en. xxxv. SCHWABE AND SABINE. 353 



than the rest, a current of electricity was caused. This he 

 called thermo-electricity ', or electricity caused by heat, and it 

 gives us another beautiful instance of the transformation of 

 energy. We saw in chapter xxxiv. that heat is altered mo- 

 tion and motion altered heat ; since then we have learnt that 

 electricity produces magnetism and magnetism electricity, 

 and now we have heat in its turn causing electricity, while 

 we know from the electric spark that electricity produces 

 both light and heat. In all these changes we see additional 

 proof that force or energy cannot be destroyed, but only 

 exhibits itself in different ways. 



But to return to the magnet. Seebeck's experiment sug- 

 gests a possible answer to the direction of the magnetic 

 needle to the north. Our globe is composed of different 

 metals and earths, and is always turning round from west to 

 east, so that one part after another comes under the heat of 

 the sun, and is made hotter than the rest. Therefore, since 

 heat produces electricity, it has been suggested that this may 

 cause the electric currents to flow round from east to west, 

 as they did through the metals in Seebeck's stirrup, thus 

 inducing magnetic currents to flow round from north to 

 south. Our knowledge on this subject is, however, very 

 imperfect, and the observations of which we shall next speak 

 seem to point to some closer connection between the sun 

 itself and magnetic currents. 



Periodicity of the Spots on the Sun, and their Effect on 

 the Earth's Magnetism, discovered by Schwabe and Sabine, 

 1825-1859. It was mentioned at p. 92 that Galileo and 

 other astronomers of the seventeenth century first observed 

 that from time to time dark spots appear on the face of the 

 sun. These spots were much studied by the astronomers 

 who came after Galileo; but Sir William Herschel was the 



