106 THE REALITIES OF MODERN SCIENCE 



both cases the energy is available, for it is possible to 

 decrease the separation, since in both cases the stone has 

 some place to which it may fall. But if the mine shaft 

 is filled up so that the stone has no place to fall the 

 conditions are not different as to the energy which the 

 system possesses, for there is still the same separation 

 existing between the attracting bodies. As to avail- 

 ability, however, the conditions are different, for the 

 energy is no longer available. 



Consider now the energy of the system which the 

 water of the ocean forms with the earth. All the drops 

 which have the same separation from the center of 

 the earth form with the latter systems having the same 

 potential energy. The energy, however, of all these 

 systems is not available. A drop of water on a cliff 

 above the ocean possesses more potential energy, 

 for it may fall to the level of the surface of the sea. 

 Of its total potential energy a part is obviously avail- 

 able, and this part is the difference between the energy 

 it has on the cliff and the energy which any drop 

 has on the surface of the ocean. Since the amount of 

 the energy which is available depends upon the dis- 

 tance above sea level, we might speak of water at sea 

 level as having zero available potential energy and 

 measure the available potential energy of water at 

 higher levels by the distance. We might further speak 

 of sea level as being the level of zero gravitational 

 potential, allowing the word " energy " to be understood. 



Let us follow this idea a little further and see where 

 it leads. We did not call attention to the drops of 

 water but rather to their " locus," namely the surface 

 of the sea. We spoke, in other words, of a locus of 



