 The caissons or gondolas always carry a combined weight of 500 tonnes ( 490 long tons ; 550 short tons ) of water and boats , with the gondolas themselves each weighing 50 tonnes ( 49 long tons ; 55 short tons ) . Care is taken to maintain the water levels on each side , thus balancing the weight on each arm . According to Archimedes ' principle , floating objects displace their own weight in water , so when the boat enters , the amount of water leaving the caisson weighs exactly the same as the boat . This is achieved by maintaining the water levels on each side to within ± 37 millimetres ( 1 @.@ 5 in ) using a site @-@ wide computer control system comprising water level sensors , automated sluices and pumps . It takes 22 @.@ 5 kilowatts ( 30 @.@ 2 hp ) to power ten hydraulic motors , which consume 1 @.@ 5 kilowatt @-@ hours ( 5 @.@ 4 MJ ) per half @-@ turn , roughly the same as boiling eight kettles of water . 
