190 Prof. Henry on Electrical Induction 



clear day ; hence if the line of the telegraph passes over an 

 elevated mountain ridge, there will be continually, during 

 clear weather, a current from the more elevated to the lower 

 points of the conductor. 



A current may also be produced in a long level line, by the 

 precipitation of vapour in the form of fog at one end, while 

 the air remains clear at the other ; or by the existence of a 

 storm of rain or snow at any point along the line, while the 

 other parts of the wire are not subjected to the same influence. 



Currents of sufficient power to set in motion the marking 

 machine of the telegraph have been observed, which must have 

 been produced by some of these causes. In one case the ma- 

 chine spontaneously began to operate without the aid of the 

 battery, while a snow storm was falling at one end of the line, 

 nnd clear weather existed at the other. On another occasion 

 a continued stream of electricity was observed to pass between 

 two points at a break in the wire, presenting the appearance 

 of a gas-light almost extinguished. A constant effect of this 

 kind indicates a constant accession of electricity at one part of 

 the wire, and a constant discharge at the other. 



3. The natural electricity of the wire of the telegraph is 

 liable to be disturbed by the ordinary electrical induction of a 

 distant cloud. Suppose a thunder-cloud, driven by the wind 

 in such a direction as to cross one end of the line of the tele- 

 graph at the elevation, say of a mile; during the whole time 

 of the approach of the cloud to the point of its path directly 

 above the wire, the repulsion of the redundant electricity with 

 which it is charged would constantly drive more and more of 

 the natural electricity of the wire to the further end of the 

 line, and would thus give rise to a current. When the cloud 

 arrived at the point nearest to the wire, the current would 

 cease for a moment; and as the repulsion gradually diminished 

 by the receding of the cloud, the natural electricity of the 

 wire would gradually return to its normal state, giving rise to 

 a current in an opposite direcdon. If the cloud were driven 

 by the wind parallel to the line of the telegraph, a current 

 would be produced towards each end of the wire, and these 

 would constantly vary in intensity with the different positions 

 of the cloud. Although currents produced in this way may 

 be too feeble to set in motion the marking apparatus, yet they 

 may have sufficient power to influence the action of the cur- 

 rent of the battery so as to interfere with the perfect operation 

 of the machine. 



4. Powerful electrical currents are produced in the wires of 

 the telegraph by every flash of lightning which takes place 



