182 THE CORRELATION OP FORCE. 



magnetism, heat, and light, are modes of action fundament- 

 ally the same, in this sense, namely, that they are equiva- 

 lent to each other in certain definite proportions, and are 

 continually interchanging one into the other, in these pre- 

 cise proportions, in many of the operations of nature around 

 us, and in ai'tificial processes instituted by man. 



In the case of this magneto-electric machine we have an 

 example of all these forces transforming themselves one 

 into another, and that again into a third, till in this single 

 process, taking it from the beginning to the end, we run 

 through the whole round. 



To bring in the whole of the circuit, however, we must 

 begin with the sun ; the heat, or other radiation comprised 

 in his beams, is transformed in the leaves of the growing 

 plant into a latent force, or " potential energy," as it is some- 

 times termed, which is stored in the grain or hay eaten by 

 the horse, if we suppose the machine to be driven by horse 

 power, or in the wood which formed the coal, if we sup- 

 pose it carried by steam. This energy comes out into ac- 

 tion in the animal, developing itself as muscular force. In 

 moving the limbs of the animal and turning the machin- 

 ery, the muscular force becomes converted into mechanical 

 motion, which is conveyed by the pulleys and bands to the 

 magneto-electric axis. There this force is expended in 

 overcoming magnetic resistance, and is transformed by 

 this action into magnetic force in the soft iron bars, and, 

 as it disappears in this form, it reappears as electric force 

 in the environing wires. From these it passes by the con- 

 ducting wires to the charcoal points where it is first trans- 

 formed into heat to make incandescent the solid particles 

 of carbon, and is then transformed into light, to radiate 

 over the surface of the sea. The whole process constitutes 

 a very complete and striking example of the correlation, or 

 reciprocal relation of the various forms of force. 



