it was now noon, but the adventurers did not think of dinner in the excitement of approaching the mysterious island. the speed of the ship was increased that they might the more quickly come to it. as they approached they could see the masses of vapor more plainly, and it appeared that some great commotion must be going on inside the big hole, since clouds of steam arose.

“i only hope it doesn’t prove too hot for us,” observed the professor. “however, i provided a water jacket for the ship, and we may need it, as well as the vacuum chambers to keep the heat from us.”

it was about three o’clock when the flying ship reached the edge of the island. from there it was about a mile to the rim of the big hole, over one side of which the waters of the ocean poured with a roar that could be heard over half a mile off.

“i think we had better halt and see that everything is in good shape before proceeding,” said mr. henderson. “jack, you and mark make a thorough inspection of the engine room, and see that all the apparatus is in working order.”

the two boys prepared to do as they were told. mark, who was walking a little ahead of jack, entered the apartment from which the storeroom opened. as he did so he saw, or thought he saw, the door of the place where the extra supplies were kept, close. without saying anything to jack he hurried forward, and tried the knob. it would not turn.

“that’s funny,” said mark to himself. “i could almost swear i saw some one go into that room. yet i know the professor did not enter, for i just left him. and none of the others would dare to. i wonder if i will ever solve the mystery.”

but he had too much to do to allow him to dwell on that matter. several of the dynamos needed adjusting and for two hours he and jack had all they could do.

in the meanwhile the professor had gone over the other parts of the ship, and gotten everything in readiness for the descent. the mermaid was lowered to within a few hundred feet of the sea, and, through a hose that was let down, the compartments, provided for this emergency were filled with water. these compartments were between the outer and inner hulls of the lower part of the craft, and were designed to prevent the interior becoming heated in case the travelers found they had to pass close to fire. there were also vacuum chambers, and from these the air was exhausted, as of course every schoolboy knows a vacuum is a non-conductor of either heat or cold.

“now i think we are ready,” the professor announced at length.

“everything’s all right in the engine room,” announced jack.

“yes, an’ everything’s all right in th’ kitchen,” put in washington. “i’ve got a good meal ready as soon as any one wants to eat.”

“it will have to wait a while,” mr. henderson remarked. “we are going to start to make the descent before we dine.”

the hose was reeled up, and the ship was sent a few hundred feet higher into the air, as mr. henderson wanted to take a last good observation before he went down into the hole.

but having risen some distance above the masses of rolling vapors he found he was at no advantage, since the strongest telescope he could bring to bear could not pierce the cloud masses.

“we’ll just have to trust to luck,” the scientist said. “i judge we’re about over the centre of the opening. lower away mark!”

the boy, who, under the watchful eye of the professor, was manipulating the levers and wheels in the conning tower, shifted some handles. the gas was expelled from the holder, the negative gravity apparatus ceased to work, and the flying mermaid sank lower and lower, toward the mysterious hole that yawned beneath her.

the hearts of all beat strangely, if not with fear, at least with apprehension, for they did not know what they might encounter. perhaps death in some terrible form awaited them. but the desire to discover something new and strange had gripped all of them, and not one would have voted to turn back.

even old andy, who seldom got excited, was in unusual spirits. he took down his gun and remarked:

“maybe i can kill some new kind of animal, and write a book about its habits, for surely we will see strange beasts in the under-world.”

lower and lower sank the ship. now it was amid the first thin masses of vapors, those that floated highest and were more like a light fog, than anything else. by means of a window in the bottom of the craft, which window was closed by a thick piece of plate glass, professor henderson could look down and see what was beneath them.

“the clouds seem to be getting thicker,” he said, as he peered through the small casement. “if they would only clear away we could see something.”

but instead of doing this the vapors accumulated more thickly about the ship. it was so dark inside the mermaid now that the electric lights had to be switched on. in the room with the floor-window the lights were not used, as had they shone one could not have seen down below.

the professor maintained his position. the descent was a perilous one, and he wanted to be on the watch to check it at once if the mermaid was liable to dash upon some pointed rock or fall into some fiery pit. his hand was on the signal levers.

suddenly he looked up and glanced at a gage on the wall. the hand of it was slowly revolving.

“we are at the earth’s surface,” the scientist said. “now we are below it. now we are fairly within the big hole! boys, we may be on the verge of a great discovery!”

an instant later it seemed as if a hot wave had struck the mermaid, or as if the craft had been plunged into boiling water.

“it’s going to be hot!” cried the professor. “lucky i provided the water jackets!”

then the lights in the interior of the ship went out, leaving the whole craft in darkness.

“what has happened?” cried mark.