196 APPLIED SCIENCE 



strand might be cut, in two and joined by a short fine wire barely 

 visible to the eye, without any difference being felt in the 

 rapidity with which signals could be transmitted, or in the mag- 

 nitude of the currents observed in the cable. The thin wire 

 would produce no sensible effect, unless the length over which it 

 formed the exclusive conductor bore some sensible proportion to 

 that of the whole cable. Six, therefore, of the seven wires of a 

 conductor may be broken in a thousand places without any 

 injury to the cable, provided any one wire at each spot remains 

 not wholly broken ; nor is it, of course, necessary that this one 

 wire should always be the same. Of course the seven wires 

 forming the strand act as one conductor, and transmit only one 

 message at a time. 



The interstices between the several wires are filled with an 

 insulating varnish known as Chatterton's Compound. The 

 object of this varnish is to prevent the percolation of water 

 along the strand, should any water ever reach it, and also to 

 produce a more perfect adhesion between the strand and the 

 gutta envelope, so that it becomes very difficult to strip off the 

 insulator, even should it be cut or abraded. In older cables it 

 was by no means difficult to pull the insulator off the copper in 

 the form of a gutta-percha tube, and in great depths water was 

 very generally found to have penetrated to the copper through- 

 out its entire length. This was not necessarily fatal to the cable, 

 for the water inside might be quite well insulated from the water 

 outside, owing to the extreme minuteness of the pores by which 

 it had gained access to the interior ; but this water was the 

 cause of serious difficulty and danger in joining a fresh piece of 

 cable to an old one during repairs, and it was also probably 

 dangerous by its tendency to produce an oxidation of the copper 

 conductor. ' In cables as now made, there is no space for the 

 water to lodge, and no water is ever found between the insulator 

 and the copper. 



The insulator employed in every cable of importance hitherto 

 laid has been gutta-percha. The copper strand is passed into 

 a vat of semi-fluid percha, and is drawn through a die of such 

 size as to allow a convenient thickness of insulator to be pressed 

 out round it. This first layer of gutta-percha receives a coat of 

 Chatterton's Compound, and the process is repeated until the 



