The effect of binary encounters is examined by restricting considerations to a sphere of finite radius R, instead of an infinite one assumed hitherto. The sphere is centered at an assigned test-particle with velocity ∇o. The corresponding modified assumption is that particles lying outside the sphere are scattered randomly with an average number density n and their velocity distribution is Maxwellian with an r.m.s. of in the space velocity. As for the particles' masses the same M is presumed.