the boys ran to attend to the engines and apparatus to which they had been assigned in view of this emergency. the professor, washington, bill, tom and andy, who had kept to themselves since the descent, came running out of the small cabin where they usually sat, and wanted to know what it was all about.

“we may hit something, in spite of all precautions,” mr. henderson remarked. “slow down the ship.”

the mermaid was, accordingly checked in her downward flight, by a liberal use of the gas and the negative gravity machine.

the bell continued to ring, and the dials pointed to the mark that indicated the ship was more than one hundred and fifty miles down.

mark, who had run to the engine room to check the descent, came back.

“why didn’t you slow her down?” asked the professor.

“i did,” replied the boy. “the negative gravity and the gas machines are working at full speed.”

“then why are we still descending?” asked the scientist. “for a while our speed was checked, but now we are falling faster than before.”

“i attended to the apparatus,” mark insisted.

just then, from without the ship, came a terrible roaring sound, as though there was a great cyclone in progress. at the same time, those aboard the craft could feel themselves being pulled downward with terrific force.

“we are caught in a draught!” mr. henderson cried. “we are being sucked down into the depths of the earth!”

he ran to the engine room. with the help of the boys he set in motion an auxiliary gravity machine, designed to exert a most powerful influence against the downward pull of the earth. as they watched the great wheels spin around, and heard the hum and whirr of the dynamos, the boys watched the pointer which indicated how low they were getting.

and, as they watched, they saw that the needle of the dial kept moving, moving, moving.

“our efforts are useless! we can’t stop!” the professor cried.

grave indeed was the plight of the adventurers. in their ship they were being sucked down into unknown regions and all their efforts did not avail to save them. it was an emergency they could not guard against, and which could not have been foreseen.

“what are to do?” asked mark.

“we can only wait,” mr. henderson replied. “the terrible suction may cease, or it may carry us to some place of safety. let us hope for the best.”

seeing there was no further use in running the engines in an effort to check the downward rush the machines were stopped. then they waited for whatever might happen.

now that they seemed in imminent peril washington was as cool as any one. he went about putting his kitchen in order and getting ready for the next meal as if they were sailing comfortably along on the surface of the ocean. as for old andy he was nervous and frightened, and plainly showed it. with his gun in readiness he paced back and forth as if on the lookout for strange beasts or birds.

bill and tom were so alarmed that they were of little use in doing anything, and they were not disturbed in their staterooms where they went when it became known that the ship was unmanageable.

the boys and the professor, while greatly frightened at the unexpected turn of events, decided there was no use in giving way to foolish alarm. they realized they could do nothing but await developments.

at the same time they took every precaution. they piled all the bedding on the floor of the living room, so that the pillows and mattresses might form a sort of pad in case the ship was dashed down on the bottom of the big hole.

“not that it would save us much,” jack observed with a grim smile, “but somehow it sort of makes your mind easier.”

all this while the ship was being sucked down at a swift pace. the pointer of the gage, indicating the depth, kept moving around and soon they were several hundreds of miles below the surface of the earth.

the professor tried, by means of several instruments, to discover in which direction they were headed, and whether they were going straight down or at an angle. but some strange influence seemed to affect the gages and other pieces of apparatus, for the pointers and hands would swing in all directions, at one time indicating that they were going down, and, again, upward.

“there must be a strong current of electricity here,” mr. henderson said, “or else there is, as many suspect, a powerful magnet at the center of the earth, which we are nearing.”

“what will you do if the ship is pulled apart, or falls and is smashed?” asked mark with much anxiety.

“you take a cheerful view of things,” said jack.

“well, it’s a good thing to prepare for emergencies,” mark added.

“if the ship was to be separated by the magnetic pull, or if it fell on sharp rocks and was split in twain, i am afraid none of us could do anything to save ourselves,” the professor answered. “still, if we were given a little warning of the disaster, i have means at hand whereby we might escape with our lives. but it would be a perilous way of——”

“i reckon yo’ all better come out an’ have supper,” broke in washington. “leastways we’ll call it supper, though i don’t rightly know whether it’s night or mornin’. anyhow i’ve got a meal ready.”

“i don’t suppose any of us feel much like eating,” observed mr. henderson, “but there is no telling when we will have the chance again, so, perhaps, we had better take advantage of it.”

for a while they ate in silence, finding that they had better appetites than they at first thought. old andy in particular did full justice to the food washington had prepared.

“i always found it a good plan to eat as much and as often as you can,” the hunter remarked. “this is a mighty uncertain world.”

“you started to tell us a little while ago, professor,” said mark, “about a plan you had for saving out lives if worst came to worst, and there was a chance to put it into operation. what is it?”

“i will tell you,” the aged inventor said. “it is something about which i have kept silent, as i did not want to frighten any of you. it was my latest invention, and i had only perfected it when we started off on this voyage. consequently i had no chance to try it. the machine works in theory, but whether it does in practice is another question. that is why i say there is a risk. but we may have to take this risk. i have placed aboard this ship a——”

the professor was interrupted in what he was about to say by a curious tremor that made the whole ship shiver as though it had struck some obstruction. yet there was no sudden jolt or jar such as would have been occasioned by that.

at the same time washington, who was out in the kitchen, came running into the dining room, crying:

“we’re droppin’ into a ragin’ fire, perfesser!”

“what do you mean?” asked mr. henderson.

“i jest took a look down through th’ hole in th’ bottom of the ship!” cried washington. “it’s all flames an’ smoke below us!”

“i wonder if it is the end,” the professor muttered in a low voice.

followed by the boys, the inventor hastened to the floor-window. the lights were turned off to enable a better view to be had of what was below them.

leaning over the glass protected aperture the boys and the professor saw, far, far down, a bright light shining. it was as if they were miles above a whole town of blast furnaces, the stacks of which were belching forth flames and smoke. the rolling clouds of vapor were illuminated by a peculiar greenish light, which, at times, turned to red, blue, purple and yellowish hues.

the effect was weird and beautiful though it was full of terror for the travelers. it seemed as if they were falling into some terrible pit of fire, for the reflection of what they feared were flames, could plainly be seen.

“i wish i’d never come on this terrible voyage!” wailed washington. “i’d rather freeze to death than be burned up.”

“washington, be quiet!” commanded the professor sternly. “this is no time for foolishness. we must work hard to save our lives, for we are in dire peril.

“mark, you and washington, with jack, start the engines. turn on every bit of power you can. fill the gas holder as full as it will hold, and use extra heavy pressure. i will see if i can not work the negative gravity apparatus to better advantage than we did before. we must escape if possible!”

the boys, as was also washington, were only too glad to have something to do to take their mind off their troubles. all three were much frightened, but mark and jack tried not to show it. as for washington he was almost crying.

soon the whirr and hum of the machinery in the mermaid was heard. the craft, which was rushing in some direction, either downward, ahead or backwards within the unknown depths, shivered from the speed of the dynamos and other apparatus. soon the boys could hear the professor starting the negative gravity engine, and then began a struggle between the forces of nature and those of mankind.

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.

just as mr. henderson was about to open the door there came a fiercer blast of heat than any that had preceded. at the same instant the conditions in the mermaid became so fearful that each of the travelers felt himself fainting away.

“go to—storeroom—get cylinder—get in——” the professor murmured, and then he fell forward in a faint.