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Losing ground

As a California resident, I have experienced my fair share of earthquakes. They’re unsettling (literally and figuratively), and yet they’re something we have all come to accept as a normal occurrence in this part of the world. We like the climate, the views, the culture—in other words, the whole vibe of the area—and we simply accept that it has its…faults. We stockpile emergency supplies, buy earthquake insurance, perform seismic retrofitting on our buildings to reduce the risk of damage, and then go on with our lives. Each time a truly devastating earthquake hits—such as the great San Francisco quake in 1906 or the Loma Prieta quake in 1989—we learn some important new lessons, and soon thereafter feel much safer about the future.

Sometimes that safety is illusory. For example, on December 11, 1995, the ground moved in a very small area of San Francisco’s Seacliff neighborhood, swallowing Howard Billman’s $1.5 million house (which, back in those days, would have been considered a very expensive property—a mansion, in fact). In this case, however, the cause was not an earthquake, but a giant sinkhole measuring 200 feet (60m) across and 40 feet (12m) deep. Sometimes sinkholes are natural occurrences; in this case, however, the culprit was an old sewer tunnel that had deteriorated. The tunnel itself, of course, was much smaller than the resulting hole. That bit of surprising geometry is just one of the things I find interesting about sinkholes. The mechanisms by which they form, and the abruptness with which they appear—often without warning, and in seemingly unlikely areas—make them even more fascinating, in a rather grim way.

Feeling Depressed and Empty?

Generally speaking, a sinkhole is a depression in the ground formed when the top layer of earth collapses into an empty space below—a process also called subsidence. (If there were no empty spaces—only layers of earth, rock, sand, and water—the ground would remain solid.) The natural question is where those cavities came from in the first place, and the usual answer is that they were always there—unnoticed until the ceiling of stone above them gave way. So what causes a layer of stone to collapse?

Sinkholes are most often found in areas where the bedrock is limestone or another relatively porous stone. Water can dissolve limestone—especially if the water is slightly acidic (due to natural or artificial causes). If acidic groundwater seeps through small cracks in a layer of limestone and into an empty cavity beneath, it can carry away the dissolved rock; the cracks eventually become wide enough that the entire layer weakens and caves in. This type of process is responsible for many of the sinkholes found in Florida and Texas.

Throwing Out the Basement with the Bathwater

But sinkholes can occur for other reasons too. In some cases, the loss of groundwater (due to pumping for municipal water supplies, say) creates new cavities large enough and near enough to the surface that sinkholes result, even though the soil or rock above the cavity was not compromised. Even more interesting, though, is the way damaged sewer pipes or other tunnels, such as the one in Seacliff, create massive underground cavities. In a typical scenario, an old sewer pipe fills to capacity after a rain storm, and due to a blockage or collapse, begins leaking excess water into the surrounding soil. When the water begins to recede, it drains back into the sewer pipe, but now saturated with minerals that have eroded from the adjacent areas. Repeat this process often enough, or with a large enough volume of water, and the empty spaces formed where the soil has eroded away become large enough to turn into sinkholes.

Yet another cause of sinkholes is salt mining—though not always for the reason you might think. Abandoned salt mine shafts can and do collapse, but some sinkholes have a more indirect cause. In Cheshire, England in the late 1800s, salt was produced in great quantities by drying brine that was pumped out of the ground. This brine was a saturated salt solution that had formed by the contact of groundwater with layers of rock salt over many years. Because the solution was saturated, it could not cause further erosion of the salt, and therefore left in place naturally occurring salt pillars that supported the earth above. However, salt producers pumped out the brine so quickly and recklessly that the salt concentration began to decrease rapidly as fresh groundwater came in. The fresh water quickly absorbed the salt that formed the supporting pillars, and the ground collapsed, forming a great many sinkholes. But because the sinkholes were located far from the sites where the pumping took place (in some cases, several miles away), it was impossible for people who had lost their homes to the subsidence to pin the blame on any particular salt producer.

None of this is in any way reassuring. Regardless of the geological and topographical features of the ground you’re walking on, there’s always the chance that some unknown empty space—whether natural or artificial—lurks below, waiting to envelop you in a sinkhole on your very next step. It happens with tragic frequency, though usually not on a scale sufficient to merit widespread attention. But then, you can’t really prepare for a sinkhole the way you prepare for an earthquake, so maybe ignorance is bliss.

Note: This is an updated version of an article that originally appeared on Interesting Thing of the Day on December 2, 2004.

Image credit: KiwiDandy [Public Domain], via Flickr

Yes, I’m aware: the Earth rotates all day, every day. But on this date in 1851, Léon Foucault proved it scientifically by setting his eponymous pendulum in motion in Paris. You can see replicas of this pendulum in science and natural history museums all over the world. They’re kind of boring to watch, as they very slowly change direction and periodically knock over little pegs. But that subtle change in direction would occur only if the Earth is rotating. This simple invention was and remains a brilliant demonstration of what to many of us is a self-evident truth.

