£6 FRAGMENTS OF SCIENCE 



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 intro- 

 duction of the term "energy," which embraces both ten- 

 sion and vis viva. Energy is possessed 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 realize itself if all hindrances were 

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

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

 to a lower one possesses no energy. It has neither mo- 

 tion nor power of motion. But the same body suspended 



