Tom Swift felt as if he was struggling in some dream or nightmare. He felt strong hands holding him and saw evil faces leering at him.

Then gradually his brain cleared. His muscles, that had been weakened by the cowardly blow, grew strong. He felt his fist land heavily on some one's face. He heard a smothered gasp of pain.

Then came the sound of footsteps running--Tom heard the "ping" of a policeman's night-stick on the sidewalk.

"Here come the cops!" he heard one voice exclaim.

"Did you get it?" asked another.

"No, I can't find it. Cut for it now!"

They released the young inventor so suddenly that he staggered about and almost fell.

The next moment Tom was looking into the face of a big policeman, who was half supporting him.

"What's the matter?" asked the officer.

"Hold-up, I guess," mumbled the lad. "There they go!" he pointed toward two dark forms slipping along down the dimly-lighted street.

The officer drew his revolver, and fired two shots in the air, but the fleeing figures did not stop.

"How did it happen?" asked the policeman. "Did they get anything from you?"

"No--I guess not," answered Tom. He saw the packages containing his purchases lying where they had fallen. A touch told him his watch and pocketbook were safe. The precious map was in a belt about his waist, and that had not been removed. "No, they didn't get anything," he assured the officer.

"I came along too quick for 'em, I guess," spoke the bluecoat. "This is a bad neighborhood. There have been several hold-ups here of late, but I was on the job too soon for these fellows. Hello, Mike," as another officer came running up in answer to the shots and the raps of the night-stick. "Couple of strong-arm-men tackled this young fellow just now. I saw something going on as I turned the corner, and I rapped and ran up. They went down that way. I fired at 'em. You take after 'em, Mike, and I'll stay here. Don't believe you can land 'em, but try! I came up too quick to allow 'em to get anything, though."

Tom did not contradict this. He knew, however, that, had the men who attacked him wished to take his watch or money, they could have done it several times before the officer arrived.

"It was the map they were after," thought Tom, "not my watch or money. This is more of the Foger's work. We must get away from here."

The policeman inquired for more particulars from Tom, who related how the hold-up had taken place. The young inventor, however, said nothing about the map he carried, letting the officer think it was an ordinary attempt at robbery, for Tom did not want any reference in the newspapers to his search for the valley of gold.

Presently the other policeman returned, having been unable to get any trace of the daring men. The two bluecoats wanted to accompany Tom back to the airship shed, for his own safety, but he declared there was no more danger, and, after having given his name, so that the affair might be reported at headquarters, he was allowed to go on his way. His head ached from the blow, but otherwise he was unhurt.

"Those fellows have been keeping watch for me," the lad reasoned, as he walked quickly toward the airship shed. "They must have been shadowing me, and they hid there until I came back. Andy Foger and his father must be getting desperate. I think I know why, too. That little dig I gave Andy about his map is bearing fruit. He begins to think it's the wrong map, and he wants to get hold of the right one. Well, they shan't if I can help it. We'll be away from here in the morning."

There was indignation and some alarm among Tom's friends when he told his story a little later that night.

"Bless my walking-stick!" cried Mr. Damon. "You'll need a bodyguard after this."

"I'd just like t' git my hands on them fellers!" exclaimed the old miner. "I'd show 'em!" and a look at his rugged frame and his muscular arms and gnarled hands showed Tom and Ned that in the event of a fight they could count much on Abe Abercrombie.

"I am glad there will be no more delays, and that we will soon be moving northward," spoke Mr. Parker, a little later. "I am anxious to confirm my theory about the advance of the ice crust, I met a man to-day who had just returned from the north of Alaska. He said that a severe winter had already set in up there. So I am anxious to get to the ice caves."

"So am I," added Tom, but it was for a different reason.

They were all up early the next morning, for there were several things to look after before they started on the trip that might bring much of danger to the adventurers. Under Tom's direction, more gas was generated, and forced into the big bag. A last adjustment was made of the planes, wing tips and rudders, and the motor was given a try-out.

"I guess everything is all right," announced the young inventor. "We'll take her out."

The Red Cloud was wheeled from the big shed, and placed on the open lot, where she would have room to rush across the ground to acquire momentum enough to rise in the air. Tom, whenever it was practical, always mounted this way, rather than by means of the lifting gas, as, in the event of a wind, he would have better control of the ship, while it was ascending into the upper currents of air, than when it was rising like a balloon.

"All aboard!" cried the lad, as he looked to see that the course was clear. Early as it was, there was quite a crowd on hand to witness the flight, as there had been every day of late, for the population of Seattle was curious regarding the big craft of the air.

"Let her go!" cried Ned Newton, enthusiastically.

Tom took his place in the steering-tower, or pilothouse, which was forward of the main cabin. Ned was in the engine-room, ready to give any assistance if needed. Mr. Damon, Mr. Parker and Abe Abercrombie were in the main cabin, looking out of the windows at the rapidly increasing throng.

"Here we go!" cried the young inventor, as he pulled the lever starting the motor, There was a buzz and a hum. The powerful propellers whirred around like blurs of light. Forward shot the great airship over the ground, gathering speed at every revolution of the blades.

Tom tilted the forward rudder to lift the ship. Suddenly it shot over the heads of the crowd. There was a cheer and some applause.

"Off for the frozen north!" cried Ned, waving his cap.

Tom shifted the rudder, to change the course of the airship. Mr. Damon was gazing on the crowd below.

"Tom! Tom!" he cried suddenly. "There's the man with the black mustache--the man who tried to rob you in the sleeping-car!" He pointed downward to some one in the throng.

"He can't get us now!" exclaimed Tom, as he increased the speed of the Red Cloud, and then, taking up a telescope, after setting the automatic steering gear, Tom pointed the glass at the person whom Mr. Damon had indicated.