Image credit: BrokenSphere [CC BY-SA 3.0 or GFDL], from Wikimedia Commons

When it comes to recording video, elements like composition, lighting, audio recording, judicious use of camera angles, panning, zooming, and so on all require practice and a certain degree of artistic sensibility; these do not come in the box with your smartphone, DSLR, or camcorder. And although virtually every modern video recording device for consumers has a built-in mechanism to compensate for the jitter caused by small hand movements, long handheld shots—especially those taken while walking, as is natural for camcorder users—typically include a fair amount of bounce and sway, making them look amateurish regardless of any other merits the recording may have. You almost never see such bumpy images on TV or in movies, unless a shot was made that way intentionally to achieve a certain effect. Professional videographers and cinematographers supplement their skill with a great piece of technology called a steadicam to smooth out the trickiest of handheld shots.

Steady as You Go

You can think of the steadicam as an extremely sophisticated shock absorber for a camera. Just as the shock absorbers on your car keep the ride smooth even when the road is not, so a steadicam keeps a camera steady despite bumps underneath. But unlike automotive shock absorbers, a steadicam must also compensate for pan (horizontal rotation), tilt (rotation up or down), and roll (rotation about the axis of the lens)—but only when these changes are not explicitly implemented by the operator. After all, a handheld shot that could only ever point in one direction would not be terribly interesting.

To do all this, a steadicam starts with a large, rigid harness or vest worn by the operator. Because the entire apparatus, including the camera, is typically quite heavy—sometimes as much as 90 lb. (about 40kg)—the vest spreads out the weight as much as possible. This not only minimizes fatigue, but also gives the camera a much more solid attachment point than the operator’s arms would provide. Protruding from this vest is a heavy-duty mechanical arm, somewhat reminiscent of the ones used in certain adjustable desk lamps. The arm has two rigid segments (called “bones”), connected by spring-loaded joints. The mechanism is designed in such a way that the bones always remain parallel to each other, but can move up, down, left, and right with the application of a small force from the operator. The camera itself (whether film or video) sits atop a postlike mounting assembly called a “sled,” which is attached to the end of the arm by a free-floating rotating joint known as a gimbal. The weight of the camera is counterbalanced partly by the tension of the springs and partly by other components mounted at the bottom of the sled such as batteries and a video screen that enables the operator to see what is being filmed while watching their step.

All these joints, springs, and weights shift the center of gravity away from the camera itself while providing multiple points where vibration and other unwanted movement can be isolated from the camera. The net effect is that the operator can walk, climb stairs, step over obstacles, or even jog while keeping the “floating” camera perfectly smooth. This makes possible shots that could never be achieved with a tripod or dolly, enabling the camera operator to walk among the actors freely while keeping all equipment out of the shot.

Gonna Film Now

The steadicam was invented in 1973 by cinematographer Garrett Brown, with its mainstream debut in the 1976 film Rocky. Since then, it has become ubiquitous in both film and TV. The most impressive use of the steadicam I’ve seen is the 2002 film Russian Ark, shot in the Hermitage, a former palace in St. Petersburg that’s now a museum. The entire 90-minute film was done as a single, continuous steadicam shot that followed the main character as he walked from one room to the next. Garrett Brown himself also famously used a steadicam to capture footage of California’s Redwood National Park in slow motion as he walked through it; when the footage was played at regular speed, it produced the backdrop for the speeder bike sequence in Return of the Jedi (1983).

As sophisticated as steadicams are, they are useless without a highly trained operator. A great deal of practice, not to mention stamina, is necessary to become proficient filming scenes with a 90-pound weight hanging from your chest. Steadicam operators, who generally work as freelancers and provide their own equipment, are both highly paid and well respected. Professional steadicam equipment is, not surprisingly, quite expensive (a complete rig can cost as much as a mid-range car)—though scaled-down versions for use with smaller consumer devices can be had for well under US$1,000. That money won’t make your vacation videos less boring, but your camera’s “off” switch was designed for just such a purpose.

Note: This is an updated version of an article that originally appeared on Interesting Thing of the Day on November 11, 2004.

Image credit: Mike1024 [Public domain], via Wikimedia Commons

Tempura refers to the Japanese technique of dipping food (usually vegetables or seafood) in a light batter and deep-frying it. There are lots of other ways to deep-fry stuff (with or without batter), but tempura has a distinctive texture and flavor. I don’t know if I’ve ever had tempura anywhere besides a Japanese restaurant, but it’s easy enough to make yourself if you’re so inclined.

Image credit: FASTILY [CC BY-SA 4.0], from Wikimedia Commons