20 HABIT AND INTELLIGENCE. [cHAP. 



press on its bed with a pressure proportionate to the 

 depth ; but no energy is due to this pressure, because there 

 is no space through which the water can fall. But to the 

 position of the water in a mill-pond a quantity of energy 

 is due, proportionate to the weight of the water multiplied 

 into the height through which it may be allowed to fall. 

 Potential Energy which is thus due to the possible action of a 

 and actual pj.jj^g^^y force^ that is not actually in action, is called 

 potential energy. When energy ceases to be potential, it 

 becomes actual. The potential energy of a raised weight 

 is proportionate, as I have just explained, to its mass mul- 

 tiplied into the height through which it is capable of 

 falling ; the actual energy of a moving body is propor- 

 tionate to its mass multiplied into the height through 

 which it must have fallen in order to acquire its velocity. 

 Their Potential and actual energy are in constant process of 



mutual transformation, the one into the other. The simplest case 



tvanstor- . 



mationin of this is the oscillation of a pendulum. Durmg the 

 of l^en°" descent of the pendulum-bob, a portion of energy propor- 

 dulum. tionate to the vertical height through which it descends is 

 transformed into the actual energy, or energy of motion, 

 due to its velocity at the lowest point of its stroke ; and 

 when the pendulum-bob rises to its former height on the 

 opposite side, this actual energy is transformed back into 

 potential energy again. Supposing no energy to be wasted 

 by friction, the pendulum-bob will always rise to exactly 

 the same height at the ends of the stroke, and will always 

 attain to exactly the same velocity in the middle : as re- 

 quired by the law of the conservation of energy, no energy 

 will be either lost or gained. This condition of the total 

 absence of friction is not perfectly attainable in our expe- 

 riments, but we can make a very near approximation to it. 

 As already stated, the energy wasted in friction is not de- 

 stroyed, but transformed into heat. This transformation of 

 energy thus effected in the oscillation of the pendulum is a 

 simple type on a small scale of what is constantly going 

 on in endless complexity and on the vastest scale through- 

 out the entire universe. 



1 For the meaning of the expression ^j-mar/z/orce see p. 32. 



