SECT. XXXII. ATLANTIC TELEGRAPH. 325 



conveys a current to the disc, the other set conducts the opposite 

 current to the copper slip. As the edge of the revolving disc is 

 always immersed in the mercury, one set of wires is constantly 

 maintained in contact with it, and the circuit is only completed 

 when a point of the copper slip dips in the mercury also ; but 

 the circuit is broken the moment that point rises above it. Thus, 

 by the rotation of the armature, the circuit is alternately broken 

 and renewed ; and as it is only at these moments that electric 

 action is manifested, a brilliant spark takes place every time the 

 copper point leaves the surface of the mercury. Platinum wire 

 is ignited, shocks smart enough to be disagreeable are given, and 

 water is decomposed with astonishing rapidity, by the same 

 means ; which proves, beyond a doubt, the identity of the mag- 

 netic and electric agencies, and places Dr. Faraday, whose experi- 

 ments established the principle, in the first rank of experimental 

 philosophers. 



A magneto-electric machine has been recently constructed by 

 Mr. Henley, of enormous power. It consists of two permanent 

 magnets, from which the induction is obtained ; each of these is 

 formed of thirty horseshoe steel magnets, two feet and a half 

 long, and from four to five inches broad, and each is surrounded 

 by a coil of wire six miles long, coated with silk to insulate the 

 coils. A shock from these wires would be instantaneous death. 

 This apparatus will ultimately be employed to send a stream of 

 electricity through long submarine and subterraneous wires ; but 

 a Volta-electric machine has hitherto been used, in which the 

 electricity is generated by a galvanic battery instead of magnets. 



Induction, or the effect of the spiral wires in augmenting the 

 power of Voltaic electricity, is admirably illustrated: in the 

 Atlantic telegraph. 



Wires that are to convey electricity under ground, or through 

 water, must be defended from injury and insulated to prevent 

 the lateral escape of the electricity. For that purpose the cable 

 that is laid at the bottom of the Atlantic, from near Valentia in 

 Ireland to Trinity Bay in Newfoundland, is formed of seven fine 

 copper wires which convey the electricity, bound together by a 

 coating of gutta percha, over which there are layers of cloth 

 dipped in pitch, and then the whole is covered by steel wires 

 twisted together in strands and twined round in long close spirals, 

 forming a cord or cable not more than an inch and a quarter in 



