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For calculation of terminal velocities it is convenient to express the Reynolds' number, Re, of a moving sphere as a function of the dimensionless group ψRe 2, where ψ is the drag coefficient. The following equations have been fitted by the method of least squares to critically selected data from a number of experimenters: Re = ψRe 2/24 -0.00023363(ψRe 2)2 + 0.0000020154(ψRe 2)3 - 0.0000000069105(ψRe 2)4 for Re<4 or ψRe 2<140. This tends to Stokes' law for low values of Re. It is specially suited to calculation of the sedimentation of air-borne particles. The upper limit corresponds to a sphere weighing 1.5 μg. falling in the normal atmosphere, that is, one having a diameter of 142 μ for unit density. logRe=-1.29536+0.986 (logψRe 2)-0.046677 (logψRe 2)2+0.0011235 (logψRe 2)3 for 3<Re<10,000 or 100<ψRe 2<4.5.107. Correction for slip in gases should be applied to Stokes' law by the following expression, based on the best results available: 1 + l/a[1.257 + 0.400exp(-1.10a/l)], where the mean free path l is given by η/0.499σc. This conveniently transforms to the following for the sedimentation of particles in air at pressure p cm. mercury 1 + l/pa[6.32.10-4 + 2.01.10-4exp(-2190ap)]

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