THE CONSTITUTION OF NATURE. 21 



gards her forces. Throughout all her regions she oscil- 

 lates from tension to vis viva, from vis viva to tension. 

 We have 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 conse- 

 quently 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 gradu- 

 ally consumed, an equivalent amount of vis viva 

 being generated. When nearest to the sun the mo- 

 tion, and consequently the vis viva, reach a maxi- 

 mum. 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 hap- 

 pily avoided by the introduction of the term ' energy/ 

 which embraces both tension 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 



