THE CONSTITUTION OF NATURE. 21 



of the orbit. Her motion, and consequently her vis 

 viva, is then a minimum. The planet rounds the 

 curve, and begins its approach to the sun. In front it 

 has a store of tensions, which are gradually consumed, 

 an equivalent amount of vis viva being generated. 

 When nearest to the sun the motion, and consequently 

 the vis viva, reach a maximum. But here the available 

 tensions have been used up. The earth rounds this 

 portion of the curve and retreats from the sun. Tensions 

 are now stored up, but vis viva is lost, to be again 

 restored at the expense of the complementary force on 

 the opposite side of the curve. Thus beats the heart 

 of the universe, but without increase or diminution of 

 its total stock of force. 



I have thus far tried to steer clear amid confusion, 

 by fixing the mind of the reader upon things rather 

 than upon names. But good names are essential ; and 

 here, as yet, we are not provided with such. We have 

 had the force of gravity and living force two utterly 

 distinct things. We have had pulls and tensions ; 

 and we might have had the force of heat, the force of 

 light, the force of magnetism, or the force of electricity 

 all of which terms have been employed more or less 

 loosely by writers on physics. This confusion is happily 

 avoided by the introduction of the term ' energy,' which 

 embraces both tension and vis viva. Energy is pos- 

 sessed by bodies already in motion ; it is then actual, 

 and we agree to call it actual or dynamic energy. It 

 is our old vis viva. On the other hand, energy is 

 possible to bodies not in motion, but which, in virtue 

 of attraction or repulsion, possess a power of motion 

 which would realise itself if all hindrances were re- 

 moved. Looking, for example, at gravity ; a body on 

 the earth's surface in a position from which it cannot 

 fall to & lower one possesses no energy. It has neither 



