72 THE SCIENTIFIC PAPERS OF 



use. The process of joining the strips of india-rubber by the 

 machine depended on the well-known property of india-rubber, 

 that when two perfectly clean fresh-cut surfaces were pressed 

 together with great force they would unite as completely as two 

 pieces of iron welded together. After many trials for effecting 

 this by machinery, he had now succeeded perfectly with the 

 machine exhibited, in which the two cut edges made by the 

 cutting wheels on each side were instantly pressed together 

 between the pressing rollers and joined without having been ever 

 exposed to the atmosphere. This was the essential point in the 

 machine, as any exposure of the cut surfaces however momentary 

 interfered with the perfection of the joint. In putting on a series 

 of coats of india-rubber for making telegraph cables, a train of 

 machines was employed through which the wire was passed in a 

 continuous line, the joints in each successive covering being in a 

 line at right angles to those in the previous covering, which gave 

 a greater security against failure at the joint. 



This insulating covering had been subjected to severe tests, 

 and proved highly satisfactory and superior to any other mode of 

 insulation. Gutta-percha, which had hitherto been the material 

 used for covering telegraph wires, was a good non-conductor ; but 

 its resistance to the passage of an electric current was only 

 relative, like that of all other insulating materials, and it would 

 conduct to a certain extent, the conducting power being about 3 

 trillion times less perfect than that of mercury, which was adopted 

 as the standard of comparison. But india-rubber had much less 

 conducting power than gutta-percha, being 16 times better as a 

 non-conductor at a temperature of 52, and 70 times better at 92. 

 The insulating power of india-rubber was moreover less affected 

 by difference of temperature than was the case with gutta-percha ; 

 and in the combination of india-rubber and Wray's mixture, 

 which he had produced, the average insulating power was not less 

 than that of india-rubber, while it was to a less extent affected by 

 change of temperature. Before, however, a current of electricity 

 could pass along the wire, it had to induce a statical charge in the 

 insulating material throughout the whole length of the wire, as 

 in a Leyden jar; and the delay or retardation thus produced 

 depended on the inductive power of the insulating material, which 

 was independent of its insulating or non-conducting power, but 



