Diagram courtesy Amazon

On December 7, 2016, a drone departed from an Amazon warehouse in the United Kingdom, ascended to an altitude of four hundred feet, and flew to a nearby farm. There it glided down to the front lawn and released from its clutches a small box containing an Amazon streaming device and a bag of popcorn. This was the first successful flight of Prime Air, Amazon’s drone delivery program. If instituted as a regular service, it would slash the costs of “last-mile delivery,” the shortest and most expensive leg of a package’s journey from warehouse…