REFERRING as briefly as possible to Prof. McAulay's letter on page 588, I contend (1) that no complex reasoning is necessary to show that the commutative and associative laws hold for the symbols of units or (better term) standards; a simple method was indicated in vol. xxxviii. p. 281; and (2) that meaningless things like the square root of a foot do not appear in any correct final result. It is true that the square of an hour is meaningless too, but the apparent occurrence of such a thing, in acceleration for instance, is otherwise explicable; for velocity is a real and simple physical quantity.