While Tom Swift was loading the Lucifer for her trip and the fire extinguishing test to occur the next morning, quite a different scene was taking place in the home of Jasper Blake, the uncle of Mary Nestor, where she had gone to spend a few weeks.

"Well, are you all ready, Mary?" asked her aunt, and it was about the same time that Ned Newton asked that same question of Tom Swift. Only Tom was in Shopton, and Mary was in Newmarket, and Tom was setting off on an air voyage, while Mary was only preparing to take a car downtown to do some shopping.

"Yes, Aunt, I'm all ready," Mary answered. "But I may be a bit late getting home."

"Why?" asked Mrs. Blake.

"I promised Uncle Barton I'd stop and call on him at his office," Mary replied. "He has something he wants me to take home to mother when I go tomorrow."

"I shall be sorry to see you go back," said Mrs. Blake. "But I imagine there will be those in Shopton who will be glad to see you return, Mary."

"Yes, mother wrote that she and dad were getting a bit lonesome," the girl casually replied, as she adjusted her veil.

"Yes, and some one else. Ah, Mary, you are a very lucky girl!" laughed her aunt, while Mary turned aside so she would not see her own blushes in the mirror.

"I thought Tom was going to call and take you home in his airship, Mary," went on her relative.

"So he is, I believe, on his way back from a city where he is going to be tomorrow making a big fire test. I am to wait for him until tomorrow afternoon. But now I really must go shopping, or all the bargains will be taken. Is there any word you want to send to Uncle Barton?"

"No," answered Mrs. Blake. "Though you might tell him to stop poking fun at your Uncle Jasper for having invested money in the Landmark Building. It's getting on your Uncle Jasper's nerves," she added.

"Uncle Barton never can give up a joke, once he thinks he has one," said Mary. "But I'll tell him to stop pestering Uncle Jasper."

"Please do," urged Mary's aunt, and then the girl left.

Mary's uncle, Barton Keith, with whom Tom Swift had been associated during the undersea search, had offices in the Landmark Building, but his home was in an adjoining suburb.

The girl was pleased with the results of her shopping, and at the close of the afternoon she stopped at the Landmark Building and was soon being shot up in the elevator to the floor where Barton Keith had his offices.

Though Mr. Keith had refrained from investing in the Landmark Building and though he laughed at Mary's Uncle Jasper for having done so, this did not prevent him from having a suite of offices in the big structure which, as we already know, was owned in large part by Field and Melling.

"Ah, Mary! Come in!" exclaimed Mr. Keith, welcoming Tom Swift's sweetheart. "It is so late I was afraid you weren't coming, and I was about to close the office and go home."

"You must blame the bargain sales for my delay," laughed Mary. "I hope I haven't kept you waiting."

"No, I still had a few things to do. One was to write a letter to your Uncle Jasper, telling him I had heard of another fire trap that was open to investors."

"Oh, and that reminds me I must tell you not to push Uncle Jasper too far!" warned Mary.

"Ha! Ha!" laughed Uncle Barton. "He made fun of me for going on the undersea search with Tom Swift. But I made good on that, and that's more than he can say about his Landmark Building deal!"

"But don't exasperate him too much!" begged Mary. "By the way, what are they doing to this building? I see the stairways and some of the elevator shafts all littered with building material."

"They are trying to make it fireproof," answered her uncle. "It's rather late to try that now, but they've got either to do it or stand a big increase in insurance rates. I'm glad I'm out of it. But now, Mary, take an easy chair until I finish some work, and then I'll walk out with you.

Mary took a seat near one of the front windows, whence she could look down into the now fast-darkening streets. She could see the supper crowds hurrying home, and out in the corridor of the big skyscraper could be heard the banging of elevator doors as the office tenants, one after another, left for the day.

Suddenly there was more commotion than usual, followed by the sound of broken glass. Then came a cry of:

"Fire! Fire!"

Mary sprang to her feet with a gasp of alarm, and her uncle rushed past her to the door leading into the hall outside his offices. As he opened the door a cloud of smoke rushed toward him and Mary, causing them to choke and gasp.

Mr. Keith closed the door a moment, and when he opened it again the smoke in the hall seemed less dense.

"It probably is only a slight blaze among some of the material the workmen are using," he said. "Come, Mary, we'll get out."

Pausing only to swing shut the door of his heavy safe and to stuff some valuable papers into his pocket, Mr. Keith advanced and, taking Mary by the arm, led her into the hall. The smoke was increasing again, and distant shouts and cries could be heard, mingled with the breaking of glass.

Mr. Keith rang the elevator buzzer several times, but when no car came up the shaft in response to his summons he turned to his niece and said:

"We'll try the stairs. It's only ten stories down, and going down isn't anything like coming up."

"Oh, indeed I can walk!" said Mary. "Let's hurry out!"

They turned toward the stairway, which wound around the elevator shafts, but such a cloud of hot, stifling smoke rolled up that it sent them back, choking and gasping for breath.

And then, as they stood there, up the elevator shafts, which were veritable chimneys, came more hot smoke, mingled with sparks of fire.

"Trapped!" gasped Mr. Keith, and he pulled Mary back toward his offices to get away from the choking, stifling smoke. "We're trapped!"