26 FRAGMENTS OF SCIENCE. 



the same play in the planetary system. The earth's orbit 

 is an ellipse, one of the foci of which is occupied by the 

 sun. Imagine the earth at the most distant part of the 

 orbit. Her motion, and consequently her vis viva, is then 

 a minimum. The planet rounds the curve, and begins to 

 approach the sun. In front it has a store of tensions, 

 which is gradually consumed, an equivalent amount of vis 

 viva being generated. When nearest to the sun the mo- 

 tion, and consequently the vis viva, is 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 em- 

 ployed more or less loosely by writers on phj'sics. This 

 confusion is happily avoided by the introduction of the 

 term " energy," embracing under it both tension and vis 

 viva. Energy is possessed by bodies already in motion ; 

 it is then actual, and Ave agree to call it actual or dynamic 

 i m /'</}/. 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 hinderances were removed. 

 Looking, for example, at gravity, a body on the earth's 

 iiiface in a position from which it cannot fall to a lower 



