The Difference Engine of Charles Babbage was conceived in 1822 and redesigned
in 1847-49; in 1991, in occasion of the bicentenary of Babbage's birth, the
Science Museum in Kensington, England, built a working copy of the whole machine
on the basis of the 21 drawings left by Babbage. The son of Babbage, Henry
Prevost, built several copies of the arithmetic unit of the Difference Engine
and sent them to various institutions; in 1995 one of those copies has been sold
for $200,000. The large-scale differential analyzer built by Vannevar Bush at
MIT in 1925 with the support of the Rockefeller Foundation (last picture)
was probably the largest computational device in the world at that time.
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