We watched them in conversation, but could not distinguish one single word of what was said. All we could gather was that the fat man was overbearing in his manner, and that the visitor was most humble and subservient against his will.

For a full half-hour we watched, but unable to gather anything further, we were compelled to return to the house next door and regain the street, where for still twenty minutes longer we waited for the visitor's exit. When at last he came forth we followed him to the corner of Knightsbridge, opposite the Hyde Park Hotel, where he boarded a motor-bus, from which he eventually descended at the corner of Gray's Inn Road walking thence to a house in Harpur Street, Bloomsbury, where we later on discovered he lodged, under the name of Leon Karff.

The nature of the mission entrusted to this man, if one had actually been entrusted to him, was a mystery, yet it was a curious fact that "Harpur Street" appeared upon that scrap of paper which to us was such an enigma.

Next morning at six o'clock, I was already idling, at the corner of Harpur Street and Theobalds Road, but not until three hours later did the foreigner emerge and walk toward Holborn. Thence he took a motor-bus back to Sloane Street, and calling upon Hartmann, spent another half an hour with him.

And afterwards he went straight home. It was then about noon, and having an engagement in Court, I was compelled to relinquish my vigil. But at a little after five Ray entered our chambers, exclaiming:

"As I expected! That man Karff has been to see Steinheim in the hospital. I was there awaiting him, believing that he might visit him. Apparently the injured man has given him certain instructions."

"About what?"

Ray shrugged his shoulders in blank ignorance. Then he said, "We have advanced one step toward the solution of the problem, my dear Jack. But we have not gone very far."

He took the copy of the cryptogram from my writing-table and again examined it. The figures "6.11" puzzled him. Many times he referred to them.

Four days passed, during which we kept strict observation upon Karff and followed him wherever he went. On the fifth day, Ray having spent all the morning watching him, to relieve him I walked along the Theobalds Road a few minutes before one and paused, as usual, before the oil shop at the corner. There was no sign of my friend, and though I waited through the whole of that cold afternoon and evening, continuing my wearisome vigil till midnight, yet he did not come.

Much surprised, I returned to New Stone Buildings, where I found a telegram from Ray, sent from Waterloo Station at three o'clock, telling me that all was right, and urging me to await further information.

This I did. For a whole week I possessed myself in patience, not knowing where Ray was or what had befallen him. That he was on the trail of a solution of the mystery was evident, but he sent me no word of his whereabouts.

It was apparent, however, that he was no longer in London.

Eleven days after his disappearance I one afternoon received another telegram, which had been handed in at Chichester, asking me to go at once to the Queen's Hotel at Southsea, where he would meet me at ten o'clock that night.

At the hour appointed I awaited him in my bedroom overlooking Southsea Common and the harbour, and at last he joined me. I saw by the serious expression upon his face that something unusual had happened.

"The fellow Karff has realised that I'm following him, Jack. Therefore you must take the matter up. He's in the service of a greengrocer in Queen Street, close to the Hard. I haven't yet discovered his game."

Thus there was left to me a very difficult matter, a mystery which I exerted every effort to unravel. For the next fortnight I watched the fellow incessantly, being relieved sometimes by the pretty daughter of the Admiral Superintendent, whose home was fortunately in the Dockyard. In all weathers and at all times we watched, but we failed to discover anything. Ray remained at the hotel impatient and inactive, and I must admit that more than once I was inclined to believe that he had been mistaken in his surmises. Leon Karff was, as far as we could discover, a hard-working foreigner, driven by force of circumstances into adopting the lowly calling of a greengrocer's assistant. His employer supplied with fruit and vegetables the officers' messes of several of the ships in the Dockyard, and on infrequent occasions he drove in the light cart with his master when on his rounds taking orders.

This round at last he was in the habit of making three times a week.

One Saturday morning, as I was idling along the Hard, I saw Karff and his master, a man named Mitchell, drive in past the policeman at the main gate. But though I waited for over three hours to watch their exit, they did not reappear.

Much surprised at this, I walked round to the Unicorn Gate, at Landport, where, on making judicious inquiries of the policeman on duty, I learnt that Mitchell had driven out – but alone! His assistant, he said, had been sent back on foot with a message through the main gate just when the dockyard men or "maties," as they are called, were leaving work at midday.

Now having stood at that gate when the throngs had poured forth, I was quite certain he had not emerged. But I kept my own counsel, and returned to Southsea, deep in my own reflections.

On taking counsel with Ray, he at once telephoned to Vera at Admiralty House, and an hour later we all three discussed the situation, it being arranged that the Admiral's daughter should contrive to admit us to the Dockyard that night, when all was quiet, in order that we might institute a search for the missing German.

Therefore, just before half-past eleven, we halted before the small private door in the Dockyard wall, used by the Admiral-Superintendent and his household, and as the clock struck the door opened, revealing Vera. Next instant we were within the forbidden zone.

