Once it became evident to the occupants of the airship what Tom Swift's plans were, they all prepared to help him. Previous to the trip certain duties had been assigned to each one, duties which were to be exercised when Tom gave the exhibition of his new aerial fire-fighting apparatus at the set fire before the fire department of Denton.

This preparation now stood the young inventor in good stead, for there was no confusion aboard the Lucifer when she winged her way toward the burning Landmark Building, where the flames were continually spouting higher and higher as they rushed through the roof, directly above the stairway well and elevator shafts.

So far the flames had confined themselves to this central part of the big structure, but it was only a question of time when they would spread out on all sides, licking up the remainder of the pile. And, for the most part, the firemen on the ground were at a great disadvantage.

They had run in lines as near as they could get to the center of the blaze, and had also attached hose to the standpipes inside the building. But this last effort was wasted, as developed later, for there was no one in the building to direct the nozzle ends of the hose attached to the standpipes on the different floors. Also the fierce heat fairly melted the pipes themselves in the vicinity of the elevator shafts, and there was no automatic sprinkling system in the building.

This was the situation, then, when Tom in his airship loaded with fire-extinguishing chemicals headed for the blaze. And this, also, was the desperate situation that confronted Mary Nestor and her uncle, Barton Keith, as well as Amos Field and Jason Melling. Those unscrupulous and cowardly men were in a veritable panic of fear, which contrasted strangely with the calm, resigned attitude of Mary and her uncle.

"We must get out! Some one must save us!" yelled Field.

"Jump from the window!" cried Melling.

"No, I can't permit that!" declared Mr. Keith, standing in their path. "It would be sure death! As it is, there may be a chance."

"A chance? How?" asked Field. "Listen to that!"

Through the closed door of Mr. Keith's office could be heard the roar and crackle of flames, while the very air was now stifling and hot, filled with acrid smoke.

"We can only wait," said Mr. Keith, and he wet Mary's handkerchief in the water and handed it to her to bind over her face.

"Is everything all right, Ned?" called Tom, as he turned on a little more power, so that the Lucifer lunged ahead toward the great pillar of fire that now reddened the sky for miles around.

"All ready," was the answer. "You only have to give the word when you want us to let go."

"Let go!" cried Mr. Damon. "Bless my umbrella, Tom! We don't have to jump out, do we?"

"He means to let go the extinguisher grenades," said Mr. Baxter. "Shall we let them all go at once, Tom?" asked the chemist.

"No, drop half when I shoot over the first time. We'll see what effect they have, and then come back with the rest."

"That's the idea!" cried Ned. "Well, give us the word when you're ready, Tom."

"I will," was the answer of the young inventor, and with keen eyes he began to set the automatic gages so those in charge of the grenades would be able to drop them most effectively.

The flames were mounting higher and higher above the ill-fated Landmark Building. It was a "land-mark" now, for miles around--a fearsome mark, indeed.

"I hope every one is out of the place," said Ned, as the airship approached nearer and the fierceness of the fire was more manifest.

"Bless my thermometer, you're right!" exclaimed Mr. Damon. "I don't see how any one could live in that furnace."

Seen from above it appeared that the fire was engulfing the whole building, while, as a matter of fact, only the central portion was yet blazing. But it was only a question of time when the remainder would ignite.

And it was to this fact--that the fire was rushing up the stairway and elevator shafts as up a chimney--that Mary and her uncle, as well as Field and Melling, owed their temporary safety.

Had Tom known that the girl he loved was in such direful danger, it is doubtful if his hand would have been as steady as it was on throttle and steering wheel. But not a muscle or nerve quivered. To Tom it was but carrying out a prearranged task. He was going to extinguish a great blaze, or attempt to do so, by means of his aerial fire-fighting apparatus. And his previous tests had given him confidence in his device. His one regret was that the fire department of the city that was contemplating the purchase of certain rights in his invention could not witness what he was about to do.

"But they'll hear of it," declared Ned, when Tom voiced this idea to his chum.

Nearer and nearer to the up-spouting column of flames the airship winged her way. Tense and alert, Tom sat at the wheel guiding his craft with her load of fire-defying chemicals. Behind him were Ned, Mr. Damon and Mr. Baxter, ready to drop the grenades at the word.

"Getting close, Tom!" called Ned, as they could all feel the heat of the conflagration in the Landmark Building, which now seemed doomed.

