104 MORRIS LOEB 



earlier investigators had actually understood these things 

 in part, but not clearly enough to explain them to others. 

 For example, there were several ideas of Galileo's about mo- 

 tion which Newton has made so clear to us that we now call 

 them Newton's Laws, namely: 



First Law: If no force acts upon a body in motion it con- 

 tinues to move uniformly in a straight line. Formerly men 

 imagined that a body must be continuously pushed to keep 

 it moving. Now we know that force is needed to stop it. 

 Ordinarily the force applied is friction, but if a body moves 

 with little friction over a smooth surface, as in skating, a 

 single push causes it to travel a long distance. 



Second Law: If force acts on a body it produces a change 

 of motion proportional to the force and in the same direction. 



Third Law: When a body exerts force upon another body 

 it experiences an equivalent reaction from the latter. 



The results of these laws were long known, but the prin- 

 ciple was not understood. When David killed Goliath, as we 

 read in the Bible, he shot a little stone at him from a sling. 

 He knew that if he swung the sling around, the pebble would 

 be held back by the sling that only allowed it to go a certain 

 distance from his hand, but as soon as it was released it did 

 not fly in a circle, but in a straight line. Here we have New- 

 ton's first law; and the sling represented the force which 

 caused the pebble to apply the second law and change its 

 direction. 



Again, the fact of gravitation was known long before New- 

 ton studied it. Many an object must have fallen upon a 

 philosopher's head before Newton was aroused by his mythical 

 apple, and we have experiments by Galileo that showed a 

 pretty clear notion of the law of gravitation as regards terres- 

 trial objects. That all objects are attracted equally to the 

 earth cannot be shown unless we remove obstructions from 



