GALILEO. 263 



orbit, and was forced to conclude that some inherent energy, perhaps 

 an angel, perpetually acted to keep it moving. Galileo's law announces 

 that if it is once set in motion it will continue to move until some 

 impressed and extraneous force causes it to stop. Motion is as ' nat- 

 ural ' as rest, therefore. It happens that on the earth there is no body 

 moving under the action of no force. Falling bodies, projectiles, and 

 the like, are perpetually attracted by the earth's mass, continually re- 

 tarded by the resistance of the air. It required abstract philosophical 

 reasoning to determine how such bodies would move were the impressed 

 forces removed, and it is this reasoning that is Galileo's chief title to 

 enduring fame. In this respect he changed the whole thought of the 

 world. His telescopic discoveries might have been made by others. 

 There was no man in Italy besides himself who could have founded the 

 new science of mechanics. 



Newton added two laws of motion which read: The alternation of motion 

 is ever proportional to the moving force impressed and is made in the right line 

 in which that force acts. To every action there is always opposed an equal 

 reaction ; or the mutual actions of two bodies upon each other are always equal, 

 and in opposite directions. His law of universal gravitation is: Every particle 

 of matter in the universe attracts every other particle with a force directly as 

 the masses of the two particles, and inversely as the squares of the distance that 

 separates them. 



With these laws as bases of calculation the question may be answered: 

 What orbit will a planet describe about the sun? The answer is, a conic sec- 

 tion, an ellipse for example. Again: What will be the law of the motion of 

 each planet in its ellipse? The answer is: Its radius- vector will sweep over 

 equal areas in equal times. Again: In a system of such planets, how will 

 their orbits be related? The answer is: The squares of their periodic times 

 will be proportional to the cubes of their mean distances from the sun. From 

 the single law of gravitation the three laws of Kepler (as above) necessarily 

 follow. Kepler's laws were empirical and were not complete until Newton's 

 discoveries. This brief note explains the logical outcome of Kepler's and of 

 Galileo's researches. 



The new laws of motion were expounded to the students of Pisa 

 with fire and eloquence. The theories of Aristotle and of his followers 

 were treated with scorn and contempt. In his zeal for the truth Gali- 

 leo branded the scientific errors of his colleagues almost as if they had 

 been moral faults. His asperity laid the foundation of enmities that 

 followed him throughout the whole of his life and led to his ruin. It 

 is as true of Galileo as of Eoger Bacon that his character was his fate. 



How the strictures of Galileo were received by the exasperated 

 Aristotelians may be imagined. If his experiments were to be believed, 

 the words of Aristotle were false. If the philosophy of Aristotle were 

 false in one part it might be false in all. The experiments must there- 

 fore be denied, and their author discredited. It is recorded that the 

 experiments were, in fact, denied. The facts of experience were met 



