MAGNETIC TELEGRAPH. 21 



that the positions of all the trains may at any 

 time be known, and protection against collisions 

 assured. To do this the metallic roof of the car 

 is used as an inductive plate, just as was the house 

 roof fifty years ago; and a wire passes from 

 it through a signaling coil to the ground by way 

 of the metal wheels and track. Near the roof 

 outside, an electric wire is stretched on poles, 

 through which electric flashes, like lightning, are 

 sent, and they set up by induction in the roof 

 electric currents similar to those passing over the 

 wire, which are read as signals by the observer; 

 and, conversely, signals are sent from the roof to 

 the wire by induction coils in the car. The ex- 

 perimental demonstration in Princeton has not been 

 lost, though buried so long, and to-day it throws 

 another safeguard around our lives.* 



The first electro-magnetic engine for generating 

 power was made by Henry, at Albany, in 1831.f 

 His clear mind was not deluded into the belief 

 that such an apparatus could supersede the steam 

 engine as an economical motor, and he warned the 

 world against that delusion. Zinc, as fuel in a bat- 

 tery, is more costly than coal in a furnace. Still, 

 he saw and said that in exceptional cases it might 

 be useful; a result now coming to pass, dependent, 

 however, upon the discovery of magneto electricity 



* See Appendix, Note D. 

 t See Appendix, Note E. 



