138 THE REALITIES OF MODERN SCIENCE 



vertical height and not upon the slope of the plane. 

 As Mach 1 points out there is contained in Galileo's 

 assumption "the uncontradictory apprehension and 

 recognition of the fact that heavy bodies do not pos- 

 sess the tendency to rise but only to fall." The ideas 

 involved, however, are really those of energy, for 

 obviously the energy acquired in falling must be 

 equal in a conservative system to the gain in energy 

 which would be consequent to a reversal of the dis- 

 placement. 



1 Mach, "Science of Mechanics," pp. 134 et seq. 



