A replica is a copy that is relatively inistinguishable from the original. Replicas are often used for historical purposes, such as being placed in a museum. Sometimes the original never existed. For example, Difference Engine No. 2, designed by Charles Babbage in the 19th century, was reconstructed from original drawings studied by Allan Bromley in the 1980s and is now on display at the Science Museum in London, England. A second example is Stephenson's Rocket where a replica was built in 1979, following the original design fairly closely, but with some adaptations.