Mechanics · pressure · carrier · station
A pneumatic postal line is a sealed pipe and a pressure difference. The carrier is a piston. The station is where clerks load, launch, catch, and empty it. Speeds come from named museum and postal sources, or are marked unknown.
Air does not “suck the mail.” A blower or steam pump raises pressure behind a carrier, lowers it ahead, or both. The carrier’s packing rings — leather, felt, later synthetics — seal against the bore so the slug of air has something to push. Reverse the gradient and the same tube runs the other way. The National Postal Museum puts the U.S. case in one sentence: cartridges were loaded into airtight tubes; compressed air propelled them; they could also travel in reverse by suction that created a partial vacuum.
European letter networks and American bulk-mail tubes used the same physics at different scales. Austria Post describes Vienna’s pipes as 65 millimetres internal diameter, laid about one metre underground. The Smithsonian’s New York canister — the object on view at the National Postal Museum — measures 24½ × 8¼ inches. Those are not interchangeable machines.
There is no universal “tube speed.” New York figures come only from Smithsonian / National Postal Museum captions. Vienna’s “around 50 km/h” is Austria Post’s wording on the 2025 stamp issue, not a dynamometer reading from 1875.
Three parts
Everything else — switches, clocks, red boxes, Brooklyn Bridge hangers — is furniture around those three.
A compressor or vacuum pump at a plant room sets the line. Early Paris used steam-powered pumps; the dedicated Paris history notes electrification of the machinery from 1927. Arrival is the dangerous part: without a cushion of air or a catcher, a carrier is a projectile. Modern hospital vendors advertise soft arrival; nineteenth-century clerks learned on the job.
Postal carriers were metal cylinders with sliding felt or leather skirts. New York’s surviving museum canister could hold, in National Postal Museum captions, as many as six hundred letters — a bulk pouch, not a single pneumatique card. Prague’s surviving pouzdra, photographed for Wikimedia, are the small European letter type.
NPM photographs and captions place U.S. apparatus in post-office basements, with clerks working around the clock to fill, send, open, and empty containers. European offices put Fortin-Hermann-style terminals on the public floor. Routing through a city network used switches and timed “trains” of capsules; Austria Post says each Vienna train could carry up to fifteen capsules.
Two families of bore
Letter tubes and bulk tubes are different machines. A 65 mm Viennese or Parisian tube moved a rolled letter, a card, or a telegram copy. An eight-inch New York tube moved a steel can of first-class mail between sorting offices. The first is closer to a telegraph extension. The second is closer to a dumbwaiter for pouches.
Latimer Clark’s 1853 London line was about 220 yards (200 m) between the Stock Exchange in Threadneedle Street and the Electric Telegraph Company in Lothbury — a building-to-building telegraph feeder, not a municipal grid. City grids came later: Berlin’s public Rohrpost is dated 1 December 1876 in German press histories (with a 1865 telegraph-office precursor in encyclopedias), Vienna 1875, Paris’s public opening 1879 after a closed official network from 1866.
Switches matter. A single point-to-point tube needs only a blower. A star network — Prague’s five main lanes meeting at Jindřišská — needs a control panel, counters, and a rule about which capsule is launched first. Photographs of that Prague panel exist; the municipal system has been inoperative since the August 2002 floods.
