36 CROONIAN LECTURES 



The idea of the conservation of energy is, 

 that the sum of the actual and potential energy 

 in the world is constant. 



The force, or, as it is better called, the actual 

 energy, that can be put into the cannon ball, is 

 exactly equal to the latent or potential energy 

 in the gunpowder, and this depends on the 

 chemical force in the oxygen, hydrogen, and 

 carbon. The ball gains only what the powder 

 loses. As the potential energy, or tension, 

 decreases, the actual energy, or motion, in- 

 creases, and no loss or gain in the total amount 

 of motion and tension can occur. 



Whatever the form of motion may be, it can 

 only come either from some other form of 

 motion or from some form of tension. 



It has been Dr. Joule's great work to exhibit 

 experimentally the measurable relation between 

 heat and gravity. He has determined that a 

 certain amount of one kind of motion does 

 produce an equivalent quantity of another kind. 

 He proved that 772 pounds weight of matter 

 falling one foot gives rise to enough heat to 

 raise a pound of water one deg. Fahr. in 

 temperature. He called the unit of mechanical 

 motion one pound weight falling one foot = one 