"You'll not dare cross it too low down, will you?"

"No, I'll have to keep pretty well up," was the answer. "There's a current of air over that fire which might turn us turtle."

Heat creates a draft, sucking in colder air from below, and making an upward-rushing column which, in the case of a big blaze, is very powerful. Tom knew he had to avoid this.

It was now almost time to act. In another few seconds they would be sailing directly into the path of the up-spouting flames. Realizing that to do this at too low an elevation would result in disaster, Tom sent his craft upward at a sharp angle. Then he turned to call to his companions.

"Be ready when I give the word!"

"All set and ready!" answered Ned, and the others signified their attention to the command that soon was to be given.

Having attained what he considered a sufficient elevation, Tom headed the Lucifer straight toward the up-spouting column of fire and smoke. If ever his craft of the air was to justify her name it was now!

Straight and true as an arrow she headed for the fiery pillar! Hotter and hotter grew the air! The darkness of the night was lighted by the awful fire, which rendered objects in the street clear and distinct. But Tom and his friends had little time for such observation.

"Get ready!" cried the young inventor, as he felt a rush of heat across his face, partly protected, as it was, by great goggles.

"All ready!" shouted Ned.

"Let go!" cried Tom, and with a click of springs the fire extinguishers dropped from the bottom of the Lucifer into the very heart of the flames in the Landmark Building.

There was a blast as from a furnace seventy times heated, a choking and gasping for breath on the part of the occupants of the airship, a shriveling, as it seemed, of the naked flesh, and then, when it appeared that all of them must be engulfed in the great heat, the airship passed out of the zone of fire.

A rush of cool air followed, reviving them all, and then, when out of the swirls of smoke, Ned, looking back, cried:

"Good work, Tom! Good work!"

"Did we hit it?" cried the young inventor. "She's half gone!" declared Mr. Baxter. "Can you give her the rest of the load?"

"I'm going to try!" declared Tom.

"Bless my bank balance!" shouted Mr. Damon, "are we going through that awful furnace again?"

"It will not be so bad this time," observed Ned. "The fire is half out now. Tom's stuff did the trick!"

Indeed it was evident, as Tom sent the Lucifer around in a sharp turn, that the fire had been largely smothered by the gas that now lay over it like a wet blanket. But there was still some fire spouting up.

"Give her all we have!" yelled Tom, as, once more, he prepared to cross the zone of fire.

"Right," sang out Ned.

Once more the Lucifer swept over the burning building. Down shot the remaining grenades, falling into the mass of flames and bursting, though the reports could not be heard because of the tumult in the streets below. For the firemen and spectators had seen the sudden dying down of the fire, they had caught sight of a shadowy shape in the night, hovering over the blazing building, and they wondered what it all meant.

"How is it?" asked Tom, as he guided the craft back to get a view of his work.

"That settles it!" answered Ned. "There isn't fire enough now to broil a beefsteak!"

This was not exactly true, for the blaze was not entirely subdued. But the flames had all been killed off in the higher parts of the Landmark Building, and what remained could easily be dealt with by the firemen on the ground. They proceeded to make short work of the remainder of the conflagration, the while wondering who had so effectively aided them from the clouds.

"Well," observed Tom, as he saw how effectively he had smothered the great fire, "it's of no use to go on now. I haven't an ounce of chemical left on board. I can't give the demonstration that I planned for tomorrow."

"You've given a better demonstration here than you ever could have in the other city," declared Mr. Baxter. "I fancy this will be all the test needed, Tom Swift!"

"Perhaps. I hope so. But we may as well land and see from the ground the effect of our work. I'd also like to inquire if any one was hurt. Let's go down."

It was rather ticklish work, making a landing in the midst of a populous city, and at night. But as it happened, there had been a number of buildings razed in the vicinity of the Landmark structure, and there was a large, vacant level space. Also several of the city's fire department searchlights were focused around the burning structure, and when it became evident that an airship was going to land--though as yet none guessed whose it was--the searchlights were turned on the vacant spot and Tom was able to make a good landing, his own powerful searchlight giving effective aid.

"What did you do that put out the fire?" demanded the chief of the Newmarket department, as he rushed up with a crowd of others when Tom and his friends alighted.

"I dropped a few grenades down that chimney," modestly answered the young inventor.

"A few grenades! Say, you must have turned a whole river of them loose!" cried the delighted chief. "It doused the fire quicker than I ever saw one put out in all my life!"

