Note 66 — THE ECONOMICAL NATURE OF PHYSICAL INQUIRY.[60]

If it be objected, that in the case of perturbations of the velocity of rotation of the earth, we could be sensible of such perturbations, and being obliged to have some measure of time, we should resort to the period of vibration of the waves of sodium light,—all that this would show is that for practical reasons we should select that event which best served us as the simplest common measure of the others.