Abstract: Controlling the velocity of a jet of foamed furnish leaving the slice of a pressurized headbox of a paper or a tissue making machine by measuring the density and pressure of a flow of foamed furnish provided by a pump to estimate an atmospheric pressure air content, measuring the pressure of the foamed furnish in the headbox, using the estimated atmospheric pressure air content and the measured pressure in the headbox to estimate the current velocity of said jet of foamed furnish, comparing the estimated current velocity with a target velocity and controlling the pump to move the estimated and target velocities closer to each other. An alternate embodiment controls jet velocity on the basis of comparing estimated and target headbox pressures.