"I'm glad I was successful," said Tom. "But was any one in the building?"

"Yes, a few," answered a policeman, who was trying to keep the crowd back from the airship. "They're bringing them out now."

"Killed?" gasped Tom.

"No. But some of them are badly hurt," the officer answered. "There was one young lady and a man named Barton Keith--"

"Barton Keith!" shouted Tom, springing forward. "Was he--Who was the young lady? I--I--"

But at that moment there was a stir in the crowd about the building, in which only a little fire flow remained, and through the throng came a disheveled and smoke-blackened young lady and a man whose clothing was also greatly disarrayed.

"Mary!" cried the young inventor.

"Tom!" gasped Mary Nestor. "How did you get here?"

"I came to put out the fire," was the answer, and Tom cooled down now that he saw Mary was unharmed. "How did you happen to be in the building?"

"I was in Uncle Barton's office when the fire broke out," answered Mary, "and we were trapped. We had to stay there, with two men from the floor above."

"Yes, and if they had stayed with us they wouldn't have been hurt," said Mr. Keith. "But, as it was, they rushed out and tried to get down the stairs. They were caught in the draft and badly burned, I believe. They are bringing them out now."

Two stretchers, on which lay inert forms, were borne through the now silent crowd by firemen and police officers, and taken to waiting ambulances.

"That's Field and Melling," said Mr. Keith to Tom. "They had offices just above me, and they were trapped, as were Mary and I. They acted like big cowards, too, though I hope they're not badly hurt. We stayed inside my office, and we were just giving up the hope of rescue when the fire seemed suddenly to die down."

"I should say it was sudden!" cried the enthusiastic local chief. "It was the chemicals from this young man's airship that did the trick!"

"Oh, Tom, was it your new machine?" asked Mary.

"Yes," was the answer. "I was on my way to give a test tomorrow in Denton when I saw this fire. I didn't know you were in it, though, Mary."

"Oh, but I'm glad you came," she said. "It was just--awful!" and she clung to Tom's arm, trembling.

When Field and Melling, whose rash conduct had caused them to be severely but not fatally burned, had been taken to a hospital and the fire was declared to be practically out, Tom made arrangements to leave his airship in the city field all night.

"And you and your friends can come to Uncle Jasper's house," said Mary.

"Of course!" said Uncle Jasper himself, who had arrived on the scene, attracted to the fire by the news that his niece and Mr. Keith were in danger. "Lots of room! Come along! We'll celebrate your rescue

So the crew of the fire-fighting Lucifer went with Mary, while the firemen, after again thanking Tom most enthusiastically, kept on playing, as a precaution, their streams of water on the still hot building.

Only the central portion of the structure, the stairs and elevator shafts, were burned away. The strong upward draft had kept the fire from spreading much to either side.

"It certainly was a fierce blaze, and I'm glad my chemicals took such prompt effect," said Tom. "I shall not fear any test after this."

It was the day following the night of excitement, and Tom and his friends, at the invitation of the fire department of Newmarket, were inspecting what was left of the Landmark Building --and there was considerable left--though access to the upper floors was to be had only by ladders, down which Mary and her uncle, Barton Keith, had been carried.

"Here are my offices," said Mr. Keith, who accompanied Tom, Ned, Mr. Damon and Mr. Baxter, as he ushered them into his suite of rooms.

"Bless my fountain pen! nothing is burned here," cried the eccentric man.

"No, the flames just shot upward," explained the fire chief, who was leading the party. "But I think those chemicals of yours would have been just as effective, Mr. Swift, if the fire had mushroomed out more."

"It was hot enough as it was," answered Tom, with a grim laugh.

"Bless my thermometer, too hot--too hot by far!" exclaimed Tom Swift's eccentric friend, and to this Ned nodded an amused agreement.

An exclamation from Mr. Baxter attracted the attention of all in Mr. Keith's office. The chemist picked up from the floor a bundle of papers.

"Here is a bundle of documents that some one has dropped, Mr. Keith," he said. "I guess you forgot to put it in your safe. Why --why--no--they aren't yours! They're mine. Here are my missing dye formulae! The secret papers I've been searching for so long! The ones I thought Field and Melling had!" cried Mr. Baxter. "How--how did they get here?" and, wonderingly, he looked at the bundle of papers he had discovered in such a strange manner.