The caisson lock can however be powered, and the caisson lifted, by buoyancy alone. Pumping ballast water in and out of the caisson is enough to float it up or down the lock chamber. The weight of ballast water is approximately that of the canal barge being lifted. The boat lift though needs a mechanical lift system. In most vertical lift systems, this must also lift or lower the weight of the caisson and its water contents as well. This is substantially more than the barge alone, and the power to lift it is supplied mechanically. At the time of their development, the steam engine was in its infancy. Steam engines had been developed as water pumps, but not yet for the supply of mechanical power (see rotative steam engine). Given the engineering constraints of the time, lifting an enclosed caisson by buoyancy was more practical than driving a boat lift.