How to Build a Natural Swimming Pool

Whether you like to practice your dolphin dives or lounge away the day on a raft, swimming is one of summer's perfect pleasures. With a minimum of materials and without so many chemicals, you can create a cooling summer retreat in your back yard.

Though fairly common in Europe, natural swimming pools, are in their early stage in the United States. You can ask most American swimming-pool contractors to build a backyard pool. 【小题1】. The list may include rebar（钢筋）, gunite(水泥砂浆), fiberglass, chlorine and an energy-sapping filtration system.

But in recent years, a few builders and a growing number of homeowners have learned how to build pools. 【小题2】. They've found it's possible to construct pools that are more about building with nature and blending into the natural landscape. Natural swimming pools use gravel stone and clay in place of concrete or fiberglass, and aquatic plants instead of harmful chemicals and complicated mechanical filtering systems. 【小题3】. They also support beneficial bacteria that consume debris and potentially harmful organisms, and give habitat to frogs, dragonflies and other water life.

【小题4】. A natural pool can he constructed for as little as $2,000 if you do it yourself, while conventional pools can cost tens of thousands of dollars.

Natural swimming pools require no harmful chemicals, are fairly low-tech. 【小题5】. You won’t have to drain the pool each autumn. Except for topping it off now and then, you'll fill the pool only once.

Many of us already live with AI a series of unseen algorithms (算法) that controls our Internet-connected devices, from smartphones to security cameras and cars that heat the seats before you’ve even stepped out of the house on a freezing morning. But, while we’ve seen the AI sun, we have yet to see it truly shine.

Researchers compare the current state of the technology to cellphones of the1990s: useful but raw. They are working on applying the largest, most powerful machine-learning models to lightweight software that can run on “the edge,” meaning small devices such as kitchen appliances or wearable devices. Our lives will gradually witness AI is everywhere.

Our interactions with the technology will become increasingly personalized. Chat bots, for example, can be awkward and disappointing today, but they will eventually become truly conversational, learning our habits and personalities, and even develop personalities of their own.

But don’t worry, the fever dreams of super intelligent machines taking over, like HAL in “2001: A Space Odyssey”, will remain science fiction for a long time to come; self-awareness and free will in machines are far beyond the capabilities of science today.

The research institute Open AI has created Muse Net, which uses artificial intelligence to mix different styles of music to create new compositions. The institute also has Jukebox, which creates new songs when given a style, artist and lyrics, which in some cases are co-written by AI. These are early efforts, achieved by feeding millions of songs into networks of artificial neurons (人工神经元), made from strings of computer code, until they internalize patterns of tune and harmony, and can recreate the sound of instruments and voices.

There are seemingly endless ways in which Al is beginning to touch our lives. from discovering new materials to new drugs to picking the fruit we eat and sorting the garbage we throw away. Self-driving cars work—they’re just waiting for laws and regulations to catch up with them.

HYDROGEN­POWERED cars have had a rough ride. Back in 2003， they were sold as “one of the most encouraging， innovative technologies of our times” by US president at the time George W. Bush. Then the Tesla revolution came along and they were left in the dust by their battery­driven electric rivals.

Now， there are signs of a comeback. A recent survey of more than 900 global automotive executives by consulting firm KPMG found that 52 percent rated hydrogen(氢) fuel cell vehicles as a leading industry trend. Japan has announced plans to put 40，000 hydrogen vehicles on the road in the next five years， and South Korea 16，000. Germany wants to have 400 refueling stations for hydrogen vehicles by 2025 and California has already opened 35.

This renewed push has its doubts. Tesla chief Elon Musk， for example， has dismissed hydrogen cars as being “extremely silly”. But Joan Ogden at the University of California， Davis， sees a future in which hydrogen and electric vehicles play complementary(互补的) roles. “There are arguments for having both，” she says.

Like electric cars， hydrogen vehicles produce zero pollutants， so they don't damage our health or the climate. The main difference is that hydrogen cars use a fuel cell instead of a battery to power an electric motor. Hydrogen is stored in a tank and fed into the fuel cell， where its chemical energy is changed into electrical energy.

Hydrogen cars are finally becoming commercially practical because fuel cells have become smaller and lighter， says Matthew Macleod at Toyota， which began selling the Mirai， one of the first mass­market hydrogen cars， in 2014 for $60，000.

We are also figuring out better ways to transport and store hydrogen， says Michael Dolan at Australia's national science organization， the CSIRO. Last month， his team showed that hydrogen gas can be changed into liquid ammonia(氨) for transportation， then changed back. Liquid ammonia takes up less space and is less flammable than hydrogen gas， making it easier to ship to refueling stations.

The ability to rapidly refuel is one of the main advantages hydrogen vehicles have， says Macleod. Filling up a hydrogen car takes about the same time as filling a petrol one， rather than the hours it typically takes to recharge an electric car's battery. You can also go further on a full tank of hydrogen—about 500 kilometers， compared with 300 kilometers for a standard fully charged battery.

But although hydrogen reacts cleanly—the only thing coming out is water—hydrogen vehicles are more energy­consuming than electric ones if you take fuel production and transport into consideration， says Jake Whitehead at the University of Queensland， Australia.

At the moment， most hydrogen is from natural gas—a fossil fuel. “Green” hydrogen can be made by splitting water using solar or wind power， but this involves multiple steps， each using energy along the way. In contrast， a single energy step is required to directly recharge a car battery at home.