THE CONSTITUTION OP NATURE. 19 



and F there is a quantity of unused tensions; beyond 

 that point the tensions have been all consumed, but 

 we have in their place an equivalent quantity of vis 

 viva. After D has passed any point, the tension pre- 

 viously in store at that point disappears, but not with- 

 out having added, during the infinitely small duration 

 of its action, a due amount of motion to that previously 

 possessed by D. The nearer D approaches to F, the 

 smaller is the sum of the tensions remaining, but the 

 greater is the vis viva; the farther D is from F, the 

 greater is the sum of the unconsumed tensions, and 

 the less is the living force. Now the principle of con- 

 servation affirms not the constancy of the value of the 

 tensions of gravity, nor yet the constancy of the vis 

 viva, taken separately, but the absolute constancy of 

 the value of both taken together. At the beginning 

 the vis viva was zero, and the tension area was a maxi- 

 mum; close to F the vis viva is a maximum, while the 

 tension area is zero. At every other point, the work- 

 producing power of the particle D consists in part of 

 vis viva, and in part of tensions. 



If gravity, instead of being attraction, were repul- 

 sion, then, with the particles in contact, the sum of the 

 tensions between D and F would be a maximum, and the 

 vis viva zero. If, in obedience to the repulsion, D 

 moved away from F, vis viva would be generated; and 

 the farther D retreated from F the greater would be its 

 vis viva, and the less the amount of tension still avail- 

 able for producing motion. Taking repulsion as well 

 as attraction into account, the principle of the conserva- 

 tion of force affirms that the mechanical value of the 

 tensions and vires vivce of the material universe, so far 

 as we know it, is a constant quantity. The universe, in 

 short, possesses two kinds of property which are mutu- 

 ally convertible. The diminution of either carries with 



