The Falkirk Wheel lies at the end of a reinforced concrete
aqueduct that connects, via the Roughcastle tunnel and a double
staircase lock, to the Union Canal. Boats entering the Wheel's
upper gondola are lowered, along with the water that they float in,
to the basin below. At the same time, an equal weight rises up,
lifted in the other gondola.
This works on the Archimedes principle of displacement. That is,
the mass of the boat sailing into the gondola will displace an
exactly proportional volume of water so that the final combination
of 'boat plus water' balances the original total mass.
Each gondola runs on small wheels that fit into a single curved
rail fixed on the inner edge of the opening on each arm. In theory,
this should be sufficient to ensure that they always remain
horizontal, but any friction or sudden movement could cause the
gondola to stick or tilt. To ensure that this could never happen
and that the water and boats always remain perfectly level
throughout the whole cycle, a series of linked cogs acts as a back
Hidden at each end, behind the arm nearest the aqueduct, are two
8m diameter cogs to which one end of each gondola is attached. A
third, exactly equivalent sized cog is in the centre, attached to
the main fixed upright. Two smaller cogs are fitted in the spaces
between, with each cog having teeth that fit into the adjacent cog
and push against each other, turning around the one fixed central
one. The two gondolas, being attached to the outer cogs, will
therefore turn at precisely the same speed, but in the opposite
direction to the Wheel.
Given the precise balancing of the gondolas and this simple but
clever system of cogs, a very small amount of energy is actually
then required to turn the Wheel. In fact, it is a group of ten
hydraulic motors located within the central spine that provide the
small amount, just 1.5kWh, of electricity to turn it.