Tom and Ned were so startled by the entrance of the excited man with his cry of "Fire!" that the young inventor nearly dropped the tank of liquid extinguisher he was helping to hoist into the aeroplane. Then, as he caught sight of his visitor, Tom exclaimed:

"Hello, Mr. Damon! We were wondering whether you'd be along to witness our first experiment."

"Experiment, Tom Swift! Experiment! Bless my Latin grammar! but you'd much better be calling out the fire department to play on that blaze down in your meadow. What is it--your barns or one of your new shops?"

"Neither one, Mr. Damon," laughed Ned. "It's only a blaze that Koku and Rad started."

"And the fire department is here," added Tom.

"Where?" inquired the eccentric man.

"Here," and Tom pointed to his airship--one of the smaller craft--into which the tank of chemicals had been hoisted.

"Oh!" exclaimed Mr. Damon. "Something new, eh, Tom?" His eyes glistened.

"Yes. Fighting fires from the air. I got the idea after the fireworks factory went up in smoke. Will you come along? There's plenty of room."

"I believe I will," assented Mr. Damon. It was not the first time, by any means, that he had gone aloft with Tom. "I happened to be coming over in my auto," he went on to explain, "when I happened to see the fire down in the meadow. I was afraid you didn't know about it."

"Oh, yes," replied Tom. "I had Rad and Koku light a big pile of packing boxes, to represent, as nearly as possible, on a small scale, a burning building. I plan now to sail over it and drop the tins of chemicals. They are arranged to burst as they fall into the blaze, and I hope the carbon dioxide set loose will blanket out the fire."

"Sounds interesting," commented Mr. Damon. "I'll go along."

The airship was wheeled out of the hangar and was soon ready for the flight. A big cloud of black vapor down in the meadow told Tom and Ned that Koku and Eradicate had done their work well. The giant and the colored man had poured oil over the wood to make a fierce blaze that would give Tom's new chemical combination a severe test.

A mechanic turned the propeller of the airship until there was an accumulation of gas in the different cylinders. Then he stepped back while Tom threw on the switch. This was not one of the self-starting types, of which Tom possessed one or two.

"Contact!" cried Tom sharply, and the man stepped forward to give the big blades a final turn that would start the motor. There was a muffled roar and then a steady staccato blending of explosions. Tom raced the motor while his men held the machine in place, and then, satisfied that all was well, the young inventor gave the word, and the craft raced over the ground, to soar aloft a little later.

Tom, Ned and Mr. Damon could look down to the meadow where the bonfire was blazing. A crowd had collected, but the heat of the blaze kept them at a good distance. Then, as many of the throng caught sight of the airship overhead, there was a new interest for them.

Tom had told Ned and Mr. Damon, before the trio had entered the machine, what he wanted them to do. This was to toss the chemicals overboard at the proper time. Of course in his perfected apparatus Tom hoped to have a device by which he could drop the fire extinguishing elements by a mere pressure of his finger or foot, as bombs were released from aircraft during the war. But this would serve for the time being.

Nearer and nearer the blaze the airship approached until it was almost above it. Tom had had some experience in bomb-dropping, and knew when to give the signal.

At last the signal came. Mr. Damon and Ned heaved over the side the metal containers of the powerful chemicals.

Down they went, unerring as an arrow, though on a slant, caused by the impetus given them by the speed of the airship.

Tom and his friends leaned over the side of the machine to watch the effect. They could see the chemicals strike the blaze, and it was evident from the manner in which the fire died down that the containers had broken, as Tom intended they should to scatter their contents.

"Hurray!" cried Ned, forgetting that he could not be heard, for no head telephones were used on this occasion and the roar of the motor would drown any human voice. "It's working, Tom!"

Truly the effect of the chemicals was seemingly to cause the fire to go out, but it was only a momentary dying down. Koku and Rad had made a fierce, yet comparatively small, conflagration, and though for a time the gas generated by Tom's mixture dampened the blaze, in a few seconds--less than half a minute--the flames were shooting higher than ever.

Tom made a gesture of disappointment, and swung his craft around in a sharp, banking turn. He had no more chemicals to drop, as he had thought this supply would be sufficient. However, he had guessed badly. The fire burned on, doing no damage, of course, for that had been thought of when it was started in the meadow.

"Something wrong!" declared the young inventor, when they were back at the hangar, climbing out of the machine.

"What was it?" asked Ned.

"Didn't use the right kind of chemicals," Tom answered. "From the way the flames shot up, you'd think I had poured oil on the blaze instead of carbon dioxide."

"Bless my insurance policy, Tom!" cried Mr. Damon, "but I'd hate to trust to your apparatus if my house caught."

"Don't blame you," Tom assented. "But I'll do the trick yet! This is only a starter!"

During the next two weeks the young inventor worked hard in his laboratory, Mr. Swift sometimes helping him, but more often Koku and Eradicate. Mr. Baxter had recovered sufficiently to leave the Swift home. But though the chemist seemed well physically, his mind appeared to be brooding over his loss.

"If I could only get my secret formulae back!" he sighed, as he thanked Tom for his kindness. "I'm sure Field and Melling have them. And I believe they got them the night of the fireworks blaze; the scoundrels!"

"Well, if I can help you, please let me," begged Tom. And then he dismissed the matter from his mind in his anxiety to hit upon the right chemical mixture for putting out fires from the air.

One afternoon, at the end of a week in which he had been busily and steadily engaged on this work, Tom finally moved away from his laboratory table with a sigh of relief, and, turning to Eradicate, who had been helping him, exclaimed:

"Well, I think I have it now!"

"Good lan' ob massy, I hopes so!" exclaimed the colored man. "It sho' do smell bad enough, Massa Tom, to make any fire go an' run an' drown hisse'f! Whew-up! It's turrible stuff!"

"Yes, it isn't very pleasant," Tom agreed, with a smile. "Though I am getting rather used to it. But when it's in a metal tube it won't smell, and I think it will put out any fire that ever started. We'll give it a test now, Rad. Just take that flask of red stuff and pour it into this one of yellow. I'll go out and light the bonfire, and we'll make a small test."

Leaving Rad to mix some of the chemicals, a task the colored man had often done before, Tom went out into the yard near his laboratory to start a blaze on which his new mixture could be tested.

He had not got far from the laboratory door when he felt a sudden jar and a rush of air, and then followed the dull boom of an explosion. Like an echo came the voice of Eradicate:

"Oh, Massa Tom, I'se blowed up! It done sploded right in mah face!"