68 THE SCIENTIFIC PAPERS OF 



method hitherto adopted consists in cutting the india-rubber into 

 strips, and winding these strips spirally upon the wire to be 

 insulated : a tedious and expensive operation, which has to be 

 repeated several times to afford any security that the water is 

 entirely excluded from the wire. The insulation of the wire 

 depends in fact upon a perfect joint being formed throughout 

 between the strips ; for it is evident that where the strips overlap 

 a spiral channel is formed, which if penetrated in any one place 

 will allow the water to spread till it may chance to find a trans- 

 verse passage into the spiral channel of the next lower coating, 

 and so forth until it reaches the wire. Formerly the layers of 

 india-rubber simply touched one another, and could readily be 

 displaced ; but lately a process of soldering the spiral layers has- 

 been introduced by Messrs. Silver, which greatly increases the 

 security of the coating, although it does not remove the objections 

 to the spiral channels which must always be formed in lapping. 

 This process of soldering consists in exposing the covered wire to 

 boiling water for about half an hour, when a most perfect cohesion 

 between adjoining surfaces is produced. The india-rubber so- 

 treated adheres to the fingers, or feels sticky ; it also loses part of 

 its elasticity and strength. It may therefore be inferred that the 

 heat produces some chemical alteration in the material, changing 

 the gum into an oil. It has been observed that india-rubber so- 

 heated has gradually changed bodily into a viscid liquid, where it 

 is in contact with the metal conductor, so as to render it unsafe 

 to be used. 



The method of covering which it is proposed to substitute for 

 the above combines the advantages of comparative cheapness and 

 certainty of result with that of rendering the application of heat 

 unnecessary. The operation is based on the well-known adhering: 

 property of india-rubber, when two fresh-cut surfaces are joined 

 together under considerable pressure. The mechanical problem 

 consisted in the construction of a machine which would draw the 

 india-rubber tight upon the wire, so as completely to exclude air ; 

 and would then cut the india-rubber at the proper inclination^ 

 and join the fresh-cut edges together at the same instant under a 

 sufficient pressure to make the joint perfect. 



The machine finally arranged for this purpose is shown in? 

 Figs. 1 to 5, Plates 2, 3, and 4, one quarter full size. Fig. 1,, 



