PKINCIPLES OF DESIGN. 71 



the various species and compositions of gutta-percha and its 

 treatment and preparation for the insulating of cables, which 

 work should be consulted for further information on the subject. 



A considerable amount of preliminary treatment of the raw 

 material is necessary. The extraneous matter which has found 

 its way into the gutta during collection has first to be removed. 

 For this purpose the raw material is heated in tanks of boiling 

 water, in which state the heavier impurities fall away. Then 

 follows the mastication process, in which the lumps, in a heated 

 and plastic state, are kneaded together in order to squeeze out 

 the finer impurities and to work the gutta into a mass of uni- 

 form consistency. The masticator machine consists of a long 

 fluted drum, mounted so as to revolve within a cylindrical 

 frame. The frame is split and the top half hinged for facilitat- 

 ing charging or emptying the machine. The gutta is kneaded 

 between the drum as it revolves and the frame of the machine 

 the latter being steam-jacketed. A second washing process 

 follows in a special machine of somewhat the same construction 

 as the above, in which water flows through at the same time 

 as the material is kneaded, so cleansing it and carrying away 

 fine impurities. The gutta is then thoroughly dried by pass- 

 ing through another masticator heated by a steam jacket. In 

 this machine the gutta is automatically exposed to the air at 

 each revolution, so that the moisture freed by the process is 

 taken up by the air. Still a further process of purification is 

 carried out by passing the material through a steam-jacketed 

 hydraulic strainer, in which it is forced through successive 

 sieves of very fine mesh to remove the small remaining impuri- 

 ties. The lumps of gutta in a refined and purified condition 

 are next rolled out in the calendering machine into sheets of 

 ^in. to fin. thick. This machine consists of steam-heated 

 rollers, each pair revolving together with small clearance in 

 such a direction as to draw the material inwards between them 

 and flatten it out. The material is then ready for the manu- 

 facture of core, described later on. 



The condition as to speed having been complied with in the 

 design of the core, it remains to provide the same with proper 

 mechanical protection. For protection against the teredo the 

 core is served with brass tape, layers of cotton tape, forming a 

 suitable cushion, being laid underneath and over the brass tape 



