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Blimp vs. Zeppelin: What's the Difference?

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The Dilemma: You're at a football game and there's a large, cigar-shaped object hovering suspiciously close to you. Question: Is it a blimp or a zeppelin? And more important, why didn't you get better seats?

People You Can Impress: whoever's perched next to you in the nosebleeds.

The Quick Trick: When in doubt, just think of Led Zeppelin. Zeppelins are heavy metal—or at least they've got metal skeletons. Blimps, on the other hand, aren't.

The Explanation: Both blimps and zeppelins work by being lighter than air—they're filled with a gas that's lighter than oxygen, so they go up like hot-air balloons. But balloons can't be steered. Realizing this, German Count (Graf) Ferdinand von Zeppelin decided he wanted to devise a "dirigible [or steerable] balloon" in the 1890s for use in military reconnaissance work. Eventually, these dirigible balloons took the generic name zeppelin and were used as bombers or scout craft through World War I. This was just one of their many uses, however. The airships doubled as a major mode of transportation between the wars, routinely making transatlantic flights, and the enormous Graf Zeppelin even circumnavigated the globe in 1929.

So just how popular were these zeppelins? Well, enough that the spire on the top of the Empire State Building was designed as a docking mast for them, although that idea proved impractical due to the serious updrafts (and besides, who wants to disembark while dangling 1,300 feet over Manhattan?).

Incidentally, anyone who's seen the footage of the Hindenburg incinerating at Lakehurst, N.J., in 1937 can see evidence of the main difference between zeppelins and blimps: zeppelins have rigid metal skeletons, making them suitable for longer trips in a wider variety of weather conditions (which also makes them expensive). Blimps, on the other hand, are simply shaped balloons with fins and an engine. Oh, and as for the name "blimp"? It dates back to 1916 and mimics the sound made when the balloon is thumped with a finger.

Led and Other Zeppelins
Led Zeppelin is to date the greatest band ever named after a flying machine (take that, Jefferson Airplane). The band's name is completely attributable to Keith Moon, the late and eccentric drummer of The Who. The pessimistic Moon thought the band, originally called the New Yardbirds, would "go over like a lead zeppelin." But the plucky young band reveled in the challenge and quickly adopted the name—with a minor change in spelling.

Excerpted from What's the Difference?

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Big Questions
What Causes Sinkholes?
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Mark Ralston/AFP/Getty Images

This week, a sinkhole opened up on the White House lawn—likely the result of excess rainfall on the "legitimate swamp" surrounding the storied building, a geologist told The New York Times. While the event had some suggesting we call for Buffy's help, sinkholes are pretty common. In the past few days alone, cavernous maws in the earth have appeared in Maryland, North Carolina, Tennessee, and of course Florida, home to more sinkholes than any other state.

Sinkholes have gulped down suburban homes, cars, and entire fields in the past. How does the ground just open up like that?

Sinkholes are a simple matter of cause and effect. Urban sinkholes may be directly traced to underground water main breaks or collapsed sewer pipelines, into which city sidewalks crumple in the absence of any structural support. In more rural areas, such catastrophes might be attributed to abandoned mine shafts or salt caverns that can't take the weight anymore. These types of sinkholes are heavily influenced by human action, but most sinkholes are unpredictable, inevitable natural occurrences.

Florida is so prone to sinkholes because it has the misfortune of being built upon a foundation of limestone—solid rock, but the kind that is easily dissolved by acidic rain or groundwater. The karst process, in which the mildly acidic water wears away at fractures in the limestone, leaves empty space where there used to be stone, and even the residue is washed away. Any loose soil, grass, or—for example—luxury condominiums perched atop the hole in the ground aren't left with much support. Just as a house built on a weak foundation is more likely to collapse, the same is true of the ground itself. Gravity eventually takes its toll, aided by natural erosion, and so the hole begins to sink.

About 10 percent of the world's landscape is composed of karst regions. Despite being common, sinkholes' unforeseeable nature serves as proof that the ground beneath our feet may not be as solid as we think.

A version of this story originally ran in 2014.

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Big Questions
How Are Speed Limits Set?
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When driving down a road where speed limits are oppressively low, or high enough to let drivers get away with reckless behavior, it's easy to blame the government for getting it wrong. But you and your fellow drivers play a bigger a role in determining speed limits than you might think.

Before cities can come up with speed limit figures, they first need to look at how fast motorists drive down certain roads when there are no limitations. According to The Sacramento Bee, officials conduct speed surveys on two types of roads: arterial roads (typically four-lane highways) and collector streets (two-lane roads connecting residential areas to arterials). Once the data has been collected, they toss out the fastest 15 percent of drivers. The thinking is that this group is probably going faster than what's safe and isn't representative of the average driver. The sweet spot, according to the state, is the 85th percentile: Drivers in this group are thought to occupy the Goldilocks zone of safety and efficiency.

Officials use whatever speed falls in the 85th percentile to set limits for that street, but they do have some wiggle room. If the average speed is 33 mph, for example, they’d normally round up to 35 or down to 30 to reach the nearest 5-mph increment. Whether they decide to make the number higher or lower depends on other information they know about that area. If there’s a risky turn, they might decide to round down and keep drivers on the slow side.

A road’s crash rate also comes into play: If the number of collisions per million miles traveled for that stretch of road is higher than average, officials might lower the speed limit regardless of the 85th percentile rule. Roads that have a history of accidents might also warrant a special signal or sign to reinforce the new speed limit.

For other types of roads, setting speed limits is more of a cut-and-dry process. Streets that run through school zones, business districts, and residential areas are all assigned standard speed limits that are much lower than what drivers might hit if given free rein.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

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