90 THE PATH OF SCIENCE 



be even more original than the concept of mass. Newton 

 showed that if a given mass is attracted toward the earth with 

 a certain force corresponding to its weight, the earth must be 

 attracted toward the mass with the same force. When a gun 

 is fired, for example, the shot is violently accelerated forward, 

 but the gun is accelerated, and not too gently, backwards. 

 Newton said: "Reaction is always equal and opposite to 

 action; that is to say, the actions of two bodies upon each 

 other are always equal and directly opposite." If these laws 

 of motion had been applied only to the observation of par- 

 ticles upon the earth, they would have produced much less 

 effect upon the minds of men than was actually the case. 

 Newton applied them to the movements of the heavenly 

 bodies and to the explanation of the law^s which Kepler had 

 deduced from those movements. 



Johannes Kepler was the successor of Tycho Brahe, the 

 great Danish astronomer. At Uranienborg in Denmark, 

 Tycho Brahe built the first modern observatory, where by 

 means of quadrants he observed the positions of stars and 

 planets. It must be remembered that this was before the 

 invention of the telescope, and these quadrants were the an- 

 cestors of the transit instruments, fixed in meridian, with 

 which the time of passage of an object across the meridian 

 can be observed. With these quadrants equipped with sights, 

 Brahe made the most astonishingly accurate observations of 

 the positions of seven hundred and seventy-seven stars. 



The cosmic theory which Brahe used was a modification of 

 Ptolemy's theory. He did not adopt the heliocentric Coperni- 

 can theory because he saw that if the positions of the stars 

 were observed six months apart, and Copernicus were right, 

 the earth would have moved in its passage around the sun a 

 prodigious distance in those six months and the stars should 

 show displacement relative to each other. Brahe's observa- 

 tions, made with the utmost precision of which he was 

 capable, showed no such movement; and he concluded that 

 the earth must be at rest. This is one of the many cases to 

 be found in the history of science where an effect which really 



