THE SIMPLE IDEA OF AN ATLANTIC CABLE 45 
ocean bottom was much smoother and more moderate in 
depth than to the south, for example around the Azores, 
where jagged peaks and valleys distorted the depths of the 
sea. The only sudden change in the bottom along the north- 
ern route was about two hundred miles off the coast of Ire- 
land, where there was a ‘“‘drop-off’ from the shallow seas of 
the island to the depths of mid-ocean. Later, however, even 
this change in depth was found to be much less sudden than 
had been supposed, and no particular trouble was experi- 
enced in the cable-laying at this place, especially if the steamer 
proceeded slowly and allowance was made for slack in the 
cable. If all the technical factors had been as favorable as the 
bottom of the sea, the undertaking would have been much 
easier. For example, soft shells of microscopic animals cov- 
ered the sharp rocks and rough sand of the ocean depths, as 
if put there by Providence. These shell-fish did not live in 
that enormous pressure but had dropped there after death in 
the shallower waters above—a quiet shower of minute shells 
to cushion the cable. 
Among the technical difficulties, those relating to the 
electric impulses to be sent through a two-thousand-mile 
cable were highly-specialized problems. An electric impulse 
does not flow along such a long insulated conductor without 
complications; it is in fact very temperamental. There has 
to be perfect insulation to keep the current from dissipating 
itself in the salt water. ‘The insulation thus lies between two 
good conductors—the copper wire within, and the salt water 
without. The cable, consequently, becomes a condenser, 
like a huge Leyden jar. Furthermore, such a long wire has 
great tendency toward self-induction, which acts to slow up 
the signals sent through it. A third disturbance comes from 
the irregular earth currents between widely-separated parts 
of the earth’s crust. ‘These were the problems on which Pro- 
fessor Thomson and Cromwell Varley worked and studied 
—problems of great scientific difficulty. 
