Dr. Taylor had said, as we have noticed, that the Copernican theory “rests on facts presently occurring before our eyes.” Mr. Youmans answers: “So does the Ptolemaic theory; and not only that, but, if the test is what occurs before our eyes, then the Ptolemaic theory is a thousand times stronger than the Copernican.” If this answer expresses the real opinion of Mr. Youmans, we must conclude that he alone, among physicists, is ignorant of the fact that terrestrial gravitation is modified by the centrifugal force due to the rotation of the earth, and that this fact is established by experiments which “occur before our eyes” when we make use of the pendulum in different latitudes. What shall we say of the aberration of light? Is not this phenomenon a proof of the movement of the earth? Or does it not “occur before our eyes”? Mr. Youmans may say that these facts do not occur before all eyes, but only before the eyes of scientific men. But Dr. Taylor had not maintained that all the facts connected with the Copernican theory occur before all eyes; and, on the other hand, Foucault’s pendulum, even though oscillating before unscientific eyes, makes visible to the dullest observer the shifting of the horizontal plane from its position at a rate proportional to the sine of the latitude of the place, thus showing to the eye the actual movement of our planet. It is true, therefore, that the Copernican theory “rests on facts presently occurring before our eyes.” But, if the Copernican theory is so obvious, “why,” asks Mr. Youmans, “did the astronomers of twenty centuries fail to discern it? Why could not the divines of Copernicus’ time see it when it was pointed out to them? And why could not Lord Bacon admit it a hundred years after Copernicus?” The _why_ is well known. The Copernican theory was at first nothing more than a hypothesis; and its truth, even after Kepler and Newton, was still in need of experimental confirmation. Had Lord Bacon or the divines of Copernicus’ time seen what we see with our eyes in Foucault’s experiment, there is little doubt that they would have recognized at last the truth of the new theory. But let this suffice about the certitude of the Copernican theory.