ch we want to send our message. D'you understand?"

"I think I do, sir; but _how_ is the exact amount of resistance tested?"

Mr Smith smiled as he looked at the earnest face of his young
questioner.

"My boy," said he, "you would require a more fully educated mind to
understand the answer to that question. The subtleties of electrical
science cannot be explained in a brief conversation. You'll have to
study and apply to books for full light on that subject. Nevertheless,
although I cannot carry you _into_ the subject just now, I can tell you
something _about_ it. You remember the testing-room which I showed you
yesterday--the darkened room between the captain's state-room and the
entrance to the grand saloon?"

"Yes, sir, I remember it well," responded Robin,--"the room into which
the conducting-wires from the ends of the cable are led to the
testing-tables, on which are the curious-looking galvanometers and other
testing machines."

"Just so," returned Smith, pleased with his pupil's aptitude. "Well, on
that table stands Professor Thomson's delicate and wonderful
galvanometer. On that instrument a ray of light, reflected from a tiny
mirror suspended to a magnet, travels along a scale and indicates the
resistance to the passage of the current along the cable by the
deflection of the magnet, which is marked by the course of this speck of
light. Now, d'you understand that, Robin?"

"I--I'm afraid not quite, sir."

"Well, no matter," rejoined Smith, with a laugh.

"At all events you can understand that if that speck of light keeps
within bounds--on its index--all is going well, but if it travels beyond
the index--bolts out of bounds--an escape of the electric current is
taking place somewhere in the cable, or what we call a _fault_ has
occurred."

"Ah, indeed," exclaimed Robin, casting a serious look at the cable as it
rose from the after-tank, ran smoothly over its line of conducting
wheels, dropped over the stern of the ship and glided into the sea like
an an endless snake of stealthy habits. "And what," he added, with a
sudden look of awe, "if the cable should break?"

"Why, it would go to the bottom, of course," replied Smith, "and several
hearts would break along with it. You see these two gentlemen
conversing near the companion-hatch?"

"Yes."

"One is the chief of the electricians; the other the chief of the
engineers. Their hearts would probably break, for their position is
awfully respo