it was only in the nick of time, for a second later and the big mammal of the ocean would have struck the ship and split it from stem to stern.

higher and higher into the air mounted the flying mermaid, while in the water below, the whale, incensed by missing his prey, was lashing the waves to foam.

“well, that was a narrow squeak; as close as i ever care to come to it!” exclaimed andy as he let go of the steel rail to which he was clinging and entered the conning tower. “i had no idea of hitting the big fish.”

“i guess he would have taken after us whether you had fired at him or not,” said mr. henderson. “he was probably looking for trouble, and took the first thing that came in his way, which happened to be us. some whales are like that, so i have read; big bull creatures, exiled from the school to which they once belonged, they get like mad creatures and know neither friend nor foe. something like rogue elephants, i imagine.”

now, having thus unexpectedly risen into the air, the professor decided to continue travel in that style for a while at least. it would require less force to propel the ship, and the going would be more comfortable, since in the upper regions the mermaid rode on an even keel, while in the water there was more or less rolling, due to the action of the waves.

once recovered from their fright caused by the whale, and having lost sight of the enormous creature, for they were now far above the ocean, the adventurers began to think of something to eat.

washington lost little time in preparing a meal, and it was eaten with a relish. the electric cooking stove worked to perfection, for the colored man had learned how to use that aboard the porpoise and monarch, and could be depended on to turn out appetizing dishes.

“what do you say to traveling through the air at night?” asked mr. henderson, as he arose from the table.

“suits me,” replied mark. “there’s less danger than in the water, i think.”

bill, tom and washington arranged to stand the night watch, and, when the professor had examined the engines and given orders about keeping the ship on her course through the air, he retired to his bunk. jack and mark soon followed.

it must have been about midnight when mark was awakened by a movement that seemed to come from the storeroom next to where his sleeping place was located. at first he thought he had been dreaming, but, as he found he was wide awake, he knew it was no imagination that had affected him.

“i certainly heard something,” he said to himself. “it sounded just as it did the other night. i wonder if i ought to investigate.”

he thought over the matter carefully as he sat upright in his bunk in the darkness. true the noise might be a natural one, due to the vibration of the engine, or to some echo from the machinery. as mark listened he heard it again.

this time he realized it was the slow movement of some heavy body. he felt a cold shiver run over him and his hair evinced an uncomfortable tendency to stand upright. but he conquered his feelings and resolved to keep cool and see if he could discover what had awakened him.

he got up and moved softly about the little room that contained his bunk. he could hear better now, and knew it was no echo or vibration that had come to his ears.

once again he heard the strange sound. it was exactly the same as before; as if some big creature was pulling itself over the floor.

“maybe it’s a snake; a water snake!” thought mark. “it may have crawled aboard when we did not notice it.”

then he remembered that the ship had not been open in any way that would enable a serpent to come on it, since it had been started on its ocean trip. before that, he was sure no snake had entered the flying mermaid. still it sounded more like a snake than anything else.

“i’m going to make a search,” decided the boy.

he took a small portable electric light, run by a storage battery, and, slipping on a pair of shoes and a bath robe, he left his stateroom.

he had decided that the noise came from the storage compartment and so made for that. the door he knew was not locked, since he had seen mr. henderson go in late that afternoon, and the professor had used no key.

moving softly, mark left his room and soon found himself in a corridor, on either side of which were located the sleeping quarters of the others. he did not want to awaken them, and, perhaps, be laughed at for his curiosity.

to get to the storeroom mark had to go first from the corridor into the dining room. he soon reached the door that guarded what he thought might be a strange secret. trying the knob softly he found it giving under his fingers.

“i wonder if i had better go in,” he thought. “perhaps, after all, it was only rats, as jack said.”

but, even as he listened he again heard the odd sound coming from the room. this determined him. he would solve the mystery if possible.

cautiously he turned the knob. the door was slowly swinging open when mark was startled by a noise from behind him. he turned suddenly to see professor henderson confronting him.

“what is it, mark? is the ship on fire? what’s the matter? is any one hurt?”

