Once more the adventurers anxiously watched the gages and indicators.  For a while the ship seemed to be holding out against the terrible influence that was sucking her down.  She appeared to hesitate.  Then, as the downward force triumphed over the mechanical energy in the craft, she began to settle again, and soon was descending, if that was the direction, as fast as before.

“It is of no use,” said the professor with a groan.  “I must try our last resort!”

He started from the engine room where Mark and Jack had gone.  As he did so, he glanced at a thermometer hanging on the wall near the door.

“Has any one turned on the heat?” he asked.

“It’s shut off,” replied Mark, looking at the electric stove.

“Then what makes it so hot?” asked the scientist.

He pointed to the little silvery column in the tiny tube of the instrument.  It registered close to one hundred degrees, though a few minutes before it had been but sixty.  And the starting of the machinery could not account for the rise in temperature, since most of the apparatus was run by electricity and developed little heat save in the immediate proximity.  The thermometer was fully ten feet away from any machine.

“It’s the fiery furnace that’s doing it!” cried Washington.  “We’re falling into th’ terrible pit an’ we’re goin’ t’ be roasted alive!”

“It certainly is getting warmer,” observed Mark, as he took off his coat.  Soon he had to shed his vest, and Jack and the professor followed his example.  The others too, also found all superfluous garments a burden, and, in a little while they were going about in scanty attire.

Still the heat increased, until it was almost torture to remain in the engine room.  Nor was it much cooler elsewhere.  In vain did the professor set a score of big electric fans to whirring.  He even placed cakes of ice, from the small ice machine that was carried, in front of the revolving blades, to cool off the air.  But the ice was melted almost as soon as it was taken from the apparatus.

“Them flames is gittin’ worser!” Washington cried a little later.  “We’s comin’ nearer!”

From the bottom window the professor and the boys looked down.  True enough the curious, changing, vari-colored lights seemed brighter.  They could almost see the tongues of flame shooting upward in anticipation of what they were soon to devour.

The heat was increasing every minute.  The sides of the ship were hot.  The heads of the travelers were getting dizzy.  They could hardly talk or move about.

“I must save our lives!  I must trust to the——­” The professor, who was muttering to himself started toward the storeroom.  As in a dream Mark watched him.  He remembered afterward that he had speculated on what might be the outcome of the mystery the professor threw about the place.  “I will have to use it,” he heard the scientist say softly.