Lovejoy's, with some medicine for his Sue," returned the boy, recognizing the farmer.

"How is the gal gettin' on?" called the other, over the canvas top of his seat.

"Fine. No danger, dad says!" answered Frank.

"That's good!" he heard the sympathetic neighbor remark, as he moved on.

Five minutes later and Frank once more found himself approaching the Whympers place. As before, the house was in complete darkness, as if the inmates were long since abed. Frank knew that the old man kept early hours, seldom sitting up, for he read much during the day, having nothing else to look after.

Then, as was only natural, the eyes of the bicycle boy turned once again with more or less affection toward the quarter where he could just dimly make out the long, squat shed out in the field, in which the precious monoplane was stored.

As he did so Frank uttered an exclamation of surprise.

"Why, there's a light over by the hangar!" he burst out. "Now, what under the sun do you suppose that old fool of a Shea can be doing? Oh, my! Look at the flame jump up! Why, as sure as you live I believe the shed's afire! And I can see the figure of a man moving about. This is no accident, but something worse! And it looks as if the little 'Bug' might be going up in smoke in a jiffy unless I can sprawl over the fence here and get on the spot mighty quick!"

CHAPTER V.

A WARM FIVE MINUTES.

"Fire! Fire!"

So Frank shouted, even as he jumped over the fence, and made a bee line for the center of the big field, where the shed lay in which the precious monoplane was stored.

He had hastily leaned his bicycle against the fence as he made the plunge. Nor did he cease to let out constant yells while running across the open as fast as his agile legs could carry him. Twice he tripped over some object and nearly fell, only to recover himself and speed on.

As he ran he kept his eyes upon the low building beyond. In this manner he plainly saw the stooping figure of a man or boy making off in a roundabout way so as to avoid him.

Frank's heart was burning with indignation because of this dastardly attempt to ruin the gallant little airship that had so nobly stood all tests and proven itself a splendid piece of workmanship.

"Oh, the contemptible coward! I'd just like to chase after him and get my hands on him once!" was the thought that passed through his brain.

But he knew he could not. The scoundrel, no matter who it was, must be allowed to escape in order that he turn his attention to the burning shed and try to save the airship from destruction. Once the fire got inside, there was enough of the dangerous gasoline about to insure the speedy burning of the whole flimsy fabric, all but the motor itself.

So Frank kept headed straight for the hangar, trying to shut out the sight of that crouching, fleeing figure. He continued to lift his sturdy young voice in repeated shouts:

"Fire! Fire!"

Those in the house must hear; yes, and the neighbors, too. He might not be able to master the flames alone and single handed, and would need help. Besides, it was only right that Andy, being part owner in the monoplane, should be made aware of its sudden peril.

As he thus drew near the low building he saw that the fire had already gained considerable headway, just as if the incendiary might have used kerosene or some other inflammable fluid, to hasten matters.

Frank's heart grew cold as ice as he contemplated the rapidity with which those hungry flames were crawling up the dry boards that constituted the side of the shed.

But he did not lose his head in this sudden crisis. It was characteristic of Frank Bird that, no matter what the emergency, he was always cool enough to think out the proper thing to be done or else jump at it through instinct.

And Frank had foreseen just some such possible need as this. He even kept several buckets of moist sand handy, where it could be snatched up at a second's warning, knowing that most fires can be smothered, when quenching them with water is out of the question.

"The buckets!" he gasped, as he arrived close to the building, one part of which was now fairly covered with the creeping tongues of ruddy fire. "I must use them on it!"

He had to turn the corner of the shed to get to where they stood. And as he did so he ran plump into a figure that was coming toward him. Just in time did Frank dodge a big fist that shot out. And in that second he recognized in the other Shea, the Irishman who had been hired to keep watch of the shed.

"Hold on, Shea!" shouted Frank. "It's me, Frank Bird. Somebody has set fire to the shed! Grab up a bucket of sand and carry it around here. We can put it out yet if we're busy!"

Shea had evidently only been aroused from a sound sleep by the approaching cries of the boy and was still in a daze. He had discovered the fire, and hearing Frank running toward him, supposed that this must be the one who had done the evil deed.

But he had sense enough to do as he was told now, snatching up one of the sand buckets and following the boy. Frank immediately commenced fighting the flames with a vim. He slapped the wet sand at the creeping fire, and wherever it struck there seemed to come a quick abatement of the conflagration. But it was by this time so extended that as fast as he succeeded in knocking it out in one place it cropped up afresh somewhere else.

His ammunition would not last if this kept up.

"Get busy, Shea!" he cried. "Find something and slap at the fire for all you're worth! Fight it, man, fight it!"

As Frank happened to turn his head to learn what the other was doing he saw something that made very little impression on his mind just then, but which had considerable bearing on the matter later.

A light was speeding along the road, heading away from town, and Frank realized that the firebug had seized upon his convenient wheel and was making his escape.

Later on he might figure out the meaning of this movement. Just then he really had no time to give it a thought, no matter if a dozen wheels were concerned. The fire demanded every atom of his attention.

Shea did get busy. Once he became stirred up, and he proved a valuable helper. He went for the flames tooth and nail, smothered them with his coat, regardless of consequences, after he had slipped that article of wearing apparel off; kicked and tore and fought until it became evident that between them they were certainly making a decided impression on the threatening conflagration.

All this while it seemed to Frank that his heart was in his throat. Not so much because he feared that they would fail to gain the mastery over the fire as that some spark might find ingress to the shed and happen to alight upon a can of the dangerous gasoline.

If such a thing occurred he knew that it would be all over. The hangar must be completely destroyed and, of course, their little darling airship would share in its fate.

So, even though he saw the end of the conflagration in sight, Frank knew he had no reason to breathe easily until every spark had been trampled under foot.

By now he was conscious of loud shouts coming from points near at hand and realized that doubtless Andy as well as others had been awakened by the racket and were coming on the run to assist. Had the safety of the airship depended on their reaching the shed in time, though, its chances would have been next