THE TELEGRAPH. 
150 
acquire very great skill, and it rarely happens that the joint 
has to be rejected by the electricians when it comes to be 
tested by the electrometer or by the method of accumulation. 
The gutta-percha keeps wonderfully under water, and would 
almost seem to have been purposely created for submarine 
telegraphy. 
As to the electrical conditions that determine the dimensions 
to be given to the conducting wire and to the insulating 
covering, it will be sufficient for us here to observe that: 
1. For any given ratio between the cost of the materials 
(conducting and insulating) there is a corresponding propor¬ 
tion between the weights or the diameters of these materials 
by which the greatest effect is obtained at the least cost. 
2. But practically the thickness of the insulator is always 
greater than its theoretical thickness. 3. If a constant ratio 
between the diameters of the conductor and the insulating 
material is maintained, the number of words a minute that can 
be sent by a given length of cable is simply proportional to 
