Directed by Captain Weston, who glanced at the compass and told him which way to steer¹ to clear the outer coral reef, Tom sent the submarine ahead, signaling for full speed to the engine-room, where his father and Mr. Sharp were. The big dynamos purred like great cats, as they sent the electrical energy into the forward and aft plates, pulling and pushing the Advance forward. On and on she rushed under water, but ever as she shot ahead the disturbance² in the phosphorescent water showed her position plainly. She would be easy to follow.  
"Can't you get any more speed out of her?" asked the captain of the lad.
 
"Yes," was the quick reply; "by using the auxiliary³ screws I think we can. I'll try it."
 
He signaled for the propellers⁴, forward and aft, to be put in operation, and the motor moving the twin screws was turned on. At once there was a perceptible increase to the speed of the Advance.
 
"Are we leaving them behind?" asked Tom anxiously, as he glanced at the speed gage⁵, and noted⁶ that the submarine was now about five hundred feet below the surface.
 
"Hard to tell," replied the Captain. "You'd have to take an observation to make sure."
 
"I'll do it," cried the youth. "You steer, please, and I'll go in the conning⁷ tower. I can look forward and aft there, as well as straight up. Maybe I can see the Wonder."
 
Springing up the circular ladder leading into the tower, Tom glanced through the windows all about the small pilot house. He saw a curious sight. It was as if the submarine was in a sea of yellowish liquid fire. She was immersed in water which glowed with the flames that contained no heat. So light was it, in fact, that there was no need of the incandescents in the tower. The young inventor could have seen to read a paper by the illumination of the phosphorus. But he had something else to do than observe this phenomenon. He wanted to see if he could catch sight of the rival submarine.
 
At first he could make out nothing save the swirl⁸ and boiling of the sea, caused by the progress of the Advance through it. But suddenly, as he looked up, he was aware of some great, black body a little to the rear and about ten feet above his craft.
 
"A shark!" he exclaimed aloud. "An immense one, too."
 
But the closer he looked the less it seemed like a shark. The position of the black object changed. It appeared to settle down, to be approaching the top of the conning tower. Then, with a suddenness that unnerved him for the time being, Tom recognized what it was; it was the underside of a ship. He could see the plates riveted⁹ together, and then, as he noted the rounded, cylindrical¹⁰ shape, he knew that it was a submarine. It was the Wonder. She was close at hand and was creeping up on the Advance. But, what was more dangerous, she seemed to be slowly settling in the water. Another moment and her great screws might crash into the Conning tower of the Swifts' boat and shave it off. Then the water would rush in, drowning the treasure-seekers like rats in a trap.
 
With a quick motion Tom yanked over the lever that allowed more water to flow into the ballast tanks. The effect was at once apparent. The Advance shot down toward the bottom of the sea. At the same time the young inventor signaled to Captain Weston to notify those in the engine-room to put on a little more speed. The Advance fairly leaped ahead, and the lad, looking up through the bull's-eye in the roof of the conning tower, had the satisfaction of seeing the rival submarine left behind.
 
The youth hurried down into the interior of the ship to tell what he had seen, and explain the reason for opening the ballast tanks. He found his father and Mr. Sharp somewhat excited over the unexpected maneuver¹¹ of the craft.
 
"So they're still following us," murmured Mr. Swift. "I don't see why we can't shake them off."
 
"It's on account of this luminous¹² water," explained Captain Weston. "Once we are clear of that it will be easy, I think, to give them the slip. That is, if we can get out of their sight long enough. Of course, if they keep close after us, they can pick us up with their searchlight, for I suppose they carry one."
 
"Yes," admitted the aged¹³ inventor, "they have as strong a one as we have. In fact, their ship is second only to this one in speed and power. I know, for Bentley & Eagert showed me some of the plans before they started it, and asked my opinion. This was before I had the notion of building a submarine. Yes, I am afraid we'll have trouble getting away from them."
 
"I can't understand this phosphorescent glow keeping up so long," remarked Captain Weston. "I've seen it in this locality several times, but it never covered such an extent of the ocean in my time. There must be changed conditions here now."
 
For an hour or more the race was kept up, and the two submarines forged ahead through the glowing sea. The Wonder remained slightly above and to the rear of the other, the better to keep sight of her, and though the Advance was run to her limit of speed, her rival could not be shaken off. Clearly the Wonder was a speedy craft.
 
"It's too bad that we've got to fight them, as well as run the risk of lots of other troubles which are always present when sailing under water," observed Mr. Damon, who wandered about the submarine like the nervous person............