hat 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