How a Pneumatic Tube Works

The metal-and-glass intricacy of pneumatic tube systems seems like a perfect example of Victorian-era technology, the sort of elaborate period gadgetry that you might see Robert Downey Jr. tinkering with in one of the Sherlock Holmes movies. But actually, the idea of pneumatics — that is, using pressurized gas to produce mechanical motion — goes back to Hero of Alexandria, a Ptolemaic Greek mathematician, inventor and author who lived in the first century A.D. [sources: Merriam-Webster, Britannica].

Hero apparently was a pretty observant guy. He noticed that the wind, even though it didn't have a visible substance, could push pretty hard on things. That led him to deduce that air was actually composed of tiny, invisible, moving "particles," what today we call molecules. He went on to figure out that if you compressed those moving molecules by jamming them into a tight space or passageway, they'd try to escape, and in the process, push a solid object that was in front of them. He also deduced that if you could create a vacuum — basically, an empty space — that air molecules would try to rush into it. By means of these principles, he wrote, "many curious and astonishing kinds of motion may be discovered" [source: Woodcroft].

Hero used his understanding of how pressurized gases behave to create gadgets like a primitive steam engine and a singing toy bird, but it wasn't until the 1810 that a British engineer, George Medhurst, published a plan for a pneumatic tube rapid transport system [source: Woodcroft].

Medhurst noted that if air was subjected to 40 pounds per square inch of pressure — only about two-and-a-half times the amount that the atmosphere exerts on us at sea level — air molecules would be propelled at 1,500 feet (457 meters) per second, or about 1,000 miles (1,609 kilometers) an hour. When pushing a canister, the speed would only be about 100 miles (160 kilometers) an hour, but that was blazingly fast for 1810 [source: Medhurst].

Unfortunately, Medhurst wasn't as good at actually building machinery as he was at designing it, and he died in 1827, before he actually could construct his vision [source: Stephen and Lee].

In the 1850s, Great Britain's General Post Office commissioned a study of Medhurst's concept, and in the 1860s, awarded a contract to engineer T.W. Rammel to build a complex system to carry mail throughout London [sources: Library – UC Berkeley, Grace's Guide] By 1886, London's tube system stretched for 34 miles (54.7 kilometers) underneath the city and transmitted 32,000 messages a day. Letters hurtled along at a speed of up to 51 miles (82 kilometers) per hour—not as fast as what Medhurst had promised, but still speedier than a horse-drawn mail wagon [source: U.S. Congress].

By the turn of the 20th century, New York had a pneumatic tube system that sent cylindrical containers containing letters and parcels zipping in a loop under Manhattan at 30 miles (48 kilometers) per hour. Other cities like Boston, Chicago, Philadelphia and St. Louis had similar systems too [source: Library – UC Berkeley].

Pneumatic transport seemed so ingenious that some people proposed using it for other things besides mail. In 1867, an inventor named Alfred Beach built and briefly operated a 300-foot-long (91-meter-long) pneumatic tube subway under a section of Broadway in Manhattan, in hopes that air-powered trains would catch on [source: Massachusetts Institute of Technology]. In 1913, Waldemar Kaempffert, managing editor of the prestigious publication Scientific American, actually proposed the idea of cooking meals in central kitchens and shipping them via pneumatic tube to people's homes. "If a letter can be shot through pneumatic tube from New York to Brooklyn, why not a five-course dinner?" he opined.

Just as Edwardian-age folks were starting to really go crazy about this newfangled pneumatic technology, World War I quickly cooled their ardor. The U.S. Post Office suspended the use of pneumatic mail transport, saying that it used too much fuel to power the air compressors that they needed. After the war, the service eventually was restored, but only in New York and Boston.

Private companies that would have built new systems stopped putting in bids because of all the Congressional regulations and gradually, the existing systems' capacity was outstripped by the growing volume of mail. Instead, the Post Office put its money into mail trucks, which had the added advantages of transporting mail to locations far more distant than a pneumatic tube system could reach and transporting larger packages. In 1953, U.S. Postmaster General Arthur Summerfield canceled pneumatic tube mail for good, and ordered the systems dismantled [source: Cohen].

Pneumatic tube transport never died out completely. As long as people used paper documents and photographs, it was still a practical method of transmitting information inside large buildings. The U.S. Central Intelligence Agency (CIA), for example, built a sprawling pneumatic tube system inside its headquarters in Langley, Va., in the 1950s, which transmitted 7,500 documents each day throughout the building's seven floors. It wasn't shut down until 1989, when CIA employees had become so reliant upon e-mail that the tubes were no longer needed [source: CIA].

In the 1970s, companies also began to use heavier, more powerful pneumatic tube systems to transport materials such as machine parts and even gravel inside plants [source: New Scientist]. They're still widely used in industry for such purposes [source: Air Link International]. And banks still have pneumatic tube systems at their drive-up windows, which enable customers to make cash deposits or withdrawals from the comfort of their cars [source: Farber].

More recently, pneumatic tube systems have become popular in hospitals because they can be used to rush tissue and blood samples from a lab in a distant part of the complex, far faster than a person could do on foot. Stanford University's hospital, for example, has installed a system with 4 miles (6.4 kilometers) of air tubes, and uses it to ship 7,000 specimens each day. They've even wedded the 19th-century wonder to 21st-century technology, by equipping the tubes with a digital monitoring system that allows hospital staff to watch the progress of their canister to its destination on a computer screen [source: Wykes].