A HISTORY OF SCIENCE 



served law of gravitation. Indeed, it is inconceivable 

 that he can have imagined a place on the earth where 

 this law does not operate. On the other hand, of 

 course, he never grasped the conception of the opera- 

 tion of this law beyond the close proximity of the 

 earth. To extend the reach of gravitation out to the 

 moon and to the stars, including within its compass 

 every particle of matter in the universe, was the work 

 of Newton, as we shall see in due course. Meantime 

 we shall better understand that work if we recall that 

 the mere local fact of terrestrial gravitation has been 

 the familiar knowledge of all generations of men. It 

 may further help to connect us in sympathy with our 

 primeval ancestor if we recall that in the attempt to 

 explain this fact of terrestrial gravitation Newton 

 made no advance, and we of to-day are scarcely more 

 enlightened than the man of the Stone Age. Like 

 the man of the Stone Age, we know that an arrow 

 shot into the sky falls back to the earth. We can 

 calculate, as he could not do, the arc it will de- 

 scribe and the exact speed of its fall ; but as to why 

 it returns to earth at all, the greatest philosopher 

 of to-day is almost as much in the dark as was 

 the first primitive bowman that ever made the ex- 

 periment. 



Other physical facts going to make up an elementary 

 science of mechanics, that were demonstratively known 

 to prehistoric man, were such as these: the rigidity of 

 solids and the mobility of liquids ; the fact that changes 

 of temperature transform solids to liquids and vice 

 versa that heat, for example, melts copper and even 

 iron, and that cold congeals water; and the fact that 



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