82 THE PROGRESS OF SCIENCE n 



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 ancient Greeks, and 

 remained dominant until the age of Galileo. The 

 publication of the " Principia " of Newton, in 

 1686-7, marks the epoch at which the progress 

 of mechanical physics had effected a complete 

 revolution of thought on these subjects. By this 

 time, it had been made clear that the old general- 

 isations were either incomplete or totally erro- 

 neous ; that a body, once set in motion, will 

 continue to move in a straight line for any con- 

 ceivable time or distance, unless it is interfered 

 with ; that any change of motion is proportional 

 to the " force " which causes it, and takes place 

 in the direction in which that " force " is exert < -d : 

 and that, when a body in motion acts as a cause 

 of motion on another, the latter gains as much as 

 the former loses, and vice versd. It is to be noted, 

 however, that while, in contradistinction to the 

 ancient idea of the inherent tendency to motion 

 of bodies, the absence of any such spontaneous 

 power of motion was accepted as a physical axiom 

 by the moderns, the old conception virtually 

 maintained itself in a new shape. For, in spite 

 of Newton's well-known warning against the 

 " absurdity " of supposing that one body can act 



