This article is from the Unicycling FAQ, by Beirne Konarski with numerous contributions by others.
Someone has built a unicycle with multiple gearings. Here is an account from
Peter Lister (email@example.com):
I know Bob Knight who built it. I haven't had the opportunity to have a go
on it (I haven't really got the hang of giraffes yet - severe lack of people
prepared to lend me theirs to practise on).
I can assure you that it's no problem to change gear. Bob can shift up and
down happily while going forward or idling, or probably backwards, knowing
him. As I understand it, the only requirement is that one reduce the pedal
pressure during a shift, but then as anyone (like me) who rides a
two-wheeler with a Sturmey hub knows, that applies to normal bikes and isn't
a problem unless one is really standing on the pedals.
Just in case it isn't clear from the photo(another person brought up this
topic when they saw a picture of the unicycle. ed.), this beast uses a fixed
Sturmey Archer 3 speed hub *not* a derailleur (it's surprising how many
people assume that the derailleur is the only possible cycle gear shift).
The hub itself is an antique (early 1950s I think), probably worth more than
the rest of the unicycle (which was made by Bob and his dad). The equivalent
wheel sizes are 20", 24" and 28".