This article is from the Unicycling FAQ, by Beirne Konarski with numerous contributions by others.
The accepted view is that the unicycle came from the penny-farthing bicycle,
which had a large front wheel and a small rear wheel. The penny-farthing had
cranks directly connected to the front axle. If a rider stopped quickly, the
rear wheel would go up in the air as the rider moved forward a bit. Some
riders no doubt found that they could ride a bit with the rear wheel up, and
then decided to see how far they could go. Pictures of unicycles from the
late 1800's show big-wheeled unicycles, which would support the idea that
the first unicycles came from penny-farthing bicycles.
The Quebec Unicycle Association has a page in French discussing the origin
of the unicycle which describes the same view (I think). The URL is
http://www.gel.ulaval.ca/~stpier02/amq/homeangl.html.
 
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