You never see them, but they're with you every time you fly. They record where you are going, how fast you're traveling and whether everything on your airplane is functioning normally. Their ability to withstand (抵挡) almost any disaster makes them seem like something out of a comic book. They're known as the black box.

When planes fall from the sky, as a Yemeni airliner did on its way to Comoros Islands in the India Ocean June 30, 2009, the black box is the best bet for identifying what went wrong. So when a French submarine detected the device's homing signal five days later, the discovery marked a huge step toward determining the cause of a tragedy in which 152 passengers were killed.

In 1958, Australian scientist David Warren developed a flight-memory recorder that would track basic information like altitude and direction. That was the first mode for a black box, which became a requirement on all U.S. commercial flights by 1960. Early model, often failed to withstand crashes, however, so in 1965 the device was completely redesigned and moved to the rear of the plane — the area least subject to impact — from its original position in the landing wells (起落架舱). The same year, the Federal Aviation Authority required that the boxes, which were never actually black, be painted orange or yellow to be visible.

Modern airplanes have two black boxes: a voice recorder, which tracks pilots' conversations, and a flight-data recorder, which monitors fuel levels, engine noises and other operating functions that help investigators reconstruct the aircraft’s final moments. Placed in an insulated（隔绝的）case and surrounded by a quarter-inch-thick panels of stainless steel, the boxes can withstand massive force and temperatures up to 2,000℉. When submerged (潜入水中), they’re also able to emit signals from depths of 20,000 ft.

Experts believe the boxes from Air France Flight 447, which crashed near Brazil on June 1, 2009, are in water nearly that deep, but statistics say they’re still likely to turn up. In the approximately 20 deep-sea crashes over the past 30 years, only one plane’s black boxes were never recovered.

The World’s Worst Airport

Like expensive watches that never break, the world’s best airports can be boring. You land, breeze through passport control and check into a hotel within minutes. The experience is pleasant, but not memorable. 【小题1】 To adapt Tolstoy, lovely airports are all alike, but every wretched (不幸的) airport is wretched in its own way.

Consider Juba, the airport in South Sudan’s capital, which is a hot tent. Planes are often late, so passengers must sweat for hours. The departure hall has no toilets, no food and no queuing system. Lucky is the traveller who finds a chair that is only half-broken. Since dirty water and tropical diseases are common, so are upset stomachs. Tough luck. Travellers should have thought twice before eating salad.

【小题2】 Big important people’s servants carry their bags, which are ostentatiously (卖弄地)) passed round, not through, the scanner. Since the machine seldom works, little peopled are in effect up-graded to big important status by not having their bags scanned for guns and explosives, either.

South Sudan is at war, so many UN planes take off from Juba carrying aid workers and emergency supplies. 【小题3】 When a journalist was booked on a UN flight, he was assured by the government that his papers were in order. Yet at the airport he was told to get a fourth permit, as well as the three expensive ones he had already obtained. This required a trip across town. Predictably, he missed his plane.

Juba has three terminals, but only one is in use. After South Sudan became independent in 2011, the government planned to build an airy structure of glass, steel and concrete. 【小题4】 In 2016 the government decided to build a more modest terminal. But it, too, stands half-completed and empty, next to the tented camp that people actually have to use. Travellers are advised to bring a good, long book.

American public schools offered transportation for their students long before the modern school bus. In the 1800s, many students attended one-room schoolhouses in rural areas where few people lived. Some students walked; others used horses or sleds to get to school. Over time, school districts used a system of horse-drawn wagons to pick up students and take them to school. In time, horse-drawn wagons gave way to cars, which picked up students before school and delivered them near home at the end of the day.

The first school buses that carried students didn’t look much like modern ones. A school bus from the 1920s, for example, looked like a long car. School buses gradually grew larger to hold more and more students. The yellow color of modern school buses was chosen because it was easy to spot. When buses stand out on the road, drivers see them and stop in time to allow students to board safely. Today, the law says that American school buses must be painted a color called “National School Bus Glossy Yellow.” The bright color helps drivers recognize school buses immediately.

Modern school buses are far safer than before. Bus drivers receive stricter safety training. What’s more, school buses have seats that help keep students safe in the event of a crash. Seat backs are made to absorb an impact when a school bus is hit hard. Buses may also include flashing stop signs, which alert drivers that students are getting off or on the bus. Some buses have cameras that help drivers see what’s happening inside the bus and outside.

Nowadays, some school districts are beginning to make changes to buses that help the environment. For example, one school bus in New York has an electric engine that doesn’t pollute the air. The bus can travel for about 120 miles (190 km) before it needs to be recharged. School buses can also cut down pollution by replacing private cars. The average school bus can drive fifty-four students, which keeps about thirty-six private cars off the road. This fact, along with newer, greener technology, can help keep the air cleaner.