118 Submarine Telegraphy. 



throughout a course of nearly 2000 miles, and irregularities in 

 the sea bottom, such as represented in Fig. 1 — though here 

 violently exaggerated — would cause the cable to hang in the 

 form of a curve of greater or less length, according to the free- 

 dom with which it was payed out, and the nature of the ground 

 over which it had to pass. The strain upon the cable then would 

 not be the simple weight of the cable itself, but would be 

 compounded of two forces. Let M N P be a section of the sea 

 bottom, HRA the cable, passing over the wheel W at the 

 stern of the ship. Then if A C represent the vertical strain 

 upon the point A, which would vary according to the depth of 

 water, and A B the horizontal strain, varying with the distance 

 of the point of suspension H, the resultant force tending to 

 break the cable would be represented by the line A R, and 

 would of course be greater with every increase of the horizon- 

 tal strain A B, or of the vertical strain A ; and would 

 invariably be greater than the mere weight due to the perpen- 

 dicular depth of the cable, which in the diagram is indi- 

 cated by the line A C. Practically, therefore, it is desirable to 

 diminish the lateral strain, especially in deep water, as far as 

 possible, so as to secure a perfect settlement in the sea bed at 

 as small a distance astern as is practicable; otherwise the 

 strain may be so great as to exceed its " breaking strain.'" 



While speaking of the nature of the ocean bed as affecting 

 the operation of telegraphic wires, it is necessary to notice the 

 action of another force of a most subtle character, one which 

 perhaps may present difficulties when all others have been con- 

 quered. We allude to the existence of magnetic currents in the 

 earth. The existence of these currents has long been known 

 to physicists, and in the course of time much has been done 

 towards determining the laws of their action. But no means 

 are at present known by which the action of the magnetic 

 force on an electric conductor can be prevented. Insulation, 

 which is so effective where electricity alone is concerned, is so 

 powerless when opposed to magnetic action, that glass, gutta- 

 percha, and other insulators scarcely interfere with the action 

 of a powerful magnet more than the air we breathe. Still, it 

 may be fairly expected that the action of " earth-currents," or 

 even of "magnetic storms," will do little more than tem- 

 porarily interrupt the working of a cable, as they did the 

 working of the terrestrial lines lying between Croydon and the 

 Royal Observatory, Greenwich, on the 3rd, 4th, and 5th 

 of the present month, on which occasion a magnetic storm 

 was experienced more violent than any that had ever been 

 previously recorded. 



Passing now from the consideration of the ocean-bed to 

 that of the action of sea-water upon a submerged cable, wc 



