Concerning the manufacture of sheet glass by the float process using, as a glass melting and refining tank furnace, a common regenerative furnace having two oppositely arranged sets of burners which are periodically alternately operated with a temporary interruption of firing operation at each reversal of firing, the invention relates to the control of the rate of feed of molten glass from the tank furnace onto a molten metal bath through a canal provided with a control tweel which can be moved upward and downward. At each reversal of firing in the furnace the tweel is temporarily lifted so as to compensate for a decrease in the volumetric rate of flow of molten glass through the canal caused by the temporary interruption of firing operation.