THE article of Prof. Schuster's in your number of March 15, entitled “A Plea for Absolute Motion,” is very interesting, but I think there are several conceptions contained therein which will not bear analysis. Partly in reference to his article, therefore, but also because the question is such an important one, I think it may be well to consider as definitely as may be what direct observational or experimental evidence we have for a zero point of motion belonging to space alone, and to which all motions of material bodies may be referred.