hypotheses, for he did. By making the hypothesis that the earth attracted bodies according to the inverse square of the distance, and calculating whether the fall of the moon toward the earth was of the amount required by this supposition, he was able to predicate the law of gravitation, and by the calculation that the orbit of a body attracted according to this law would be an ellipse he was able to explain the law of planetary motion discovered by Kepler. It is difficult to see how Kepler could have arrived at his law of elliptic motion if he had not first guessed that the orbits of the planets were circles or conic sections, and then verified it by comparison with the observations on their apparent positions.
The chief test of the success of a scientific hypothesis and of a train of reasoning therefrom is found in the ability to make predictions. Of this probably the most striking example in all science is the law of gravitation just alluded to. All the observations of the last two hundred } r ears have only resulted in confirming Newton's conclusion, while the accuracy of astronomical prediction exceeds that of any part of science. Such is an example of scientific faith. Another famous example is Hamilton's famous discovery of conical refraction. On looking through a piece of Iceland spar at an object one sees it doubled. The laws of this double refraction had been thoroughly described by Fresnel, who related them to a certain geometrical surface invented by him. By the study of the geometry of this surface, which was found to possess two singular points, Hamilton showed that on looking through the crystal in a certain direction at a point, one would see not two points but a whole continuous circle. This experiment was made by Hamilton's friend Lloyd, who saw the circle, confirming in the most brilliant manner the wonderful imagination of Hamilton, who saw in his mind's eye what never yet man had seen.
Another example of successful hypothesis is afforded by the kinetic theory of gases, which explains the properties of gases by the hypothesis that they consist of extremely small particles in very rapid motion, which by striking each other and the walls of the containing vessel by the impacts give rise to the pressure which the gas exerts. On this theory the friction which a current of gas exerts on a portion moving less rapidly, thereby setting it in motion, is of the same nature as the action that a crowd of men jumping from a moving train to a car upon a parallel track would have, their momentum tending to set the second car on which they alighted in motion. One of the remarkable predictions of this theory is the result of Maxwell that the viscosity of the gas is independent of its density, a result which has been well verified by experiment.
As a final example of scientific thought, let me briefly refer to the hypothesis of the luminiferous ether. About one hundred years ago,