“i was just going in this room to——” began mark.

“don’t do it! don’t do it!” exclaimed the professor in an excited whisper. “no one must go in that room. i forgot to tell you and jack about it. no one must enter. it contains a secret!”

“i heard a strange noise and——” mark began again.

“it could make no noise! it would be impossible for it to make a noise!” the professor exclaimed.

“i heard something,” the boy insisted.

“you were dreaming!” said the professor. “now go back to bed, mark, and don’t let this happen again. remember, no one must enter that room unless i give permission!”

somewhat crestfallen at the outcome of his investigations, but realizing that the professor could do what he wanted to aboard his own ship, mark went back to bed. but he could not sleep. all the rest of the night he was wondering whether mr. henderson had some strange creature hidden aboard the mermaid. he feared lest the old scientist’s mind might be affected and, in his wildness he had made some infernal machine that would, in time, blow the whole ship apart.

but tired nature asserted itself at last, and, weary with vain imaginings, mark fell into a slumber. the next morning he awoke with a start from a dream that he was being devoured by an immense water snake.

he said nothing to the others about his night’s adventure, for, as it transpired, no one else had been awakened by his investigations. the professor did not refer to his conversation with mark.

“there’s something queer going on aboard the ship this trip,” said mark to himself. “but i guess it’s none of my business. professor henderson seems to know what he is doing and i guess i can trust him.”

he resolved to think no more of the strange noises and movements, and, for several nights thereafter he was not disturbed by them.

the weather, which, up to this time had been fair, took a sudden turn for the worse about the fourth day after mark’s little night expedition. one evening the sun sank in a mass of dull lead-colored clouds and a sharp wind sprang up.

“we’re going to have a storm,” said mr. henderson. “it’s liable to be a bad one, too, from the way the barometer is falling.”

he looked at the glass, and scanned the various instruments that told how high up the mermaid was and how fast she was traveling.

“we’re pretty high up in the air,” he said, “and scooting along at about fifty miles an hour. we are going against the wind, too, but fortunately it is not blowing hard.”

at that moment there sounded from without a peculiar howling sound, as if a siren whistle was being blown.

“'pears like there’s goin’ t’ be a tumultuous demonstration of sub-maxiliary contortions in th’ empherial regions contiguous t’ th’ upper atmosphere!” exclaimed washington, entering from the engine room into the conning tower.

“what’s the trouble?” asked mr. henderson.

“terrible big black cloud chasin’ us from behind!” exclaimed the colored man.

noting the alarm in washington’s voice the professor glanced from the rear window. what he saw caused him to exclaim:

“it’s a cyclone! we must drop down to avoid it!”

he sprang to a lever controlling the gas and yanked it toward him. there was a shrill hissing sound, and a second later the mermaid began to sink. the boys watching the gages on the wall of the tower, saw that the craft was falling rapidly.

but, with a rush and roar, the terrible wind was upon them. it caught the craft in its fearful grip and heeled it over as a ship careens to the ocean blast.

“it’s a storm in the upper regions! we’ll find it calm below!” cried the professor above the howling of the gale. he opened the gas outlet wider and the ship fell more rapidly.

“are you sure we’re over the ocean?” asked mark.

“positive!” the professor called back. “we have been traveling straight south over the atlantic for the last week. we will land in the midst of the waters and float safely.”

lower and lower went the mermaid. the wind was now blowing with the force of a tornado, and, as the craft had to slant in order to descend, it felt the power of the gale more than if it had scudded before it. but, by skilful use of the directing tube, the professor was able to keep the boat from turning over. as they came further down toward the earth the force of the wind was felt less and less, until, as they came within two hundred feet of the water which they saw below them in the gathering dusk, it died out altogether.

“now we are free from it,” said the professor as the mermaid came down on the waves like an immense swan.

“are you going ahead or going to stop here?” asked mark.

“we’ll keep right on,” mr. henderson answered. “no telling when the storm may strike down here. we’ll go as far as we can to-night.”