THE BALANCE OF NUTRITION 217 



balance of energy, however, it is important to have as definite 

 a conception as possible of what is meant by the term. It is 

 not altogether easy to give a simple general definition of energy, 

 but for the present purpose that given by Noyes l may be 

 adopted, viz., " That which gives rise to changes in the prop- 

 erties of bodies and to the power to produce such changes." 

 For the present purpose, however, the conception of energy may 

 be more readily apprehended from illustrations than from defi- 

 nitions. 



The subject may be conveniently approached from the side 

 of mechanics. A moving body is capable of producing certain 

 effects by virtue of its motion. The falling weight of a pile 

 driver, for example, forces the pile downward against the re- 

 sistance of the ground and at the same time produces heat at 

 the point of impact. The projectile fired from a sixteen inch 

 gun striking the side of the armored ship overcomes the cohesive 

 force of the armor plate and deforms or penetrates it, while 

 the blow also gives rise to an evolution of heat. The blows of 

 the blacksmith, if rapid and heavy enough, may raise the iron 

 on his anvil to a red heat. Accordingly, it is said that a moving 

 body possesses energy in the form called kinetic energy, or energy 

 of motion. 



If a body suspended above the earth is set free it falls to the 

 ground, and at the moment of contact with the earth possesses 

 a certain amount of kinetic energy which was generated during 

 its fall from something which was not energy of motion. This 

 other form of energy, which the body possessed previous to 

 its fall by virtue of its position, may be called gravitation energy. 

 The same relation is illustrated by a swinging pendulum. Dur- 

 ing the downward swing, the gravitation energy which it pos- 

 sessed when at its highest point is converted into kinetic energy, 

 while when it rises the kinetic energy which it possesses is re- 

 converted into gravitation energy. When we lift a weight we are 

 conscious of expending work, which is stored up as gravitation 

 energy, to be liberated again as kinetic energy when the weight 

 falls. 



304. Forms of energy. In general, whenever the rate of 

 motion of a body is increased (or, to use a more familiar if less 

 accurate expression, whenever motion is produced) it is to be 



1 General Principles of Physical Science, 1902. 



