MR. STREET is in error in attributing to me (NATURE, October 25, p. 145) the “contention that viscous action in a solid earth cannot be an appreciable cause of the slowing of its rotation.” I have never made any such assertion, it is opposed to my personal opinion, and in the present state of knowledge it is quite impossible either to affirm or to deny a statement of such definiteness. His criticisms of the law of viscosity used by me (M.N., R.A.S., vol. lxxvii., pp. 449–56) are confined to its precise mathematical form, to which I attach little importance, and do not touch the physical conceptions underlying it, which are fundamental. The mathematical argument was only a numerical illustration of the order of magnitude of the effects to be expected from these.