No. 6. DEPARTMENT OF AGRICULTURE. 673 



kiiietk' lucrgy or energy of inoliou is used to do work upon the pen- 

 dulum; at the other extremity o! the swing all the aetuul energy 

 has been converted back into potential energy and so the alternate 

 conversion goes on as long as the peuduhim swings. In the case 

 of the mill, the water in the reservoir contains potential energy due 

 to its position; as long as the water stands in the reservoir, it does 

 not actually do any work, but it contains the possibility of doing 

 work. When the gate is opened and it begins to set the wheel in 

 motion, this potential energy, or energy of position, becomes con- 

 verted iijto energy of motion or kinetic energy. In the case of the 

 burning coal under the boiler, the coal itself, as long as it does not 

 burn, does no work, but it has stored up within it the possibility of 

 doing work; that is, it contains potential energy. When it burns, 

 this potential energy or latent energy becomes actual or kinetic in 

 the form of heat and this heat in its turn gives rise to the various 

 other forms of energy such as that of the expanding steam, the mov- 

 ing engine, the electric current, the moving trolley, etc. 



What I have just said suggests another important fact regarding 

 energy, namely, its capacity for taking varying forms. The case of 

 the electric plant is perhaps as good an illustration as any. Here 

 we ordinarily start either with the potential energy of coal or the 

 potential energy of water in an elevated reservoir. The potential 

 energy of the coal by the process of combustion is converted into 

 the actual or kinetic energy of heat; some of this heat escapes, but 

 a part of it is used in forcing the particles of the water in the boiler 

 further apart and converting them into steam. The steam thus 

 produced, therefore, contains potential energy; that is, a part of the 

 potential energy of the coal has now taken the form of the potential 

 energy of steam. In the engine this steam expands and moves the 

 piston from one end of the cylinder to the other, thus setting the 

 engine in motion; in this process the potential energy of the steam, 

 derived from the heat of the fire, is again converted into the actual 

 or kinetic form of visible motion. In the dyuamo, this energy is 

 changed from the form of motion to that of electricilv and in this 

 form passes out over the conducting wires. Following it up still 

 further, a part of this energy is perhaps used in driving a trolley 

 car or a motor acd thus finally takes the form of visible motion; 

 another part of it is perhaps employed in an electric lamp, produc- 

 ing heat and light, while still a third may be used to produce chemi- 

 cal changes in the vats of an electro {)later, while by proper appara- 

 tus a portion of the heat radiating from the electric lamp may be 

 re-converted into electricity and start on another cycle of change. 

 Still further, the liglit from the electric lam{) falling on the green 

 leaves of a plant may take the place of sunlight and expend its en- 

 ergy in building up the tissues of the plant, finallv producing some- 

 43_r)— 1901 



