to produce a pneumatic tube postal service for the city of Rome, with the air pumps powered by water-wheels, be plausible?

Only for government buildings and districts occupied by wealthy citizens of course.

How the Romans made lead pipes

Copper pipes (if used) can be made the same way (hammered around an iron or steel mandrel) so any issue of standardization is more or less inherently solved by the production method.

Message-tubes to feed into the pipes can also be made this way hammered around a slightly thinner pole with leather (rather than rubber) washers to give a relatively airtight fit in the pipes.

I'm voting for Yes

Lead pipes can be hermetically sealed. It's lead. It could be hermetically sealed with a candle (well... not a candle... but if they can extrude lead pipe they can obviously melt it).

It's possible that the Romans had springs. This is good! Because the manufacturing tolerances of the pipe probably wouldn't allow even a loose-fitting canister. BUT! Throw some spring-loaded legs with wheels on the thing such that a smaller canister can roll along with shock absorption... now we're cooking with gas!

But, those springs have consequences. No vacuum. But... that could easily be fixed with a leather collar to seal around the canister. More complex than I like... but it's solving problems.

I'm not super fond of pneumatics for this purpose. I don't believe sealing the pipe is the problem. I do believe creating the vacuum in the first place could be. Pumps are good for moving fluids, but you need really, really, really well sealed pumps to create even a mediocre vacuum.

Water, on the other hand.... Fill the pipe with water from the sending end. When the canister arrives, the recipient closes a ball valve, allowing the water to drain on both sides. How does the sender know when to let the water drain? When the pressure equalizes.

Now, officially, measuring pressure wasn't something that happend until the 1500s. But it wouldn't be hard to have a small pipe affixed to the main line, probably pointing up, such that when the line pressurized a spray of water came out of it ringing a bell. That would tell the sender to turn off the water and let it drain. The line would need to be designed such that the canister didn't fall back as the water drained. Hey... you might not even need the valves I mentioned. All you need is the ability to turn water on from both sides of the pipe.

So, yes, I believe it's plausible for an alternate-history story.

Reality check, sadly, I'd have to say is:

IMPLAUSIBLE

The only thing truly implausible about the scenario is slaves. Rome had a lot of them. It becomes a simple matter of economics: it's much cheaper to build an aqueduct than it is to supply Rome with water via lines of slaves forming a bucket chain all the way up into the mountains; whereas it's much cheaper to tell a slave to "pop on over to M Lucius Cicero's office with this packet of letters" than it is to build a pneumatic tube system on a large scale.

Is it technologically possible? Absolutely! Does it satisfy the cool factor? Sure! But is it plausible in a heavily slave dependent Roman Empire? That I'd have to say is no.

If you want to use pneumatic post, you need to be able to rely on vacuum. This implies that whoever manufactures the system has to ensure tight tolerances on both the pipes and the vessel, to prevent leakages disrupting the vacuum system.

I am not sure manufacturing processes in the Roman Empire could deliver a combination of narrow tolerances and large production, of the order of hundreds of meters of piping on a single building.

Moreover, lead is a poor material for this application: being really a soft metal, I doubt it would withstand the pressure differential between the vacuum on the inside and the atmosphere on the outside. You would either get breakages/deformations, or would need to use thicker walls, making again tolerances more broad.

It is entirely possible to have a series of pneumatic tubes working in ancient Rome. The technology did not come into play until the early 1800's, but there is nothing that could have stopped it from happening at an earlier time.

Romans had lead pipes. They had waterways to transport water to homes. By adding leather and grease gaskets to connect the pipes, you can send objects down the pipe. You could use the existing aqueducts to run the pipes. The issue is having a standard circumference, say 6", a cylinder with leather gaskets on each end and a source of air pressure. This isn't much different than the version first operating in London in 1850. The air pressure on modern systems is only 15 psi. You could use a windmill, a water wheel to provide blower power. A blower is a simple squirrel cage fan turning fast enough to push air. Your bathroom fan is powerful enough to do the job.

Good luck with your series of tubes.

• https://en.wikipedia.org/wiki/Pneumatic_tube


• https://www.blowervacuumbestpractices.com/system-assessments/conveying/importance-particle-velocity-dilute-phase-pneumatic-conveying