"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 quant.i.ty, 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 a.s.sociated 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.

CHAPTER XIII

A SUCCESSFUL TEST

"It took you long enough," Ned remarked as Tom entered the main office of the plant, having been to see Mary off on her trip to Newmarket.

This was following his call of the night before to learn more particulars of her unexpected visit.

"Yes, I didn't plan to be gone so long," apologized Tom. "But I thought while I was there I might as well go all the way with her."

"And did you?"

"Yes. In the electric runabout. I wanted to come back and get the airship, but she said she wanted to look nice when she met her relatives, and as yet airship travel is a bit mussy. Though when I get my cabined cruiser of the clouds I'll guarantee not to ruffle a curl of the daintiest girl!"

"Getting poetical in your old age!" laughed Ned. "Well, here is that statement you said you wanted me to get ready. Want to go over it now?"

"No, I guess not, as long as you know it's all right. I'm going to start right in and get ready for a bang-up test."

"Of what--your new aerial fire fighting apparatus?"

"Yes. Mr. Baxter and I are going to make up a lot of the chemical compound that--we discovered through using it on the blazing tree--will best do the trick. Then I'm going to try it on a pit fire, and after that on a big blaze with an airship."