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Almost every machine with moving parts has wheels, yet no one knows exactly when the first wheel was invented or what it was used for. We do know, however, that they existed over 5,500 years ago in ancient Asia.
The oldest known transport wheel was discovered in 2002 in Slovenia. It is over 5, 100 years old. Evidencesuggests that wheels for transport didn't become popular for a while, though. This could be because animals did a perfectly good job of carrying farming tools and humans around.
But it could also be because of a difficult situation. While wheels need to roll on smooth surfaces, roadswith smooth surfaces weren't going to be constructed until there was plenty of demand for them. Eventually,road surfaces did become smoother, but this difficult situation appeared again a few centuries later. There had been no important changes in wheel and vehicle design before the arrival of modern road design.
In the mid-1700s, a Frenchman came up with a new design of road - a base layer (层)of large stones coveredwith a thin layer of smaller atones. A Scotsman improved on this design in the 1820s and a strong, lastingroad surface became a reality. At around the same time, metal hubs (the central part of a wheel) came intobeing, followed by the pneumatic tyre(充气的) in 1864. Alloy wheels were invented in 1967, sixty years after the appearance of tarmacked roads(泊油路). As wheel design took off, vehicles got faster and faster.
