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move round the sun along curved lines, from whence 

 we can perceive that they lie under the uninterrupted 

 action of some force. Founding our argument upon Kep- 

 ler's law, it is possible to prove, by a purely mathema- 

 tical method, without any hypothesis, that this force acts 

 in the direction of a line uniting the centre of the 

 planets and the centre of the sun. At the same time 

 this force evidently attracts the planet towards the 

 sun, and does not repulse it, since in the latter case 

 the planetary orbit would present a convexity to- 

 ward the sun and not a concavity as in fact. The 

 same remark applies to the satellites ; between the planet 

 and its satellite exists a force tending to approximate 

 these bodies. There are also stars revolving round one 

 another, and consequently these also are not excluded 

 from this general law. The primary cause, in conse- 

 quence of which the heavenly bodies are attracted to one 

 another, is called Universal Gravitation. 



By Kepler's law, aided with mathematical reasoning, 

 we can prove that universal gravitation is 1) in direct 

 proportion to the mass of the acting bodies, and 2) in 

 inverse proportion to the square of their distance from 

 one another. These two laws are called Newton's laws, 

 from the name of their discoverer, the celebrated Eng- 

 lish scientist of the seventeenth century - - Sir Isaac 

 Newton. 



The story of the discover}^ of these laws is simply 

 told. The young Englishman, Newton, was sitting one 

 day on a bench in his garden when, suddenly, an apple 

 fell at his feet, and, at the same moment, the moon emer- 

 ged from behind the clouds. Such very ordinary and 

 natural phenomena might easily have passed without 

 remark, but Newton, struck by them, began to ask him- 

 self why the apple should necessarily fall to the ground, 

 and not fly off into space, and why the moon steadily 

 accompanies the earth, and no other globe. An immense 

 number of experiments were tried with falling bodies 

 of various sorts, and, at last, the laws of gravitation were 



