The covering of the conductor with its dielectric or insulating sheath was effected as follows:--The centre wire of the copper strand was first covered with a coating of gutta percha, reduced to a viscid state with Stockholm tar, this being the preparation known as “Chatterton’s Compound.” This coating must be so thick that, when the other six wires forming the strand were laid spirally and tightly round it, every interstice was completely filled up and all air excluded. The object of this process was two-fold; first, to prevent any space for air between the conductor and insulator, and thus exclude the increase of inductive action attendant upon the absence of a perfect union of those two agents, and, second, to secure mechanical solidity to the entire core; the conductors of some earlier Cables having been found to be to some extent loose within the gutta percha tube surrounding them, and thereby much more liable to permanent extension, mechanical injury, and imperfect centricity than those in which the preliminary precaution just described had been made use of. The whole conductor next received a coating of Chatterton’s Compound outside of it; this, when the core was completed, quickly solidified, and became almost as hard as the remainder of the subsequent insulation. It was then surrounded with a first coating of the purest gutta percha, which being pressed around it while in a plastic state by means of a very accurate die, formed a first continuous tube along the whole conductor. Over this tube was laid by the same process a thin covering of Chatterton’s Compound, for the purpose of effectually closing up any possible pores or minute flaws that might have escaped detection in the first gutta percha tube. To this covering of Chatterton’s Compound succeeded a second tube of pure gutta percha, then another coating of the compound, and so on alternately until the conductor had received in all four coatings of compound and four of gutta percha. The total weight of insulating material thus applied was 400 pounds to the nautical mile, against 261 pounds in the Cable of 1857-8.