The plastic must also be flat and smooth for the reading to be accurate, and the object must be brought to the machine (which is not very portable) for analysis, which takes yet more time.

ECONOMIST: Recycling

The mariner uses a 250 watt free-spinning motor mounted in the front wheel for propulsion and high capacity 36 volt 9 amp hour advanced lithium battery (LiFEPO4) which provide a smooth, powered ride on flat and hilly terrain.

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Over the centuries, the river had slowly carried away mountainsides, tumbling and cracking rock, reducing boulders into a rainbow of smooth, flat stones the size of a child's hand.

CNN: The seven stones of simpler living