as tom passed down the hall on his way to the side door, from which he could more quickly reach the aeroplane shed, he saw his father coming from his room.

"what's the matter? what is it?" asked mr. swift, and alarm showed on his pale face.

"it's nothing much, dad," said the youth, as quietly as he could, for he realized that to excite his father might have a bad effect on the invalid.

"then why are you in such a hurry? why have you that revolver? i know there is something wrong, tom. i am going to help you!"

in his father's present weakened state tom desired this least of all, so he said:

"now, never mind, dad. i thought i heard a noise out in the yard, and i'm not going to take any chances. so i roused mr. jackson, and i'm going down to see what it is. perhaps it may only be eradicate's mule, boomerang, kicking around, or it may be rad himself, or some one after his chickens. don't worry. mr. jackson and i can attend to it. you go back to bed, father."

tom spoke with such assurance that mr. swift believed him, and retired to his room, just as the engineer, partly dressed, came hurrying out in response to tom's summons. he had his rifle, and, had the invalid inventor seen that, he surely would have worried more.

"come on!" whispered tom. "don't make any noise. i don't want to excite my father."

"what was it?" asked the engineer.

"i don't know. burglar alarm went off, that's all i can say until we get to the shed."

together the two left the house softly, and soon were hurrying toward the aeroplane shed.

"look!" exclaimed mr. jackson. "didn't you see a light just then, tom?"

"where?"

"by the side window of the shed?"

"no, i didn't notice it! oh, yes! there it is! some one is in there! if it's andy foger, i'll have him arrested, sure!"

"maybe we can't catch him."

"that's so. andy is a pretty slippery customer. say, mr. jackson, you go around and get eradicate, and have him bring a club. we can't trust him with a gun. tell him to get at the back door, and i'll wait for you to join me, and we'll go in the front door. then we'll have 'em between two fires. they can't get away."

"how about the windows?"

"they're high up, and hard to open since i put the new catches on them. whoever got in must have forced the lock of the door. there goes the light again!"

as tom spoke there was seen the faint glimmer of a light. it moved slowly about the interior of the shed, and with a peculiar bobbing motion, which indicated that some one was carrying it.

"go for eradicate, and don't make any more noise than you can help in waking him up," whispered tom, for they were now close to the shed, and might be heard.

mr. jackson slipped off in the darkness, and tom drew nearer to the building that housed his humming-bird. there was one window lower than the others, and near it was a box, that tom remembered having seen that afternoon. he planned to get up on that and look in, before making a raid to capture the intruder.

tom raised himself up to the window. the light had been visible a moment before he placed the box in position, but an instant later it seemed to go out, and the place was in darkness.

"i wonder if they've gone away?" thought tom. "i can't hear any noise."

he listened intently. it was dark and silent in the shop. suddenly the light flashed up brighter than before, and the young inventor caught sight of a man walking around the new aeroplane, examining it carefully. he carried, as tom could see, a large-sized electric flash-lamp, with a brilliant tungsten filament, which gave a powerful light.

as the youth watched, he saw the intruder place the light on a bench, in such a position that the rays fell full upon the humming-bird. then, adjusting the spring switch so that the light would continue to glow, the man stepped back and drew something from an inner pocket.

"i wonder what he's up to?" mused tom. "i wish eradicate and mr. jackson would hurry back. who can that fellow be, i wonder? i've never seen him before, as far as i know. i thought sure it was going to turn out to be andy foger!"

tom turned around to look into the dark yard surrounding the shed. he was anxious to hear the approach of his two allies, but there was no sound of their footsteps.

as he turned back to watch the man he could not repress a cry of alarm, for what the intruder had drawn from his pocket was a small hatchet, and he was advancing with it toward the humming-bird!

"he's going to destroy my aeroplane!" gasped tom, and he raised his revolver to fire.

he did not intend to shoot at the man, but only to fire to scare him, and thus hasten the coming of mr. jackson and the colored man. but there was no need of this, for an instant later the two came running up silently, eradicate with a big club.

"whar am he?" he asked in a hoarse whisper. "let me git at him, massa tom!"

"hush!" exclaimed the young inventor. "we have no time to lose! he's in there, getting ready to chop my aeroplane to bits! go to the back door, rad, and if he tries to come out don't let him get away."

"i won't!" declared the colored man emphatically, and he shook his club suggestively.

"come on! we'll go in the front door," whispered tom to the engineer. "i have the key. we'll catch him red-handed, and hand him over to the police."

waiting a few seconds, to enable eradicate to get to his place, tom and the engineer stole softly toward the big double doors. every moment the youth expected to hear the crash of the hatchet on his prize machine. he shivered in anticipation, but the blows did not fall.

tom pushed open the door and stepped inside, followed by mr. jackson. as they did so they saw the man standing in front of the humming-bird. he again raised the little hatchet, which was like an indian tomahawk, and poised it for an instant over the delicate framework and planes of the air craft. then his arm began to descend.

"stop!" yelled tom, and at the same time he fired in the air.

the man turned as suddenly as though a bullet had struck him, and for a moment tom was afraid lest he had hit him by accident; but an instant later the intruder grabbed up his flashlight, and holding it before him, so that its rays shone full on tom and mr. jackson, while it left him in the shadow, sprang toward them, the hatchet still in his hand.

"look out, tom!" cried mr. jackson.

"out of my way!" shouted the man.

bravely tom stood his ground. he wished now that he had a club instead of his revolver. the would-be vandal was almost upon him. mr. jackson clubbed his rifle and swung it at the fellow. the latter dodged, and came straight at tom.

"look out!" yelled the engineer again, but it was too late. there was the sound of a blow, and tom went down like a log. then the place was in darkness, and the sound of footsteps in rapid flight could be heard outside the shed.

the intruder, after wounding the young inventor, had made his escape.