WITH reference to the very favourable notice in your last number of my edition of Deschanel's “Traite de Physique,” will you allow me to remark that my reason for rewriting the section on mass (§ 42) was that Deschanel, in accordance with what has been till recent years an almost universal custom, employs a variable unit of force, and, as depending upon this, a variable unit of mass, so that the number denoting the mass of one and the same body is diminished as the body is carried from the equator to the poles, and would increase up to infinity if the body fell to the centre of the earth.