there was a sudden tremor all over the airship as washington, in the engine room, in obedience to the signals, turned off the power. then sounded a hiss as the captain let some gas from the bag. the ship began to sink toward the earth.

the black cloud that the professor had been gazing at came nearer. it grew larger and seemed to be made up of a number of small moving objects.

"quick, andy!" cried the old inventor. "we shall need your services now!"

"what's the matter?" exclaimed the old hunter, as he hurried forward with his gun in readiness.

"eagles!" cried amos henderson.

"eagles?"

"yes! a whole flock of them. just ahead! see that dark cloud! they are coming this way! they think the ship is a rival bird and they will attack it. strong as the monarch is, the silk in the gas bag is frail. if the birds tear that we will fall to the earth and be killed! use your gun! see if you can drive them off!"

andy kneeled down on the forward part of the ship. he aimed at the black mass, in which scores and scores of birds could now be seen. then his gun sent out fire and lead.

bang! bang! it spoke, and two birds dropped toward the earth. again the gun belched forth, and more of the eagles were killed. as fast as andy could pull the trigger he fired.

"we must all get guns!" cried the professor. "it is the only way to save the ship! come on, boys! you'll find weapons in the dining-room lockers!"

mark and jack hurried after the rifles. the professor was greatly excited. bill and tom came running forward. the inventor rapidly handed out the guns.

in the meanwhile the ship was slowly settling toward the ground. the captain hoped to get low enough to escape the onward rush of the big birds, but he had counted without the anger of the eagles. they thought the airship was a rival in the realms of space and were determined to destroy it.

on and on they came in spite of the number among them that were killed. every one on the ship, except washington, who had to attend to the engines, was firing. the birds never stopped or swerved from their course.

then with a rush and roar, a flapping of wings that sounded like thunder, and shrill cries and screams that almost drowned the noise of the guns, the eagles surrounded the monarch. they struck at it with their talons. they opened wide their sharp beaks and snapped at the wood and iron.

some of the fierce birds even attacked the men, and boys, and were beaten off with the butts of the rifles. others of the eagles rose higher in the air and struck at the oiled silk bag. at first the yielding surface offered no resistance and was not damaged. then one fierce bird, with wide-opened beak, struck at the thin cloth and tore a hole in it as large as a man's hand.

the sudden settling of the airship told that something was wrong. then the professor, glancing aloft, saw what had happened, and hastened to his helper.

"quick, washington!" he shouted. "start the gas generator at full speed! we must pump lots of the gas in to keep us afloat! we are in great danger!"

"why not try the machine gun on the eagles?" shouted jack.

"good idea!" exclaimed the inventor. "you two boys work it!"

at last the eagles, alarmed by the number killed, and frightened by the noise of the guns and the shots, halted in their rushes at the airship. some of the wounded ones wheeled away. then others followed until, finally, the whole colony of birds sailed off.

"there they go!" cried jack.

"yes, but i fear too late to do us any good," spoke the professor. "the airship is slowly settling."

"can't it be fixed?" asked mark.

"i suppose i could let it down to earth and patch up the hole, but i fear to do so," answered the inventor. "the monarch is not under control, and if i attempt to make a landing i may smash her all to pieces. she may settle down until within a few hundred feet of the earth and then plunge like a meteor. we would all be killed then."

"is there no other way?" asked jack.

"none, unless we could patch up the hole in the gas bag while we are up aloft. i can hold the ship there for a while yet. another reason why i do not want to land is that we are over a thickly settled portion of the state now, and if i go down to earth we will be surrounded by a curious crowd that will delay us."

"is that netting strong?" asked mark, suddenly, pointing to the cords that confined the gas bag.

"two strands would support a man's weight," said mr. henderson.

"and have you anything to mend the silk bag with?" went on the boy.

"yes, but why do you ask?"

"because," answered mark, "if you'll let me i'll climb up and mend the hole the eagle made."

"dare you do it?" cried the old professor, hope shining in his face.

"try me and see."

the professor quickly prepared a piece of silk, kept on hand to repair breaks in the bag. it was coated with a very strong and fresh cement. the silk was to be inserted in the tear made by the eagles, when it would at once harden and prevent the further escape of gas.

mark made ready for the perilous ascent. he took off his coat, and removed his shoes so his feet could better cling to the frail-looking though strong cords.

"slow down the ship!" commanded the captain. "now, mark, try! i hope you succeed! move cautiously. you don't want to lose your life!"

mark said nothing. he grasped the piece of oiled silk, coated with the cement, in his teeth, clinching it by a strip that was free from the sticky substance. then he stood on the rail of the monarch and began his climb aloft. surely few ascents were made under such fearful conditions. the airship was now more than a mile above the earth. one false step and the boy would plunge into eternity. nothing could save him.

up and up he went, testing every cord and mesh before he trusted his weight to it. on and on he advanced. the frail gas bag swayed in the wind that was springing up. it seemed like a thing alive.

"careful! careful!" cautioned the professor in strained tones. everyone on the ship held his breath. up and up mark went. at last he reached the place where the eagle's beak had torn the bag.

he braced himself in the meshes of the net. then, leaning forward, he fixed the patch under the rent, and pressed it into place. the cement did not take hold at first. mark pressed harder. would the leak be stopped?

"will he make it?" asked one.

"i don't think so."

"he must make it!"

"if not we are lost!"

"you are right!"

for a moment there was a doubt. then the sticky stuff adhered to the silk bag, and the patch was made fast. a shout from washington in the engine room told that the gas had ceased to rush out. mark had succeeded.

washington hastened to turn the gas generator to half speed. before he could do so, however, there had been a great increase in the volume of vapor in the bag, caused by the sudden stopping off of the vent. up shot the airship, the accumulation of gas lifting it higher from the earth. so suddenly did it shoot up, from having been almost at rest, that there was a tremor through the whole craft.

"look out, mark!" cried jack. he looked up to where his comrade clung to the netting.

"hold fast! we'll stop the ship in a second," exclaimed the captain.

but it was too late. the sudden rising of the craft had shaken mark's hold, which was not of the best at any time, since the gas bag was a yielding surface to lean against.

the next instant the boy, vainly clutching the air for some sort of grip for his hands, toppled over backward. his feet slid from the meshes of the net, and he plunged downward toward the earth, more than a mile below!