ENERGY IN 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 Diffe>ent Kinds of Mechanical 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 =yj-) or as vis viva (half the product of the mass 



ini>^ 

 into the square of the velocity U = ^• 



^ We therefore notice that the measures of force and work 

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

 given by w — mg. On the other hand, we have by the laws of 



IS 2 S 



falling bodies v=gt; s = \f[t'^; whence,^— -, ; «/ = /« — ^;or, if 



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



time, we have F = MLT~^, 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 : — 



imp' 

 Work = / X s= = M L2 T--. 



