All phenomena occurring in a glass tank furnace, i.e., melting the batch, refining and conditioning the molten glass, and other chemical and physical processes are connected inseparably with the convection current of molten glass. It may, therefore, be said that the effort to understand these complicated processes cannot be successful without learning the fundamental characteristics or trends of convection current.From this view point the authors carried out some series of model experiments using glycerin in place of the molten glass. In a box having a simple rectangular shape the glycerin was heated uniformly from above and cooled through side and bottom walls of the box. Temperature distributions in the fluid were measured with Cu-Constantan thermocopules whose junctions were fixed at twenty points in the vessel, and the flow patterns were observed from the photographic records of the tracks of tracer particles moving in the illuminated area of the liquid. Various factors, for example, heat input from the top surface, arrangement of heaters, viscosity of the fluid, heat transfer coefficient of the wall, and depth of the liquid were changed successively for the purpose of understanding the influence of these factors on the convection current and the temperature distribution. From the results obtained the authors pointed out some important characteristics of the convection current. Some of them are digested as follows:(1) The change of heat input causes the proportional change of temperature gradients both in vertical and horizontal directions as well as the amount of heat discharged from unit surface area of side and bottom walls, but it has not so much effect on the flow velocity of convection current.(2) Increase of heat input indicates a tendency to shift upward the circulating current and consequently the fluid near the bottom remains stagnant. These tendencies were also observed when the viscosity of fluid was decreased and when the heaters concentrated at the center of the vessel.(3) Decreases of the overall heat transfer coefficient of wall, depth of the fluid and increase of viscosity of the fluid result in a reduction in the velocity of convection current. Generally it could be pointed out that the change of the flow velocity is not so large as one would expect at first. This may be regarded as self-controlling effect of the convection current.