The manufacture and testing of the “core” of the Atlantic Cable having been completed at the Gutta Percha works as described, a telegraphic line was thereby produced which, without further addition of material or substance, beyond that of copper and gutta percha, proportionable to any required increase in its length, would be perfect as an electrical communicator through the longest distances and in the deepest water, in which element moreover it appears to be chemically indestructible, if the experience of some fourteen years may be taken as evidence. At this point, however, the final form to be assumed by the deep-sea Cable was subject to important mechanical considerations, which came into play across the path of those purely electrical; and upon the manner in which these considerations are met and dealt with, depend, not merely the primarily successful submersion, but the ultimate durability and commercial value of deep-sea Cables. The problem in the case of the Atlantic Telegraph enterprise may be thus stated. Given a submarine telegraph core like that already described, constructed on the best known principles and perfect as to its electrical conductivity and insulation--it is required to lower the same through the sea to a maximum depth of two and a-half miles, so as not merely not to allow the insulating medium to be torn or strained, but so as not even to bring its normal elasticity into play against the more tensile but perfectly inelastic material of the conductor. For if the core were lowered into very deep water like that referred to without further protection, even supposing it to escape actual fracture by the adoption of extraordinary precaution and by the aid of fine weather, it is evident that whenever, as would be highly probable, either in the act of paying out, during the lifting or manœuvring of the ship, or even from the effects of its own weight, the gutta percha sheath became extended to the limit of its elasticity, the copper in the centre would be stretched to a corresponding extent, and, the tension being removed, the gutta percha in returning to its original length would pull back the now elongated copper, which thenceforward would in every such case “buckle up,” and exert a constant lateral thrust against the gutta percha; ending, probably, in its ultimate escape to the outside, and the consequent destruction of the core as an electrical agent. Moreover, in the event of an electrical fault being discovered in any submerged portion of the Cable during the process of “paying-out” in deep water, it is of paramount importance towards its recovery and repair, that the engineer should have such an assurance in the quality and strength of his materials as will enable him confidently to exert a known force in hauling back the injured part, without apprehension of damage to the vital portion of the Cable.