The night was frosty and a good deal too bright to suit our purpose. Vera gave some instructions to her lover, pointing to a row of long, dark sheds with sloping roofs on the opposite side of the Dockyard, saying:

"If he's inside, he's almost certain to be hidden somewhere near No. 4 shed. But be careful of the police; they are very watchful over yonder."

And after refastening the gate she disappeared into the darkness.

In the deep shadows we both crept noiselessly forward, negotiating in safety a pair of lock-gates in the open, and pursuing our way until in the vicinity of the shed which the Admiral's daughter had pointed out we discovered an old boiler, in which we both secreted ourselves.

Hardly had we crept inside when we heard the measured tramp of a policeman, who passed actually within a few feet of us. From the round hole in which we lay we could see Gosport – a pale row of lamps across the harbour.

We waited there, scarcely daring to whisper, until at last the clock struck one. If Karff was in the vicinity of that shed beside which we were secreted, he made no sign. All was silent. Once the shrill siren of a ship out at Spithead broke the quiet. Then its echoes died away.

"I really think we might have a careful look round," Ray suggested after a long silence.

With great care, therefore, we both emerged from our hiding-place, and keeping well within the shadows, passed round shed No. 4, which we found was completely closed in from view, its door being strongly barred and padlocked.

Unable to see anything, we decided to halt in the darkness behind a heap of scrap-iron and to listen for any sound of movement.

The cutting wind chilled us both to the marrow, for a white rime had gathered on the ground. The only sound we heard was that of the measured footsteps of another constable, which advanced and then died away again. There was, however, no sign of the German spy.

"To get in by the door yonder would be impossible. Therefore, he would try the roof," my companion remarked.

"You're right," I said. "You remain down here and watch while I try and get up above."

So I left him, and after considerable difficulty succeeded in gaining the roof of the shed adjoining, crouching in the gutter between the sloping roofs of the two sheds.

On each side of me sloped upwards skylights which lighted the interiors of the building-sheds, but all were thickly coated with a composition of dockyard dust and soot, which had been poured forth from many a warship's funnel as well as from the dozens of furnaces around. All was dark below; therefore I could see nothing.

I had been in my elevated position for fully twenty minutes before I was prompted to creep along to the further end of the gulley, where, to my surprise, I saw that close to where I stood two panes of glass had been neatly removed and laid aside.

Through the hole I gazed down into the interior of the shed, when I was startled to see the small glow of an electric lamp in the hand of the man of whom we were in search.

He was standing beside the long, spindle-shaped hull of a new submarine boat which lay on a very elevated set of stocks on the far side of the shed. Another boat similar, but not so nearly complete, lay at the bottom of the dock alongside her.

As Karff with his electric lamp moved slowly and noiselessly along, carefully examining England's newest submarine, which rumour had said was the most silent and perfect craft of its kind, I was able to make out vaguely that, differing considerably from photographs of other submarines I had seen, the boat on the elevated stocks had a bow which ran out into a kind of snout, while instead of the usual small circular or oval conning-tower she had what looked like a long, narrow superstructure running along the greater part of her length. This, however, was much higher forward than aft. She seemed, too, to have a great number of propellers.

I watched the man Karff making some rapid memoranda, and so occupied was he with his work that he never looked upward. Had he done so, he would certainly have detected my head against the sky.

In a manner which showed him to be fully acquainted with the construction of submarine vessels, he moved to and fro, examining both boats. Then, after about half an hour's minute investigation, he seated himself upon a bench and with his little lamp shaded to throw no reflection he took out a piece of paper and leisurely made a rough sketch of England's newest war-craft, both side and horizontal views.

Leaving him thus occupied, I descended to Ray, and finding him secreted near the water's edge, described what I had discovered.

"Good!" he exclaimed. "So I was not mistaken in that cryptogram after all! We will allow the fellow to complete his work and then compel him to disgorge his notes. They will furnish us with very excellent evidence."

So we waited, keeping our eyes fixed upon the spot where he must descend, and hardly daring to breathe lest we should prematurely alarm him.

The Dockyard clock chimed three, but the spy had not emerged. After another half-hour of watchful silence I saw that Ray began to be anxious. At last the bell rang out four, and scarcely had the last sound died away when we were startled by a splash near us, and next moment discerned a man in white shirtsleeves swimming away.

"Why! That's him!" I gasped. "He's cut a way out of the side of the shed!"

But next moment a boat shot forth from the darkness pulled by a woman who had apparently been waiting close by. The woman was Vera!

In a moment we were both down the steps and pulling in the boat towards the swimming man, who, we saw, was being rapidly approached by a second boat which had also been in waiting until the chiming of the clock.

The spy was exerting every muscle to reach the boat, but we soon overtook him.

Ray called upon him in German to surrender, but he refused, and kept on. Quickly, however, we cut him off from the boat which he was trying to reach, while the rower, seeing the discovery of his friend, pulled away into the darkness.

For some time the spy struggled on, but at last, abandoned and exhausted, he was compelled to obey us and come aboard in order to save his life.

Half dead and helpless he submitted to our search, when in his belt, preserved in an oilskin pocket, we discovered the memoranda and the drawing which I had seen him prepare.

