"Say no more about it," said Jack. "I was just as much to blame."

Frank now broke into the conversation.

"What vessel is this?" he asked, pointing to the low-lying bulk of the submarine, against which the small boat now sc.r.a.ped.

"German submarine X-9," replied Lieutenant Stein, "where, until we put into port again, you will be our prisoners."

The four now clambered to the top of the submarine. Lieutenant Stein led the way to the entrance through the combined bridge and conning tower, and all went below. At the foot of the short flight of steps stood a man in captain's uniform.

"The sole survivors of a British cruiser, sir," said Lieutenant Stein to the captain, indicating the two lads. "I have not learned their names nor rank."

The two lads hastened to introduce themselves.

"I am Captain von Cromp, commander of this vessel," said the captain gruffly. "You are my prisoners until I put into port and can turn you over to the proper authorities."

Jack and Frank bowed in recognition of their fate. The captain turned to Lieutenant Stein.

"You will see that the prisoners are well cared for," he said.

"They are in your custody."

The lads glanced curiously about as they were led along toward the lieutenant's cabin. It was the first time either had been inside a submarine vessel, and both felt a trifle squeamish. The boat was upon the surface of the sea now, however, and a dim light penetrated below.

The lieutenant's cabin, well forward, was fitted up luxuriously.

There were several bunks in the little room, and the lieutenant motioned to them.

"You will sleep there," he said quietly. "Make yourselves perfectly at home. I guess there is no danger of your attempting to escape. However, you must remain below and not ascend to the bridge under any circ.u.mstances."

He bowed, and left them.

"I don't know as I am particularly fond of this kind of travel,"

Frank confided to Jack. "It's all right as long as we remain on the surface, but I'll bet it would feel queer to be moving along under the water."

"Right you are," replied Jack. "However, we are here and we shall have to make the best of a bad situation. Then, too, perhaps we can learn something that may prove of use to us later on."

The lads dined that night at the officers' mess and became quite well acquainted with all of them. They found Captain von Cromp not half so gruff as he had been when they first came aboard.

They were questioned about the service they had seen, and their story greatly surprised all the officers.

Upon Lieutenant Stein's request, the commander granted the lads permission to look over the vessel.

The lieutenant showed them how the vessel was submerged, by allowing one of the tanks to fill with water; how it rose again by forcing the water from the compartment by means of compressed air; how the air was purified when a lengthy submersion was necessary, and how the vessel was handled in times of action.

He showed them the periscope, and allowed them to peer through, although there was no need to use this, as the vessel was above water.

"When the submarine is submerged," explained Lieutenant Stein, "the periscope is the eye of the vessel. Peering over the waves, it reflects what it sees into the watching human eye in the conning tower. Destroy it, and the submarine is a blind thing, plunging to destruction."

"Then the periscope is the one weak spot in a submarine?" asked Frank.

"Exactly," was the reply. "Of course, if it were destroyed, the vessel might rise immediately to the surface and so gain its bearings. But in the midst of battle it would probably mean certain destruction; for when it rose the submarine would naturally be so close to the enemy that a single big sh.e.l.l would put it out of business."

The boys looked long at this strange mechanical eye. Shaped like a small pipe, it ran up from the conning tower and protruded above the vessel. A large lens at the top turned off as does an elbow in a stove pipe. This portion, when necessary, moved in all directions. When raised to its maximum height everything within a radius of ten miles is reflected in it.

"The shaft can be lowered to within a few inches of the top of the water," the lieutenant explained, "thus guarding against the danger of being hit. The officer in the conning tower peers into the binoculars and sees just what the periscope sees."

"Will you explain just how it works?" asked Jack. I

"Certainly. The periscope consists, as you may see, of a slender tubular shaft extending up through the conning tower of the submarine. Each submarine is equipped with a pair--thus if one is shot away the other can be put in immediate use. At the upper end of the shaft is a mirror lens. Upon this mirror lens is reflected the surrounding surface of the ocean. The image reflected there is carried down the tube to other lenses and then conveyed to enlarging binoculars. Now do you understand?"

"Perfectly," replied Jack; "and now as to the manner in which a submarine fights. It is by torpedoes, as I understand it."

"Exactly," replied the lieutenant, "and the torpedo is the most deadly, effective and, it may be also said, intelligent of modern warfare. One torpedo, striking the right kind of a blow, can destroy a battleship. The submarine has no other effective, weapon than the torpedo, which is delivered from a small tube.

There is this advantage in favor of the battleship, however: the submarine is a slow craft. It is slower than the slowest battleship when it proceeds under water. When it gets to the surface its speed is doubled, but then it is an easy target for the guns of the threatened battleship and also for the swift torpedo boats and torpedo destroyers which are always thrown out as escorts when a submarine attack is antic.i.p.ated. Some submarines are equipped with light rapid-firing guns, but these are of no more use in attacking on-water boats than would be a popgun. Do I make myself clear?"

"Perfectly," said Jack.

"It is indeed interesting," said Frank. "Can you tell us more?"

The lieutenant continued: "Beyond these factors--the superior speed, the protection of torpedo boats and the weakness of the periscope--there has been no protection yet devised against the attack of a submarine."

"But the torpedo nets--?" interrupted Frank.

"There is of course," the lieutenant went on, "a crudely defensive measure called the torpedo net. These are meshes of strong steel which are dropped down from the side of the warship and are supposed to catch the torpedo before it hits the side of the ship."

"Well, don't they?" asked Frank.

"In theory," said the lieutenant, "the torpedo explodes within the net and the force of its attack is more or less diminished.

As a matter of fact, however, torpedo nets are not dependable.

Why, most of our submarines are equipped with a formidable device for cutting these nets. This device, in one form, resembles an enormous pair of sheers which cut through the nets like paper.

In another form they are equipped with powerful tearing arms which drag the net away and expose the sides of the battleship to the deadly messenger from the torpedo tube. Am I tiring you?"

"I should say not," replied both lads in one breath, and Frank added: "I don't just understand how a submarine sinks and rises."

"It's very simple," said the lieutenant, "and at the same time I'll tell you something else. The submarine is unaffected by tempests, and for this reason also is more deadly than a battleship. The submarine can dive down into the depths where there is no movement of the waves, and it can remain under water for fourteen hours continuously. This is accomplished by tanks which can be filled with water and, overcoming what is known as the 'margin of buoyancy,' submerge the vessel. The air is replenished by special purifying devices and by tanks of oxygen.

When the vessel wants to rise, it simply pumps out the water from the tanks."

"It certainly is a wonderful invention," said Frank, when the lieutenant had concluded his explanation.

"Indeed it is," agreed Jack.

"You should be aboard when we are in action," smiled the lieutenant. "I am sure you would be greatly interested."

"I don't doubt it," said Jack, "although from what you have told us regarding the deadliness of submarines, I believe that I should rather witness action on a British submarine."

"Nevertheless," said the lieutenant, "you are likely to see action aboard the X-9, for I do not believe Captain Von Cromp will return to port until he has at least tried the effect of his torpedoes, on a ship or so of your countrymen."

"May he go to defeat if he tries it!" said Jack fervently.

"In which case," said the lieutenant with good natured tolerance, "you would undoubtedly go with us."

"Even so," replied Jack, "I still could not wish to see you get away."