NATURAL PHILOSOPHY. 121 



failure is considered to be attributable to the cable having been de- 

 signed "without regard to the conditions of the climate, or the char- 

 acter of the bottom of the sea over which it had to be laid, and to 

 the insufficiency of the agreement with the contractor for securing 

 effectual supervision during manufacture and control of the manner 

 of laying." 



Looking at these circumstances, and similar ones in connection 

 with other lines, the committee point out that the failures in every 

 case are assignable to defined causes which might have been guarded 

 against. 



In regard to the best material for conductors, the committee state, 

 that after much experimentation, no substance has been found which 

 added to pure copper will increase its conducting power. 



As regards the material for covering telegraph cables, it 



^j CU CJ -L ' 



appears that India-rubber was almost the first substance that had 

 been used for covering overland wires ; and the report says " it is 

 remarkable that the first really efficient insulating substance that 

 was used, after falling into disuse, should be now again brought 

 forward. As in the copper for the conductor, so India-rubber ap- 

 peared almost specially intended for the purpose of insulation. It 

 possesses insulating qualities of the highest order. It is tough, highly 

 elastic, of less specific gravity than water, easily manipulated, ex- 

 tremely durable under water, nearly impervious to moisture, and 

 it appeared on its first introduction as though nothing further could 

 be desired." 



The reason set forth for its disuse is stated to havQ. been defective 

 application. After the first failure of India-rubber, gutta-percha was 

 introduced to take its place ; and up to the present time it has been 

 used as the chief insulating agent. 



The report states that the committee made numerous experiments 

 with both India-rubber and gutta-percha as a coating for submarine 

 cables. It was found that pressure consolidated the material and im- 

 proved the insulating qualities of both gutta-percha and India-rubber. 

 Temperature was found to produce a marked effect upon these sub- 

 stances in relation to the insulating powers. Thus, with the gutta- 

 percha, the insulation was not half as good at a temperature of 

 seventy-five degrees Fah. as at fifty-two degrees, and not one-fourth 

 as good at ninety-two degrees. At a temperature of thirty-two 

 degrees its insulating qualities were three times as good as at fifty-two 

 decrees. At a temperature of one hundred and thirty-two degrees, 

 gutta-percha-covered wire was entirely spoiled. Temperature does 

 not affect India-rubber so much as gutta-percha. 



Both these substances, however, were found to be porous under great 

 pressure in water, and this seems to be the great difficulty to over- 

 come so as to make them more perfect insulators. 



In conclusion, the committee repeat their belief that the exercise 

 of due care might have prevented all the unsatisfactory results that 

 have thus far attended this branch of enterprise, and that if proper 

 regard be henceforth bestowed upon the question the results will 

 prove as successful as they have hitherto been disastrous. The evi- 

 dence appended is extremely voluminous, and occupies five hundred 

 and twenty pages. 

 11 



