The scattering processes between the quasiparticles in spin- up superfluid
with the quasiparticles in spin-down normal fluid are added to the other
relevant scattering processes in the Boltzmann collision terms. The Boltzmann
equation has been solved exactly for temperatures just below T_c_1. The shear
viscosity component of the A_1- phase drops as C_1(1-T/T_c_1)^(1/2). The
numerical factor C_1 is in fairly good agreement with the experiments