SCIENTIFIC FAITH AND WORKS 115 



hypotheses, for he did. By making the hypothesis that the earth at- 

 tracted bodies according to the inverse square of the distance, and cal- 

 culating 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 hun- 

 dred } 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 ex- 

 ample is Hamilton's famous discovery of conical refraction. On look- 

 ing 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 Hamil- 

 ton'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 predic- 

 tions 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, 



