IN regard to my estimate of viscous dissipation of oceanic energy in its bearing on the slowing of the earth's rotation (Proc. Roy. Soc., A, 93, p. 348), I quite assent to Mr. Jeffreys's position (NATURE, vol. xcix., p. 405, July 19, 1917) that it is still open to doubt whether the viscosity of the ocean causes a very appreciable part of the earth's retardation. The formula for calculating the rate of dissipation of internal energy by viscosity, in the absence of disturbing forces, from a knowledge of the surface currents alone, is not challenged. But, irrespective of his argument, this internal energy, especially that associated with the longer-period motions, will be in part energy of currents arising from temperature differences, and therefore supplied by the solar heat and not by the energy of the earth's rotation. The contribution from the shallower parts of the ocean may have more chance of falling under the latter head. In any case, the whole question can be discussed only when more is known as to the distribution of the oceanic currents. At present the only motion known at a considerable distance from land is the residual drift, and this only in a few places, chiefly where it exceeds one knot. But as this does not change with the tide, its energy is of thermal origin.