88 GREAT MEN OF SCIENCE 



matter, could certainly not have been foreseen; Newton had 

 rather to provide the proof by successfully developing his 

 mechanics of the heavens, and this meant, even without the 

 simultaneous discovery of the general law of gravitation, in 

 itself a great and new insight into the nature of the world. 

 It may be remarked by the way, that the universal validity 

 of the laws of motion is better assured than the action of 

 gravitation over space of any extent. We know from the 

 motion of the farthest double stars, which can be determined 

 by Doppler's principle, that the phenomena of motion there 

 taking place are of the same kind as, and take place under 

 forces similar to, those in our solar system, but we have no 

 definite assurance that the gravitational forces between our 

 sun and stars at any distance act according to the square of 

 the distance; the case might also be otherwise. 



Newton's three laws of motion are exhaustive even to-day 

 for all known phenomena of the motion of matter, and in part 

 even further; and in so far, dynamics (the doctrine of the 

 motion of matter) reached with Newton its full and funda- 

 mental development. 



By the strict application of the laws of motion to the 

 heavenly bodies, Newton arrived at a knowledge of the forces 

 which act between them. The fact that such forces must 

 exist, say between earth and moon, was shown by the curva- 

 ture of the moon's path; without the action of force, it would 

 only possess uniform motion in a straight line, according to 

 the law of inertia. The case of the moon is the same as that 

 of a body projected horizontally near the earth; this body like- 

 wise describes a path curved in the direction of the force 

 acting upon it. If the velocity of the body projected hori- 

 zontally could be made sufficiently great, it would no longer 

 fall to the ground, but travel around the earth like the moon, 

 which is drawn towards the centre of the earth by the effect 

 of gravity.^ Forces must also be present in the case of all 



^ Principia, lib. \, discussion of definition 5. 



