The work done by the buoyancy force in the Boussinesq approximation is customarily interpreted as corresponding to the conversion between potential and kinetic energy. Studies on the energy balance of the thermohaline circulation in the oceans have been conducted on the basis of this interpretation; the circulation, as a result, is believed to revolve by consuming the potential energy of the oceans. This document shows, however, that the work done by the buoyancy force in the approximation corresponds to the conversion between internal and kinetic energy; the internal energy of the oceans is thus shown to be the source of the kinetic energy of the thermohaline circulation.