“I suppose by putting on more coals,” answered Kearns cautiously.
“Indeed!” sniffed the Professor contemptuously, “does it not occur to you—O man of self-vaunted thickness!—that the servant might indulge in some operation prior to putting on those fresh coals?”
“Huh!” exclaimed Kearns, with a puzzled expression; “I suppose you mean that there would first be a raking out of the ashes.”
“Good!” exclaimed the Professor; “you are beginning to betray an intelligence which is almost human. Now, so it is in the case of the air-ship. In that secondary storage box to which I have referred are certain chemicals which furnish power. When a portion of these chemicals have exhausted their activity, a residuum gradually forms, corresponding to the ashes in your grate, only it is in liquid form instead of solid. This residuum is allowed to escape through a valve and is thus carried away and, of course, the storage box is fed from time to time, as may be required, with fresh energy-producing chemicals.”
“Let me see if I understand it,” persisted Kearns with obstinacy. “This liquid residuum, as you call it, leaks away through the bottom of the air-ship, doesn’t it?”
“You may put it that way,” said the Professor, smiling.
“And that blamed old dripping liquid residuum would either burn into, or else make a deep brown stain upon any fairly soft material it touched, wouldn’t it?”
“Yes, yes,” exclaimed the Professor excitedly; “a dark brown stain. But how—how did you know this?”
“Not bad, eh?” retorted Kearns, “for a marvel of density! In course of time—O sage of a century of Wisdom!—and under your able direction, you’ll find that almost human intelligence of mine gradually developing itself.”
“But how did you know this?” persisted the Professor.