In the soil, right under its the surface, there is a tank, approx. 8m (8yd) long. It produces partial vacuum (low pressure air). Just by water power!

Start:  The tank is flooded from the upper water (left) and the connection to the basin (right) is closed.
Operation: The connection to the basin is open and the flooding connection closed. The tank would normally empty now because of the difference in the water levels: The tank is a few meters higher than the canal (lower water). But as the tank is closed, the water just runs out that far, because there is a balance between air pressure, hydrostatic pressure and partial vacuum in the tank.
(At a water level distance of 10m between tank and lower water a total vacuum would be achieved but in this case the vacuum could not drag up the water in the water lifters - see next page).
The volume of air under partial vacuum in the tank is automatically regenerated each time the lock is used.