Two days ago, while we were well out in the gulf, I opened the letter containing the first part of my sealed orders. Those orders, as you know, sent us to Belize. Before we reach there and open the envelope containing the rest of our orders, I think it necessary to test out the Grampus thoroughly. Unless I am greatly mistaken, the instructions yet to be read may call for work that will demand the last ounce of preparation we can give the submarine. I have stopped the motor, and we are lying motionless on the surface of the sea. The lead shows that there are two hundred and twenty-five feet of water under us. The steel shell of the Grampus is warranted to stand the pressure of water at that depth. Do you follow me?"

"Certainly, captain."

"Now, Matt, I have been watching you for a long time, and I believe that you know more about the gasoline motor than I do, and fully as much about maneuvring the submarine. We are going to dive to two hundred and ten feet—the deepest submersion by far the Grampus ever made. I wish you to take entire charge. If you get into difficulties, you must get out of them again, for I intend to stand by and not put in a word unless tragedy stares us in the face and you call on me for advice."

A thrill ran through Motor Matt. The submarine, with all her complicated equipment, was for a time to be under his control. This move of Captain Nemo, Jr.'s, perhaps, was a test for him no less than for the Grampus.

For a brief space the young motorist bent his head thoughtfully.

"Do you hesitate, Matt?" asked Captain Nemo, Jr.

"Not at all, sir," was the calm answer. "I was just running over in my mind the things necessary to be done in making such a deep dive. The pressure at two hundred and ten feet will be terrific. At that depth, the lid of our hatchway will be supporting a weight of more than thirty-two tons."

"Exactly," answered the captain, pleased with the way Matt's mind was going over the work.

"If there happened to be anything wrong with the calculations of the man who built the Grampus, captain, she would be smashed like an egg shell."

"We are going to prove his calculations." The captain seated himself on a low stool. "Gaines is at the motor, Clackett is at the submerging tanks, Speake has charge of the storage batteries and compressed air, and Cassidy is here in the periscope room with us to drive the Grampus in any direction you desire."

"Dick Ferral is with Gaines," added Matt, "and Carl Pretzel is with Clackett."

"Exactly. Every man is at his station, and some of the stations are double manned. Now, then, go ahead."

Matt whirled to a speaking tube.

"We're going to make a record dive, Clackett," he called into the tube, "and Captain Nemo, Jr., has placed me in charge——"

"Bully for the captain!" came back the voice of Clackett, echoing weirdly distinct in the periscope room.

"Hoop-a-la!" bubbled the exultant tones of Matt's Dutch chum. "Der king oof der modor poys iss der poy for me."

"Our submergence will be two hundred and ten feet," went on Matt. "You and Carl, Clackett, will put the steel baulks in place. I'll have Dick and Gaines help you."

Another order was called to the engine room, and presently there were sounds, forward and aft, which indicated that the metal props, to further strengthen the steel shell, were being dropped into their supports.

"Cassidy," said Matt, "see that the double doors of the hatch are secured."

"Aye, aye, sir," answered the mate, darting up the conning-tower ladder.

"Speake," ordered Matt, through another tube, "see that the tension indicators are in place."

"Double doors of the hatch secured," reported Cassidy a moment later.

"Pressure sponsons in place," came rattling through the tube from Clackett.

"Tension indicators in position," announced Speake.

"Dive at the rate of twelve yards to the minute, Clackett," ordered Matt.

A hiss of air, escaping from the ballast tanks as the water came in, was heard. A tremor ran through the steel fabric, followed by a gentle downward motion. Matt kept his eyes on the manometric needles. Twenty yards, twenty-five, thirty, and forty were indicated. A pressure of ten pounds to the square centimeter was recorded.

"Plates are beginning to bend, captain," called Speake.

This was not particularly alarming, for the baulks would settle down to their work.

"Close the bulkhead doors, Dick!" called Matt.

"Aye, aye, old ship!" returned Dick, and sounds indicated that the order was immediately carried out.

"Sixty yards," called Clackett; "sixty-five, seventy——"

"Hold her so!" cried Matt.

"What is the danger point in the matter of flexion, captain?" asked Matt, turning to Nemo, Jr., whose gray head was bowed forward on his hand, while his gleaming eyes regarded the cool, self-possessed young motorist with something like admiration.

"Ten millimeters," was the answer.

"We still have a margin of three millimeters and are at the depth you indicated."

"Bravo! We are five yards from the bottom. Do a little cruising, Matt. Let us see how the Grampus behaves at this depth."

The entire shell of the submarine was under an enormous pressure.

Matt gave the order to start the motor, and the popping of the engine soon settled into a low hum of perfectly working cylinders. A forward motion was felt by those in the submarine.

"Not many people have ever had the novel experience of navigating the ocean seventy yards below the surface," remarked the captain, with a slow smile.

"It's a wonderful thing!" exclaimed Matt. "The Grampus seems equal to any task you set for her, captain."

The air of the periscope room was being exhausted by the breathing of Matt, Nemo, Jr., and Cassidy. Matt ordered the bulkhead doors opened, in order that fresh oxygen might be admitted from the reservoirs. Just before the doors were opened, Captain Nemo, Jr.'s face had suddenly paled, and he had swayed on his seat, throwing a hand to his chest.

"You can't stand this, captain!" exclaimed Matt, jumping to the captain's side. "Hadn't we better ascend?"

The captain collected himself quickly and waved the youth away.

"Never mind me, my lad," he answered. "I feel better, now that a little fresh oxygen is coming in to us. Go on with your maneuvring."

All was silent in the submarine, save for the croon of the engine, running as sweetly as any Matt had ever heard. Aside from a faint oppression in the chest and a low ringing in the ears, the Grampus might have been cruising on the surface, so far as her passengers could know.

Cassidy was at the wheel, steering, his passive eyes on the compass.

Matt turned away from the manometer with a remark on his lips, but before the words could be spoken there was a shock, and the submarine shivered and stopped dead.

"Shiminy grickets!" whooped the voice of Carl. "Ve must haf run indo vone oof der moundains in der sea."

"Full speed astern, Gaines," cried Matt.

The blades of the propeller revolved fiercely. The steel hull shook and tugged, but all to no purpose.

Captain Nemo, Jr., sat quietly in his seat and never offered a suggestion. His steady eyes were on Motor Matt.

The king of the motor boys realized that they were in a terrible predicament. Suppose they were hopelessly entangled in the ocean's depths? Suppose there was no escape for them, and the shell of the Grampus was to be their tomb?

These reflections did not shake the lad's nerve. His face whitened a little, but a resolute light gleamed in his gray eyes.

"How are the bow plates, Speake?" he demanded through one of the tubes.

Speake was in the torpedo room.

"Right as a trivet!" answered Speake.

After five minutes of violent