The mathematical derivation of relations between currents, gradients and dimensions in idealised containers leads to certain rules: (a) the maximum speed of currents in spouts and surface currents in single containers is proportional to the temperature gradient and the cube of the height of spout and tank, respectively. b) The transport of thermal energy is proportional to these. In real tanks a vertical temperature gradient and the ensuing differences in viscosity must be considered. Qualitatively, this consideration leads to the concept of a stagnant layer on the bottom of the tank and a smaller "efficient depth." The neutral zone between current and counter current, ideally placed at six tenths of the depth from the bottom, is placed higher in the real case.
