the formulas for viscous drag force experienced by a porous sphere falling in a viscous fluid at low reynolds numbers which have been given by several authors are compared with one another and with the stokes formula. the problem in applying these formulas to real porous media is discussed. it is shown that in many cases of $left double quote$ standard $right double quote$ porous material (for example, sandor felt) the differences between the viscous drag force due to the stokes formula and others are insignificant, regardless of the assumptions made with regard to the boundary conditions and the filtration equation during the derivation of these formulas. a fibrous medium with very large porosity (for example, cotton) is given as an example of a porous medium in the case of which some of these formulas may have practical meaning. a new method of experimentally determining both the constant appearing in the beavers-joseph condition and the effective viscosity in the brinkman equation is given. 22 refs