The British submarine D-16 was, at this time at any rate, the only under-the-water vessel that could remain under the sea indefinitely. The one real weakness of beneath-the-sea fighters had always been their inability to remain long under water, and for this reason they could operate only within a certain radius of their base.

In discovering that there was a means by which a submarine could remain indefinitely under water, Sir John H—— had overcome this difficulty. The new invention had been tried on the D-16, and it had worked.

According to Sir John, a submarine in order to remain submerged indefinitely, must be able to extract air from the water, as does a fish, for it is air that is needed most under water. Up to the time of Sir John H——’s discovery this had never been accomplished.

Reasoning that it must be a peculiarity about the gills of a fish that permitted it to extract air from the water, Sir John had experimented along this line, and his experiments finally terminated successfully. When the D-16 had proved the practicability of Sir John’s theory, the vessel had immediately been put into commission, and the construction of others begun.

The secret of the D-16 was known to only a trusted few besides the crew of the vessel—the King, Winston Churchill, and David Lloyd George, Chancellor of the Exchequer. Besides her ability to remain submerged, D-16 was also many knots faster than any other submarine.

From a naval viewpoint, perhaps, the war up to this time had not progressed as favorably as had been expected. While the allied forces on the continent had been hemming in the Germans and their allies, driving them back from beneath the very walls of Paris, back across the Marne and across the Aisne, and while the legions of the Czar had been attempting to force a passage of the Carpathian mountains for an invasion of Hungary and Austria, and making determined assault within the borders of East Prussia, the British, French and Russian fleets had been practically inactive.

True, there had been several important naval battles, but none which could be called decisive, although whatever advantage there was was with the British.

The French, with the Austrian fleet cooped up in their base in the Adriatic, had little to do but to see that the Austrians were not allowed to escape. The Russian fleet had had one or two brushes with the Turks, but these were unimportant.

In the North Sea England was having more difficulty. German submarines, from their base at Ostend, had made several successful raids into Dover harbor, on the British coast; three unfortified towns on the coast had been razed by German shells and German aeroplanes had been seen flying about in the vicinity of London.

Huge Zeppelin balloons, upon which, England believed, the German Emperor planned to risk all should other means fail, had been seen over the British Isles, but had been driven off. One had been sunk. After this, England, ever fearful of an air raid, took heart, and agreed with the nobleman who said that a raid by air was not feasible.

Besides the ships of war that had been sunk by the Germans, British merchant vessels had also been sent to the bottom by the enemy’s submarines.

But the main German fleet was cooped up in Heligoland and in the Kiel canal by the British blockade, which, in itself, was proof positive of Great Britain’s naval supremacy. The Kaiser had no mind to give open battle to England on the sea.

This was the situation, then, when submarine D-16 set forth from London, to do, with her new power, what damage she could to the enemy’s fleet.

One day, while the lads had been looking over the vessel, as she lay in drydock, they had seen a man run furtively from the place as they approached; but they were unable to catch him. A second time they had seen him, though not close enough to identify him.

Although both had thought considerably of the matter, neither had mentioned it to the other, and it had been allowed to drop. Nor had they come upon the man again before they put to sea.

The sinking of the German vessel related in the first chapter had been the first venture of the D-16, and now the vessel was heading toward the coast of England again, having gone as far toward the strong fortress of Heligoland as Lord Hastings had deemed advisable at that time.

All day the little vessel continued on her way, traveling upon the surface, for there was now no need to submerge. She went very slowly, and night found her not many miles from the scene of her first encounter.

A sharp lookout was kept for some sign of an enemy, but there was none.

With the coming of the first light of day, Jack and Frank ascended the bridge together, and turned their eyes toward the west. A faint cloud of smoke on the horizon gave evidence of a ship of some sort.

“Probably a British vessel,” said Jack.

“Can’t tell,” returned Frank. “A German cruiser may have succeeded in running the blockade and getting in behind.”

