"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."

"Let me know when you do," begged Ned. "I want to see you do it."

"I'll send you word," promised the young inventor.

Then he began several days and nights of hard work. And he was glad to have the chance to occupy himself, for, though Tom professed not to be much affected by the departure of Mary Nestor, he really was very lonesome.

"How is her uncle, Barton Keith, by the way?" asked Ned, when he called on his chum one day, to find him reading a letter which needed but half an eye to tell was from Mary.

"About as usual," was the answer. "He sends word by Mary that he'll be glad to see us any time we want to call. He has some nice offices in the Landmark Building."

"Those papers proving his right to the oil land, which you recovered from the sunken ship for him, must have made his fortune."

"Well, yes--that and other things," agreed Tom. "Say, we had some exciting times on that undersea search, didn't we?"

"Did you call on Mr. Keith when you went to Newmarket with Mary?" Ned wanted to know, for he and Tom had taken quite a liking to Miss Nestor's uncle.

"No, I didn't get a chance. Besides, I wanted to keep away from the Landmark Building."

"Why?"

"Oh, I might run into Field and Melling, and I don't want to see them until I can accuse them, and prove it, of having taken Mr. Baxter's dye formulae."

"Oh, yes, they're in the same building with Mr. Keith, aren't they? Why do they call it the Landmark? Though I suppose the answer is obvious."

"Yes," assented Tom. "It's a big building--the tallest ever erected in that city, and a fine structure. Though while they were about it I don't see why they didn't make it fireproof."

"Didn't they?" asked Ned, in surprise. "Then the insurance rates must be unusually high, for the companies are beginning to realize how fire departments, even in big cities, are hampered in fighting blazes above the tenth or twelfth stories."

"Yes, it was a mistake not to have the Land mark Building fireproof," admitted Tom. "And Mr. Keith says the owners are beginning to realize that now. It is what is called the 'slow burning' construction."

"Insurance companies don't go much on that," declared Ned, who was in a position to know. "Well, let us hope it never catches fire."

These were busy days for the young inventor. He laid aside all his other activities in order to perfect the plans for manufacturing his new chemical fire extinguisher on a large scale. For Tom realized that while a small quantity of chemicals in a compound might act in a certain way on one occasion, if the bulk should happen to be increased the experimenter could not always count on invariably the same results.

There appeared to be at times a change engendered when a large quantity of chemicals were mixed which was not manifest in a small and experimental batch.

So Tom wanted to mix up a big tank of his new chemical compound and see if it would work in large quantities as well as it did with the small amount Ned had dropped on the blazing tree.

To this end Tom worked at night, as well as by day, and finally he announced to Ned and Mr. Damon, who called one evening, that he believed he had everything in readiness for an exhaustive test the next day.

"There's the stuff!" exclaimed Tom, not a little proudly, as he waved his hand toward an immense carboy in the main shop. "That's what I hope will do the trick. Just take a--"

"Hold on! Stop! That's enough! Bless my hair brush!" cried Mr. Damon, holding up a protesting hand. "If you take that cork out, Tom Swift, you and I will cease to be friends!"

"I wasn't going to open it," laughed the young inventor. "It has a worse odor and seems to choke you more in a big quantity than when there's only a little. I was just going to shake the carboy to let you realize how full it was."

"We'll take your word for it!" laughed Ned. "Now about your test. How are you going to work it?"

"There are to be two tests," answered Tom. "The first, and the smaller, will be in the pit, as before, only this time we shall have what, I believe, will be the successful combination of chemicals to drop on it.

"The second test will be the main one. In that I plan to have an old barn which I have bought set ablaze. Then Ned and I will sail over it in the airship and drop chemicals on it. The barn will be filled with empty boxes and barrels, to make as hot a fire as possible. You are invited to accompany us, Mr. Damon."

"Will there be any smell?" asked the eccentric man, who seemed to have a dislike for anything that was not as agreeable as perfume.

"No, the chemicals will be sealed in containers, which will be dropped from my airship as bombs were dropped in the war," said Tom.

"On those conditions I'll go along," agreed Mr. Damon. "But bless my wedding certificate, Tom! don't tell my wife. She thinks I'm crazy enough now, associating with you and flying occasionally. If she thought I would help you battle with flames from the air she'd likely never speak to me again."

"I'll not tell," promised Tom, laughing.

Preparations for the test went on rapidly. In the morning a fire was to be started in the same pit where the experiment had partly failed before.

From the platform over the blazing hole some of the new combination of chemicals was to be dropped. If it acted with success, as Tom believed it would, he proposed to go on with the more important test in the afternoon.

To this end he had purchased from a farmer the right to set on fire an old ramshackle barn, standing in the midst of a field about three miles outside of Shopton. The barn was on an untilled farm, the house having been destroyed some years before, and it was not near any other structures, so that, even in a high wind, no damage would result.

Tom had filled the barn with inflammable material, and was going to spare no effort to have the test as exhaustive as possible.

The time came for the preliminary trial, and there were a few anxious moments after the oil-soaked boards and boxes in the pit were set ablaze.

"Let her go!" cried Tom to his man on the elevated platform, and down fell the container of chemicals. It had no sooner struck and burst, letting loose a mass of flame-choking vapor, than the fire died out.

"You've struck it, Tom! You've struck it!" cried Ned.

"It begins to look so," agreed the young inventor. "But I'll not call myself out of the woods until this afternoon. Though we can consider it a success so far."

Quite a throng was on hand when the old barn was set ablaze. Tom and Ned and Mr. Damon were there with the airship which had been especially fitted to carry the bombs filled with the extinguisher.

In order to insure a quick, hot blaze the barn was fired on all four sides at once by Tom's men. When it was seen to be a veritable raging furnace of fire, Tom and his two friends took their places in the airship and rapidly mounted upward.

Necessarily they had to circle off away from the blaze to get to the necessary height, but Tom soon brought the airship around again and headed for the black pall of smoke which marked the place of the blazing barn.

"We'll all three send down bombs at the same time," Tom told his friends, as they darted forward. "When I give the word press the levers, and the chemical containers will drop. Then we'll hope for the best."

Higher mounted the flames, and more fiercely raged the fire. The heat of it penetrated even aloft, where Tom and his friends were scudding along in the airship.

"Now!" cried Tom, as his craft hovered for an instant in a favorable position for dropping the bombs. The young inventor, Mr. Damon, and Ned Newton pressed the levers. Looking over the sides of the craft, they saw three dark objects dropping into the midst of the burning barn.