but mark was certain it was nothing like that, though a careful search failed to reveal anything or any person near the ship. it was too dark to examine for footprints, and even mark, after taking a look all about, felt he might have been deceived by shadows. still he was a little nervous, and could hardly sleep for imagining what the thing he saw could have been.

the next day every one was so busy that no one, not even mark, recalled the little excitement of the night before. shortly after noon, final preparations having been made, they all got aboard the mermaid and started off.

it was a bright sunshiny day, and the craft, speeding away from the island where it had been constructed, over the dancing blue waves, must have presented a strange sight had there been any spectators. for surely no such ship had ever before sailed those waters.

however, there was no other vessel in sight, and the island, as far as the professor and his friends knew, had never been inhabited.

“we will not try for any great speed,” mr. henderson remarked as he, with mark and jack, stood in the conning tower managing the mermaid. “we don’t want to strain any joints at the start or heat any engine bearings. there will be time enough for speed later.”

“yes, and we may need it more when we get into the centre of the earth than we do now,” observed mark.

“why so?” asked jack.

“no telling what we may run up against underneath the ground,” went on mark. “we may have to fight strange animals and stranger beings. besides, the atmosphere and water there can’t be the same as up here; do you think so, professor?”

for a few minutes the scientist was silent. he seemed to be thinking deeply.

“i will tell you what i believe,” he said at length. “i have never spoken of it before, but now that we are fairly started and may eventually have a chance to prove my theory, i will say that i think the centre of this earth on which we live is hollow. inside of it, forming a core, so to speak, i believe there is another earth, similar to ours in some respects which revolves inside this larger sphere.”

they were well out to sea now, as they could observe when they emerged on the little deck. above their heads was the aluminum gas holder, which served as a sort of protection from the sun that was quite warm. the mermaid rode with an easy motion, being submerged just enough to make her steady, yet not deep enough to encounter much resistance from the water. in fact it could not have been arranged better for speed or comfort.

“i think we will sail well to the eastward before making our course south,” mr. henderson said. “i do not care to meet too many ships, as those aboard will be very curious and i do not want too much news of this venture to get out. we will take an unfrequented route and avoid delays by being hailed by every passing vessel whose captain will wonder what queer craft he had met with.”

the boys enjoyed the sail, for the weather could not have been better. even old andy, who seldom said much, seemed delighted with the prospect of having strange adventures. he had his rifle with him, and, indeed, he seldom went anywhere that he did not carry it.

“for there’s no telling when you may see something you want to shoot or that ought to be shot,” he used to say, “and it’s always the man without a gun who needs it most. so i’m taking no chances.”

they sailed all that afternoon without meeting with a craft of any kind. straight to the east they went, and when night began to settle down washington got supper. it was decided to run slowly after dark until all hands were more familiar with the ship.

morning found the mermaid about a hundred miles from the island where she had been launched. the night had been uneventful, except that mark told jack he heard some strange noise near his bunk several times. he was nearest the storeroom where spare parts, and the curious cylinder the professor had brought aboard, were kept.

“i guess it was rats,” said jack. “they are always in ships.”

“old wooden ships, yes,” admitted mark. “but i’ll bet there’s not a rat aboard the mermaid.”

“then you were dreaming,” said jack, as if that settled it.

mark did not speak further of the noise, but he did considerable thinking. however, the next night there was no further disturbance.

the fourth day out, when everything had passed off well, the engines doing their best, the professor decided to speed them up a bit, since he was satisfied they had “found” themselves as mechanics term it.

“we’ll see how fast we can go through the water,” said mr. henderson, “and then i think we can safely turn our course south. we are well beyond the ordinary lines of travel now.”

having oiled the bearings well, and seen that everything was in place and properly adjusted, the professor and the boys took their places in the conning tower, while washington, tom and bill remained in the engine room. andy stayed on deck with his gun.

“i might see a big fish, and we could vary our bill of fare,” he said with a laugh.

