Suddenly, above the noise of the conflict, Tom Swift heard an ominous sound.  It was a hissing in the air, and well he knew what it was.

“The gas bag!” he cried.  “They’ve punctured it!  The vapor is escaping.  If they put too many holes in the bag it will be all up with us!”

“What’s to be done?” asked Mr. Durban.

“If we can’t drive them back we must retreat ourselves!” declared Tom desperately.  “Our only hope is to keep the airship safe from harm.”

Once more came a rush of the savages.  They had discovered that the gas bag was vulnerable, and were directing their arrows against that.  It was punctured in several more places.  The gas was rapidly escaping.

“We’ve got to retreat!” yelled Tom.  He hurried to the engine-room, and turned on the power.  The great propellers revolved, and sent the Black Hawk scudding across the level plain.  With yells of surprise the red dwarfs scattered arid made way for it.

Up into the air it mounted on the broad wings.  For the time being our friends has been driven back, and the missionaries whom they had come to rescue were still in the hands of the savages.

CHAPTER XXII

A NIGHT ATTACK

“Well, what’s to be done?”

Tom Swift asked that question.

“Bless my percussion cap!  They certainly are the very worst imps for fighting that I ever heard of,” commented Mr. Damon helplessly.

“Is the gas bag much punctured?” asked Ned Newton.

“Wait a minute,” resumed the young inventor, as he pulled the speed lever a trifle farther over, thereby sending the craft forward more swiftly, “I think my question ought to be answered first.  What’s to be done?  Are we going to run away, and leave that man and woman to their fate?”

“Of course not!” declared Mr. Durban stoutly, “but we couldn’t stay there, and have them destroy the airship.”

“No, that’s so,” admitted Tom, “if we lost the airship it would be all up with us and our chances of rescuing the missionaries.  But what can we do?  I hate to retreat!”

“But what else is there left for us?” demanded Ned.

“Nothing, of course.  But we’ve got to plan to get the best of those red pygmies.  We can’t go back in the airship, and give them open battle.  There are too many of them, and, by Jove!  I believe more are coming every minute!”

Tom and the others looked down.  From all sides of the plain, hastening toward the village of mud huts, from which our friends were retreating, could be seen swarms of the small but fierce savages.  They were coming from the jungle, and were armed with war clubs, bows and arrows and the small but formidable blowguns.

“Where are they coming from?” asked Mr. Damon.

“From the surrounding tribes,” explained Mr. Durban.  “They have been summoned to do battle against us.”