548 
POPULAR SCIENCE REVIEW. 
occur at the same spot_, and^ if a single wire were broken, it 
would not interfere, to any sensible degree, with the passage 
of the current. In constructing the Atlantic cable, however, 
the utmost care prevailed in every department. Every por- 
tion of the copper wire was submitted to exact examination, 
and no wire was used which possessed a lower conducting 
power than 85 per cent, of that of pure copper. The insula- 
tion of this conductor was thus effected. The centre wire of 
the copper strand was first covered with Chatterton^s com- 
pound (that is, gutta-percha rendered viscid with Stock- 
holm tar) . This first coating was so thick that, when the other 
six wires forming the strand were laid spirally around it, every 
interstice became filled and air excluded. The mechanical so- 
lidity of the core thus formed was very great. This cable is 
nearly three times as heavy as the old Atlantic conductor, its 
weight being 300 lbs. to the nautical mile against 107 lbs. The 
complete exclusion of air, which is a most important object, is 
believed to be effected by this arrangement. The conductors of 
some cables have suffered from their being to some extent loose 
within their insulating covering. The core being thus formed 
next received another coating of Chatterton^s compound, which 
quickly solidifies, and thus thoroughly protects, the whole. It 
was then surrounded with a coating of the purest gutta-percha, 
which was pressed, around it in a plastic state, by means of a 
very accurate die, and sheathed the core in a continuous tube. 
Over this was again laid another coating of Chatterton^s com- 
pound, for the purpose of effectually closing up any pores or 
flaws which may have escaped detection in the gutta-percha 
tube. To this covering succeeds a second tube of gutta-percha, 
then another coating of ChattertoAs compound, and so on 
alternately, until the wire was covered with four coatings of 
gutta-percha, and four of the compound. This being com- 
pleted, it weighs 400 lbs. to the nautical mile. It is well 
known that the insulating power of gutta-percha sensibly 
decreases by heat. The whole of the conductor was immersed 
in water of a temperature of 75°Fahr., in which it remained for 
twenty-four hours, and during the time it was constantly sub- 
mitted to electrical tests. The core, after having been sub- 
jected to this and other tests, to prove the perfect character 
of its insulation, was then submitted to the process of being 
protected by its iron-wire armour. 
Jute, which had been submitted to the action of catechu, in 
order to preserve it from decay, was very carefully wrapped 
around the core, and as fast as the wrapping proceeded it was 
coiled in water. If the water at any time penetrated to the 
wire, it would at once conduct away the electric current to the 
earth, and the loss of insulation would be at once detected. 
