ENERGY IX GENERAL. 79 



play of natural forces evaporation under the action 

 of the sun, the formation of clouds, transport by winds, 

 etc. And we here again see that a complex energy 

 has been transformed, in its first phenomenal con- 

 dition, into potential energy, and that this potential 

 energy is always expended in the second phase with- 

 out loss or gain. 



The Different Kinds of MecJianical Energy ; of 

 Motion, of Position. There are as many forms of 

 energy as there are distinct categories of phenomena 

 or of varieties in these categories. Physicists dis- 

 tinguish between two kinds of mechanical energy 

 energy of motion and energy of position. The energy 

 of position presents several variants energy of dis- 

 tance, which corresponds to force : of this we have just 

 spoken ; energy of surface, which corresponds to par- 

 ticular phenomena of surface tension ; and energy of 

 volume which corresponds to the phenomena of pressure 

 Energy of motion, kinetic energy, is measured in two 

 ways: as work (the product of force and displacement, 

 W =fs) or as vis viva (half the product of the mass 



into the square of the velocity U = ^-.* 



1 We therefore notice that the measures of force and work 

 bring in mass, space, and time. The typical force, weight, is 

 given by ^.=mg. On the other hand, we have by the laws of 



falling bodies v=gt ; s=\gf*', whence g= ^-, v/=m ^-^-;OT,\{ 



F be the force, M the mass, L the space described, and T the 

 time, we have F = MLT- S , which expresses what are called the 

 dimensions of the force that is to say, the magnitudes with their 

 degree, which enter into its expression. We may thus easily 

 obtain the dimensions of work : 