“That’s true, too,” Jack agreed. “It’s a mystery to me how they do it. England is supposed to have them safely bottled up. It’s beyond me how they get out.”

“It’s beyond me, too. Of course, it’s easy enough for the submarine; but you’d think we’d be bound to spot a big cruiser.”

“Something will have to be done, sooner or later. What’s the use of a blockade if it doesn’t blockade?”

“Well,” said Frank dryly, “I have no doubt that if you have a plan, the admiralty would be glad to know it.”

“I haven’t any plan; but England will have to do something. That’s certain.”

“There is no question about that. Hello!”

“What’s up?” asked Jack, looking at his friend in some surprise.

For answer Frank pointed toward the east. Jack peered intently into the distance and also uttered an exclamation of surprise.

“Germans?” he asked.

“Must be,” replied Frank briefly.

He glanced ahead again. Then turned to his friend with an exclamation of satisfaction.

“If they are,” he said, “they’ll get more than they bargained for this time.”

For what his eyes had made out, through his glass, was the outline of five large battle cruisers, convoyed by a flotilla of torpedo boats.

“British?” asked Jack eagerly.

“Look like it,” replied Frank, “but I can’t tell for sure.”

It was plain to both lads that neither fleet had made out the presence of the other, and both—the one from the West and the one from the East—were steaming directly toward the little submarine, which lay squarely between them, though out of sight, because she sat so low in the water.

“Call Lord Hastings,” said Jack, and Frank hastened to obey.

A moment later the commander of the D-16 appeared on the bridge. He took in the situation at a glance.

“The fleet to the east is German,” he said, after a careful scrutiny through his glass, “and the other must be the British fleet commanded by Admiral Beatty.”

“Then there will be a fight,” said Frank.

“There will be unless the Germans perceive our ships soon enough to give them a chance to escape,” returned Lord Hastings. “Another hour undiscovered, however, and we’ll get them sure, for if I am not mistaken, the leading British ships are the Lion and the Tiger—the fastest cruisers afloat today. Also their guns are greatly superior to those of the enemy.”

“But what are we going to do?” asked Frank, somewhat impatiently. “We are not going to stand by and look on, are we?”

Lord Hastings smiled.

“Don’t you worry,” he said quietly. “We’ll take our part, whatever it may be. We’ll go below now.”

The three descended. The conning tower and bridge were closed behind them, and soon the tanks were opened. The D-16 submerged until the top of her periscope barely protruded above the edge of the water.

Lord Hastings, Frank and Jack took turns watching the approach of the two fleets, now that they were in range of the periscope; and it was while the latter was at the instrument that sudden signs of commotion became noticeable on the German vessels. At the same instant Jack perceived that the British ships had increased their stride, and were making directly for the Germans.

“They have sighted each other, sir,” he cried, turning to Lord Hastings in great excitement.

Lord Hastings sprang to the periscope.

“And the Germans are turning to run,” he said, after a quick glance.

It was true. The German commander, realizing that he was probably no match for the powerful British squadron, had no mind to give battle when the odds were as nearly even as they were now. Evidently he had more confidence in the power of his enemy than he had in his own. Therefore, immediately the British fleet was sighted, he gave the command to come about and make for the protection of the mined area about Heligoland at full speed.

But the time lost in coming about was to prove a severe blow to the Germans. The British fleet, led by the Tiger, Admiral Beatty’s flagship, had sighted its prey, and was making after it at full speed.

“Guess we might as well take a hand in this,” remarked Lord Hastings coolly. “Submerge another five fathoms, Mr. Templeton.”

More water was let into the tanks and the D-16 dropped rapidly lower into the sea.

“Full speed ahead, Mr. Templeton,” came the next command.

The D-16 seemed to leap forward like a live thing, as she dashed in pursuit of the fleeing German fleet.

“More notches for the table, I guess,” said Frank to Jack.

“We’ll see,” was the latter’s reply. “I hope so.”