“here we go!” exclaimed the professor as he shifted the levers and turned some wheels and valves. “now we’ll see how fast we can travel.”

as he spoke the mermaid responded to the added impulse of the compressed air and shot through the water at a terrific speed. the sudden increase in momentum almost threw the boys from their feet, and they would have fallen had they not grasped some projecting levers.

“i guess that will do,” said the scientist. “i think we have speed enough for almost any emergency. i’ll let her run at this rate for a while, and then we’ll slack up.”

looking ahead, the boys could see the green waters parting in front of the bow of the mermaid, as if to make room for her. two huge waves were thrown upon either side.

suddenly, dead ahead, there loomed up a big black object.

“look out you’ll hit the rock!” cried mark to the professor, who was steering.

with a turn of his wrist mr. henderson moved the wheel which controlled the tube. it was deflected and sent the boat to larboard.

at that instant from the rock two small fountains of water rose in the air, falling back in a shower of spray through which the sun gleamed.

“that’s not a rock! it’s a big whale!” cried jack. “and we’re going to hit him!”

the professor had miscalculated the speed of the craft, or else had not thrown her far enough to larboard, for, a second later, the mermaid was almost upon the big leviathan.

with a desperate twirling of the steering wheel the professor veered the craft as far as possible. but all he could do did not suffice, for the craft hit the whale a glancing blow on the side, and the ship careened as if she would turn turtle.

at the same time there rang out from upon deck the sound of a rifle shot. old andy had taken a chance at the enormous creature of the deep.

“hurrah!” the boys heard him shout. “i give him one plumb in the eye! a fine shot! and we hit him besides with the boat. i guess he’s a goner!”

“i’m afraid not,” muttered the professor. “that was a bad blow we struck him, but i think it will only ruffle his temper. we’ll have to look sharp now, boys.”

by this time the ship had rushed past the whale, but the boys, looking through a window in the rear of the tower could see the huge body. now the fountains of water which the whale spouted were tinged with red.

“he’s bleeding!” exclaimed the professor. “i guess andy hit him in a vital spot.”

“but not vital enough!” cried mark. “see! he’s coming after us!”

and so it proved. the whale, angered, and, probably half crazed by the pain of the bullet and the blow, was coursing after the ship, coming on with the speed of an express train. straight at the mermaid he lunged his huge bulk.

“we must escape him!” cried mr. henderson. “if he hits us he’ll send us to the bottom!”

he had made ready to slow up the mermaid to see if it had sustained any damage from the impact with the whale, but when he saw the monster coming after the boat he knew the only safety lay in flight.

“let us go up into the air and so escape him!” cried jack, with sudden inspiration.

for an instant neither mark nor the professor grasped what jack meant. then, with an exclamation, the professor pulled forward the lever that generated the gas and set working the gravity neutralizer, which would enable the ship to rise.

faster through the water went the mermaid, and faster after her came the whale. above the hum of the engines was heard the hiss of the powerful gas. the ship trembled more violently.

“we are rising!” exclaimed the professor, as he looked at a gage.

the boys could feel the craft lifting from the waves which clung to her as if they hated to lose her. the boys knew the gas was beginning to operate.

“if it is not too late!” whispered mark, half to himself.

for the monster of the seas was coming on, lashing the water to foam with his terrible flukes, and sending aloft a bloody spray. his speed was awful.

now he was but ten feet away from the fleeing craft—now but eight—now five! ten seconds more and the big head, like the blunt stern of a battle ship, forced forward by the tons of blubber, flesh, bone and fat behind it would strike the mermaid and crush it like an egg shell.

now if ever was the need for the flying mermaid to prove herself worthy of the name. now, if ever, was the time for her to leave the watery element and take to the lighter one.

and she did. with a last tremble, as if to free herself from the hold of the waves, the gallant craft soared up into the air, leaving the water, which dripped from her keel like a fountain’s spray, and shooting aloft like a bird, escaped her terrible enemy which passed under her, so close that the lower part of the flying mermaid scraped the whale’s back.

“saved!” exclaimed the professor.