IN THE LAST HALF-CENTUKY. 65 



of the thrower. So that, the weight of 

 the stone remaining the same, it looks as 

 if the motive power communicated to it 

 were measured by the distance to which 

 the stone travels — as if, in other words, 

 the power needed to send it a hundred 

 yards was twice as great as that needed 

 to send it fifty yards. These, apparently 

 obvious, conclusions from the everyday 

 appearances of rest and motion fairly 

 represent the state of opinion upon the 

 subject which prevailed among the an- 

 cient Greeks, and remained dominant 

 until the age of Galileo. The publica- 

 tion of the ' Principia ' of Newton, in 

 1686-7, marks the epoch at which the 

 progress of mechanical physics had ef- 

 fected a complete revolution of thought 

 on these subjects. By this time, it had 

 been made clear that the old generalisa- 

 tions were either incomplete or totally 

 erroneous; that a body, once set in mo- 

 tion, will continue to move in a straight 



