72 LIGHT AND COLORS PRODUCED BY HEAT. 



arranged the plate of hot iron, E, representing the 

 torrid region of the earth. With these extremes 

 of temperature, the excitation is intensified suffi- 

 ciently to produce bright electric flashes, and 

 other electric phenomena, corresponding with 

 those developed by electrical machines. By 

 transmitting this electric excitation through void 

 space in a bell-glass, the flashings of the aurora 

 borealis are represented, as before stated. Be- 

 tween the polar seas composed of salt water and 

 ice, and the torrid zone heated by sunshine, there 

 are north and south currents, above the terrestrial 

 currents circulating from east to west, which ap- 

 pear occasionally as " the northern lights " or 

 aurora, crossing the terrestrial currents, and con- 

 sequently disturbing magnetic needles on the 

 earth's surface beneath them. Every local dis- 

 turbance of temperature of the earth's surface, 

 by inducing excitation of currents transverse to 

 those regularly circulating from east to west 

 about the earth, disturbs and agitates the com- 

 pass needle. 



The following experiment is of practical inter- 

 est. An experimenter arranged a sheet of copper 

 and of iron, with one of the ends of each in con- 

 tact, within the flue from a furnace. The other 

 end of each plate he connected by a wire, extended 

 to an office one hundred and fifty feet distant, 

 making a circuit about a galvanometer needle. 

 He states that he was thus enabled to know the 



