“don’t be alarmed,” spoke the calm voice of the professor. “i have only turned off the electrics. i want to switch on the search lights, to see if we can learn anything about our position.”

as he spoke he turned a switch, and, the gloom below the ship, as the boys could see by glimpses from the floor-window, was pierced by a dazzling glare. in the bottom of the mermaid were set a number of powerful electric arc lights with reflectors, constructed to throw the beams downward. the professor had built them in for just this emergency, as he thought that at some time they might want to illuminate what was below the craft.

not that it was of much avail on this occasion, for, though the lights were powerful, they could not pierce the miles of gloom that lay below them. the beams only served to accentuate the darkness.

“i guess we’ll have to trust to luck,” the professor said, after a vain attempt, by means of powerful glasses, to distinguish something. “there is too much fog and vapor.”

“what makes it so warm?” asked mark, removing his coat.

“well, you must remember you are approaching the interior of the earth,” the professor answered. “it has been calculated that the heat increases one degree for every fifty-five feet you descend. we have come down several hundred feet and of course it is getting warmer.”

“then if we go down very far it will get so hot we will not be able to stand it,” jack put in.

“i do not believe we will suffer any great inconvenience,” mr. henderson went on. “i believe that after we pass a certain point it will become cooler. i think the inner fires of the earth are more or less heated gas in a sort of inner chamber between two shells. if we can pass the second shell, we will be all right.”

“but aren’t we liable to hit something, going down into the dark this way?” asked mark.

“we will guard ourselves as far as possible,” the scientist answered.

the mermaid seemed to be going down on a side of the immense shaft a good way distant from the strange waterfall. when they had first dropped into the hole the travelers could hear the rush of waters, but now the noise was not audible.

“i think the hole must widen out the farther down we go,” the professor said. “we are probably many miles from the fall now.”

“i’m sure i hope so,” put in jack. “it would be no fun to have to take a shower bath in this place.”

after a meal, the boys and the professor took some more observations, but with all their efforts nothing could be seen below the ship but a vast black void, into which they were steadily descending.

“i wonder when we’re going to stop,” asked mark. “it’s like playing the game ‘going to jerusalem,’ you keep wondering when the music will cease and you will have a chance to grab a chair. i only hope we have a chair or something else to sit on, in case we go to smash.”

“we’re not liable to have any accidents with the professor in charge,” jack answered. “didn’t he bring us safe out of some pretty tight holes when we went to the north pole in the airship, and again when we found the south pole in the submarine?”

“yes, but this is different,” objected mark.

“well, i’m not worrying,” jack went on. “it doesn’t do any good, and only makes you lie awake nights. by the way, i wonder what time it is getting to be.”

he looked at his watch and found it was close on to eight o’clock in the evening. so late had dinner been served, and so varied were the happenings of the last few hours, that time had passed quickly.

“why it’s almost bed-time,” said jack. “i wonder if we are to go on dropping into the depths of nowhere all night.”

at that moment the professor entered the room where the boys were. he seemed quite pleased over something, and was smiling.

“everything is going along famously,” he said. “i have just tested the air and find it is rich in oxygen. we shall suffer nothing on that score. the heat too, seems to have decreased. on the whole, everything favors us.”

“are we going on down?” asked mark.

“as far as we can,” mr. henderson answered. “let me see how far we are below now.”

he went to the gage that indicated the vertical position of the ship. because of the changed conditions, the craft now sinking below the surface of the earth instead of rising above it, as was its wont, some calculations were necessary. these the scientist made as quickly as he could.

“we are now ten miles underground!” he exclaimed. “that is doing very well. my theories are working out. i think we shall land somewhere before long.”

“i hopes so!” exclaimed washington coming in at this point. “i’m mighty skeered shootin’ down int’ this dark hole, and no time-table t’ show when we’s due t’ arrive.”

“we ought to land in a couple of days more,” the professor answered. “never mind about worrying washington, i’ll take care of you.”

“i hopes so, perfesser,” the colored man said. “i got a little girl waitin’ for me back in georgia, an’ i’d like t’ see her 'fore i git burned up.”

accompanied by the professor, the boys made a tour of the ship to see that all the machinery and apparatus were in working order. owing to the changed conditions the negative gravity engine had to be worked at faster speed than usual, since the downward pull of the earth was greater the farther they descended into the interior and they did not want to fall too swiftly. but this was easily provided for, since the professor had made the apparatus capable of standing a great strain.

the ten miles had become fourteen when the professor, finding that everything was in good shape, proposed that the boys go to bed. they did not want to, though they were sleepy, and they feared to miss some strange sights.

but when the professor had promised to call them in case anything unusual developed, they consented to turn in, and bill and tom assumed their duties, which were light enough, now that the ship was merely falling into the immense shaft.

when mark turned into his bunk he could not go to sleep at once. it may have been the excitement over their new position, or because he had eaten too hearty a supper, but the fact was he remained awake for some time.

while thus tossing restlessly on his bed, wondering what ailed him, he thought he heard a noise in the main apartment out of which the storeroom opened. he crawled softly from his bed, and looked from his stateroom door.

in the light of a shaded electric mark saw the figure of some one glide across the floor and take refuge in the room, which professor henderson always was so particular about.

“i wonder what or who that was,” reasoned mark. “there is some mystery in this. can the professor have concealed some one on this ship whose presence he does not want to admit? it certainly looks so.”

not wanting to awaken the ship’s crew, and remembering what mr. henderson had said about any one entering the storeroom, mark went back to bed, to fall into an uneasy slumber.

“breakfast!” called washington breaking in on a fine dream jack was having about being captain of a company of automobile soldiers. “last call for breakfast!”

“hello! is it morning?” asked jack.

“not so’s you could notice it,” washington went on. “it’s as dark as a stack of black cats and another one throwed in. but breakfast is ready jest the same.”

the boys were soon at the table, and learned that nothing of importance had occurred during the night. the mermaid had been kept going slowly down, and about seven o’clock registered more than fifty miles below the earth’s surface.

still there was no change in the outward surroundings. it remained as black as the interior of egypt when that country was at its darkest. the powerful electrics could not pierce the gloom. the ship was working well, and the travelers were very comfortable.

down, down, down, went the mermaid. the temperature, which had risen to about ninety went back to sixty-nine, and there seemed to be no more danger from the inner fires.

they were now a hundred miles under the surface. but still the professor kept the mermaid sinking. every now and again he would take an observation, but only found the impenetrable darkness surrounded them.

“we must arrive somewhere, soon,” he muttered.

it was about six o’clock that night that the alarm bell set up a sudden ringing. the professor who was making some calculations on a piece of paper jumped to his feet, and so did a number of the others.

“we are nearing the bottom!” he cried. “the bell has given us warning!”