28 Prof. Henry on Electrical Induction 
ally produced in this way; I merely wish to convey the idea 
that known principles of electrical. action might, under certain 
circumstances, lead us to anticipate such results. 
2. The state of the wire may be disturbed by the conduction | 
of a current of electricity from one portion of space to another, 
Without the presence of a thunder-cloud ; and this will happen in 
case of a long line, when the electrical condition of the atmes- 
phere which surrounds the wire at one place is different from that 
at another. Now it is well known that a mere difference in ele- 
vation is attended with a change in the electrical state of the at- 
mosphere. A conductor, elevated by ig of a kite, gives 
sparks of positive electricity in a perfectly 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 along level line, by wpe 
precipitation of vapor 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 ma- 
chine 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 bat- 
tery, while a snow storm was falling at one end of the line, and 
clear weather existed at the other. On another occasion, a con- 
tinued stream of electricity was observed to pass between two 
pune at a break in the wire, presenting the appearance of a gas- 
ght almost extinguished, A constant effect of this kind indi- 
cates 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 telegraph 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 farther 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 re- 
pulsion gradually diminished by the receding of the cloud, the 
natural electricity of the wire would gradually return to its nor- 
mal state, giving rise to a current in an opposite direction. If the 
were driven by the wind parallel to the line of the tele- 
graph, a current would be produced towards each end of the 
a 
