ELECTRICITY AT THE WORLD'S FAIR. 39 



The failure of tlie theory proposed in the case of the earth and 

 moon is no less striking when applied to Mars, Jupiter, or any- 

 other planet. In every instance the position of the satellite as- 

 sumed to afford permanent moonlight would be one of instability. 

 This striking fact renders the oversight of Laplace the more re- 

 markable. It may be stated, however, that by the arrangement 

 of several moons about the same planet almost, if not entirely, 

 perpetual moonlight might be possible. The system of Jupiter 

 and his moons furnishes a clear illustration. 



In conclusion, we have seen, then, that where one of the great- 

 est mathematicians of all time suggested a change a so-called 

 improvement in the system of the world the modification would 

 have left us without tides, or, worse still, the earth in the system 

 proposed would have lost control of her satellite, and we would 

 not only have been deprived of moonlight, but also of the moon 

 itself. 



ELECTRICITY AT THE WORLD'S FAIR. 



By CHARLES M. LUNGEEN. 

 II. 



THE facility with which a high temperature may be obtained 

 with electricity, and the heat controlled and located just 

 where it is wanted, makes this agent peculiarly well adapted to 

 the heating of metals for welding and forging purposes. This 

 was early recognized by Prof. Elihu Thomson, to Avhom the de- 

 velopment of the art is chiefly due, and who has devised a great 

 variety of apparatus capable of performing all classes of work, 

 from the simple welding of two wires to the making of large and 

 complicated joints. 



The principle involved is very simple. If a current be passed 

 through a rod or wire, heat will be developed in it if the current 

 be of sufficient volume. If this circuit, instead of being formed 

 of a continuous conductor, be a broken one, such as would be 

 furnished by two rods whose ends abut, the heat will be developed 

 first at the surface of con' act, as this is the point of greatest re- 

 sistance, and then spread along the rods. And if, while the rods 

 are in a heated condition, they be pressed together, they will be- 

 come strongly united and form a perfect joint. On account of 

 the radiation of heat from the surface and the cooling effect of the 

 air, the rods become hotter at the center than at the surface, which 

 is the reverse of what happens with a forge-heated bar, where the 

 heating begins at the outside and gradually extends to the inte- 

 rior. This feature of the electric welding process has an impor- 

 tant advantage in producing a firmer and more perfect joint, and 



