Caen Hill locks, at Devizes, provides an insight into the engineering needed to build and maintain the canal. These 16 locks at Caen Hill form part of the Devizes flight of 29 locks. The total rise is 237 feet (72 m) in 2 miles (3.2 km) or a 1 in 30 gradient. Because a large volume of water is needed for the locks to operate a back pump was installed at Foxhangers in 1996, capable of returning 32 million litres (7 million imperial gallons) of water per day to the top of the flight, equivalent to one lockful every 11 minutes. Because of the steepness of the terrain there was no space to use the normal arrangement of water pounds between the locks. As a result, the 16 locks utilise unusually large side ponds to store the water needed to operate. The locks take 5–6 hours to travel in a boat, and lock 41 is the narrowest on the canal.