366 



THE ELECTRIC TELEGRAPH. 



over this, with a coat of gutta-percha, gave mean results 

 between those found with the two materials separately. 



From all the observations made in carrying out these ex- 



Gutta-percha. 



_u i_ 



Pressure. 



boo atmospheres. 



Fig. 168. 



periments the means of the co-efficient a, from these three 

 modes of insulation, are for 



Gutta-percha ........................ + 0,0041 



India-rubber ..................... 0,0009 



Combined ........................... -|- 0,0016 



to be inserted in the formula 



aj p) 



. by which as good an approximation to the true resistance is 

 obtained, as the expressions of the curves as straight lines 

 allow ; and the observations were too rough and too few to 

 make it worth while to calculate the formula with more 

 members. 



The values obtained for the Carthagena-Oran core agree 



