Page:Ballantyne--The Battery and the Boiler.djvu/94

 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?"