In Part I, an attempt is made to determine the coefficient of viscosity of flowing lava. H. JEFFREYS already studied the flow of viscous liquid in an inclined channel of infinite breadth. Hitherto the formula deduced by him has been applied to the approximate determination of the coefficients of viscosity of flowing lava extruded in volcanic eruptions. Here, to make a more precise discussion, the flow of viscous liquid in an inclined channel of triangular section is studied: a formula to determine the coefficient of viscosity of the above flow is obtained by the method of dimensional analysis and the experimental coefficients contained in the formula are determined for various apex-angles of sections by the experiments with the flow of glycerine. As an example, the formula is applied to the lava flow extruded in the 1951 eruption of Mihara. In Part II, upwelling of lave from a central crater vent or a linear fissure is discussed as a Hagen-Poiseuille's flow. If the rate of lava flow, the height of lava accumulation corresponding to the residual pressure head at magma reservoir, the coefficient of viscosity of lava and the radius of the vent are known, the length of the vent from the crater to the reservoir can be estimated. This principle is applied to a few central eruptions, i.e. the dome formation on the summit of Tarumai in 1909, the overflow of lava from the Mihara crater in 1950 and the formation of lava fountain in the Sakurazima crater in 1958. The results obtained by these discussions are plausible and suggestive.
