"This is certainly the strangest sight I ever saw," remarked Ned, as he and his chum flew nearer and nearer to the smoking and blazing tree. "Is the world turning upside down, Tom, when fires start in this fashion?"

"I fancy it can easily be explained," answered the young inventor. "We'll go into that later. Here, Ned, grab hold of that tin can on the floor and take out the screw plug."

"What's the idea?"

"I want you to drop it as nearly as you can right into the midst of the tree that's on fire."

"Oh, I get your drift! Well, you can count on me."

Ned picked up from the floor of their aeroplane a metal can similar to those Tom used to hold oil or perhaps spare gasoline when he was experimenting on airship speed. The opening was closed with a screw plug, with wings to afford an easier grip. As Ned unscrewed this his nostrils were greeted by an odor that made him gasp.

"Don't mind a little thing like that," cried Tom. "Drop it down, Ned! Drop it down! We're going to be right over the tree in another second or two!"

Ned leaned over the side of the craft and had a good view of the strange sight. The tree that was on fire was a dead oak of great size, dwarfing the other trees in the grove in which it stood. In common with other oaks this one still retained many of its dried leaves, though it was devoid, or almost devoid, of life. Ned noticed in the branches many irregularly shaped objects, and it appeared to be these that were on fire, blazing fiercely.

"It looks as though some one had tied bundles of sticks in the tree and set them on fire," Ned thought as he poised the opened tin of the evil-smelling compound on the edge of the aeroplane's cockpit.

"Let her go, Ned!" cried Tom. "You'll be too late in another second!"

Ned raised himself in his seat and threw, rather than let fall, the can straight for the blazing tree. Like a bomb it shot toward earth, and Ned and Tom, looking down, could see it strike a limb and break open, the rupture of the can letting loose the liquid contained in it.

And then, before the eyes of Tom and Ned, the fire seemed to die out as a picture melts away on a moving picture screen. The smoke rolled away in a ball-like cloud, and the flames ceased to crackle and roar.

"Well, for the love of molasses! what happened, Tom?" cried Ned, as the young inventor guided his craft about in a big circle to come back again over the tree. He wanted to make sure that the fire was out.

It was!

"What sent that blaze to the happy hunting grounds?" asked Ned.

"My new aerial extinguisher," answered Tom, with justifiable pride in his voice. "This fire happened in the nick of time for me, Ned. I had a tin of my new combination in the car, not with any intention of using it, though. I intended to pour it in the new containers I am having made in Newmarket to see if it would corrode them, a thing I wish to avoid.

"But when I saw that tree on fire I couldn't resist the temptation to use my very latest combination of chemicals. It is so recent that I haven't actually tried it on a blaze yet, though I had figured out in theory that it ought to work. And it did, Ned! It worked!"

"Well, I should say so!" agreed his chum. "That blaze was doused for fair. The test could not have been better. But what in the name of a volunteer fire department set that tree to blazing, Tom?"

"I'll tell you in a moment. I want to make some notes before I forget. That combination seems to be just of the right strength. It did the trick. Here, take the wheel and hold her steady while I jot down some memoranda before they get away from me."

Ned was capable of managing an airship, especially under Tom's watchful eye, and as this craft was one with dual controls there was no difficulty in shifting from one steersman to the other.

So while Ned guided, now and then gazing down at the tree from which some smoke still arose, though the fire was all out, Tom made the necessary scientific notes for future amplification.

"And now," observed Ned, as his chum resumed the wheel, "suppose you enlighten me on how that tree came to be on fire--if you didn't set it yourself."

"No, I didn't do that," Tom said, with a laugh. "And I only have a theory as to the cause of the blaze. But suppose we go down and take a look. There's a good field around this grove, and we can get a fine take off. I'll have to go back to Shopton anyhow, to get some more of the chemical."

So the aeroplane made a landing, and then the mystery was explained. The dead oak, to which some of its last year's foliage still clung, was the abiding place of thousands of crows that had built their nests in it. There were hundreds of the big nests, made of dried sticks, mostly, and these made an ideal fuel for the fire.

"But where are the crows, and what started the fire?" asked Ned.

"I fancy the birds flew away as soon as they saw their homes on fire," said Tom. "Or they may not have been at home. Flocks of crows often go to some distant feeding ground for the day, returning at night. I fancy that is what happened here.

"As for the cause of the blaze, I believe it was set by some mischievous boys, who saw a good chance to have some fun without thought of doing any real damage. For the dead tree was of no value, and I imagine the farmers would be glad to see the flock of crows dispersed. Some boys probably climbed up and set fire to one of the nests, and then, when they saw the whole lot going, they became frightened and ran away."

And Tom's theory was, eventually, proved to be true. Some lads, wandering afield, had set fire to the crows' nests and then, frightened as they saw a bigger blaze than they intended, ran away.

Tom and Ned did not remain to see what the returning crows might think about the destruction of their homes, provided they saw fit to return, but, starting the aeroplane, were again on their way.

Tom had lingered long enough to make sure that his latest combination of chemicals had been just what was needed. He felt sure that by using a larger quantity, no fire, however fierce, could continue to blaze.

"But I want to give it a good trial, Ned, as we did from the tower," said Tom. "Though I don't believe there'll be a fizzle this time."

It did not take long for Tom to secure another supply of the new chemical. He then went with it to the firm in Newmarket that was making his containers, or "bombs" as he called them.

On his return he consulted with Mr. Baxter as to the ingredients of the fluid that had put out the blaze in the tree.

"I believe you have at last hit on the right combination," said the chemist. "You are on the road to success, Tom. I wish I could say the same of myself."

"Perhaps your formulae may come back to you as suddenly as they disappeared, or as quickly as I discovered that I had the right thing to put out the fire," said Tom hopefully.

Busy days followed for the young inventor. Now that he was convinced he had at last evolved the right mixture of chemicals, he prepared to make a test on a larger scale than merely a blazing tree.

"I'll try it with a fire in the pit," he said to his chum.

Preparations were made, and the day before Tom was to carry out his plans he received a letter.

"What's the matter? Bad news?" asked Ned, as he saw his friend's face change after reading the epistle.

"Nothing much. Only Mary is going away, and I had expected her to be at the test," Tom answered.

"Going away?" echoed Ned. For long?"

"Oh, no, only for a couple of weeks. She is going to visit an uncle and aunt in Newmarket, or just outside of that city. Another uncle, Barton Keith, has offices in the Landmark Building, I believe."

"Landmark Building," murmured Ned. "Isn't that where Field and Melling hang out?"

"Yes. But don't mention Mary's uncle in connection with them," laughed Tom. "He wouldn't like it."

"I should say not!"

Ned well remembered Mary's uncle, who had been associated with Tom in recovering the treasure in the undersea search.

"Well, if she can't be here, she can't," said Tom, as philosophically as possible. "I'd better run over and bid her goodbye."

This Tom did, though Ned noticed that his chum acted as though lonesome on his return.

"But when he gets to work testing his new chemical he'll be all right," decided Ned.