The man, sullen and half drowned, refused to make any statement, though he could speak English well and write it perfectly, as shown by the note on his plan of the new boat; therefore we landed him at the Stony Steps across at Gosport. Before we left him we gave him to understand that if he did not at once leave the country he would be arrested. Yet so absurd is our law that I doubt whether we could have given him in charge even though we had wished!

We rowed back across to the landing-stage at Portsmouth Harbour Station, and after we had seen Vera safely home we returned together to the "Queen's" at Southsea, where, in the secrecy of Ray's bedroom, we examined the spy's plan of the new submarine, and read his memoranda, which were in German, but which translated were as follows:

"Report by Leon Karff, late foreman-fitter at Kiel Dockyard, on Submarine 'F 2,' now building in Shed No. 4, Portsmouth Dockyard.

"This boat would appear to me to be of about 700 tons displacement when complete, possibly rather over. She is, as far as I am able to measure, about 180 feet long with an extreme beam a little forward of amidships of 20 feet. She is fitted with three propeller shafts with three small four-bladed propellers on each. As she is provided with what appear to me to be some kind of turbine engines, I imagine that the centre shaft is for going astern only. The propellers on this shaft seem to be attached in such a way that they could be 'feathered' by suitable gearing on board so as not to retard the vessel's way when going ahead. The engines of this boat are of a type which I have never before seen. I imagine that they are a combination of the new 'gas-producer' engine and the turbine system, the explosion of the combined gas and air being split up and passing into the turbine through a number of different channels simultaneously. This would be a very economical system if the necessary power can be obtained, and would be much safer for use below than petrol engines.

"The boat is evidently intended to operate a good deal in an 'awash' position, for there is fairly thick armour-plating over the greater part of the upper side of the bow, while the fore end of the superstructure is made of two 6-inch Krupp steel plates meeting at an acute angle, and so forming a kind of stem when the boat is moving in this way. The space enclosed between these two plates is evidently intended to be used as the conning-tower. Here there are a periscope, steering-wheel, voice-tubes, and everything necessary for the control of the vessel. There are two horizontal propellers or fans, which seem to be driven by electricity derived from an installation of accumulators, and which are certainly intended to secure horizontal immersion, so the vessel will not plunge or dive, but immerse herself horizontally by means of these propellers, which, by the way, work in vertical shafts running completely through the boat, one forward and the other aft, as was the case in the Nordenfeldt, Waddington, and other early submarines.

"Forward there is an air-lock and diving-chamber, as in the 'Lake' boats, so that divers can get in and out of the vessel whilst under water. It would also afford a means of escape for the crew in the case of accident. This is further provided for by a detachable boat or caisson at the after end of the superstructure capable of holding ten men, I should say, or possibly a dozen. There are also appliances which I suppose are telephone buoys for communicating with the surface. There are six torpedo tubes fitted, one forward, one aft, and the others two on either broadside. And there seems to be provision for six other torpedoes of the 18-inch type.

"There is a long rudder for ordinary steering, and four horizontal ones or planes which are placed abreast the horizontal screws and which, I imagine, act automatically in conjunction with them, as they seem to gear up with the shafts for these propellers. There is a big safety detachable weight which fits loosely into a recess amidships, and four broad wheels with ball bearings which do not fold up as in the 'Lake' boats, but always protrude nearly half their diameter. After all they would not obstruct her way when water-borne more than a keel – or very little more. They are quite independent and unconnected with the interior of the vessel, which while resting on them would receive forward impetus from her propellers. In the 'awash' position she would offer a very small and almost invulnerable target."

"Well," I said, marvelling at what we had translated. "What induced you to believe that the cryptogram had any reference to the new submarine."

"Those figures '6.11' puzzled me greatly," he replied; "but at last I deciphered them as 'F. 2' – F being the sixth letter of the alphabet – the number of our newest and most formidable submarine, which was being kept such a strict secret by the Admiralty. 'Royal Pier' is the name of the hotel in which Steinheim stayed at Southsea, and 18 the number of his room. From facts I elucidated, it was made plain that Max Steinheim was about to embark upon the investigation, being in secret communication with Hartmann, and was to meet Karff at Charing Cross Station. This Steinheim had already, by an ingenious device, secured from a private of engineers named James Ward – whom I have seen – certain information regarding the new boom defences of Portsmouth Harbour. Ward, whose home is at Great Weldon, suddenly discovered to his horror that the man was a German spy, followed him to Guildford, attacked him in the train, and left him for dead. For that reason Steinheim has refused to make any statement to the police. When I saw Ward a week ago, he explained how innocently he had fallen into the trap which the cunning Steinheim had laid for him."

"The evidence you have here in black and white will surely prove convincing," I remarked. "You will go and see Steinheim again, I suppose? He is still in the hospital."

"No. We shall remain silent. To show our hand will only place Hartmann on the alert. To do that is needless. We have prevented the plan of our new submarine going to Germany, and for the present that is sufficient."

And my friend drew up the blind and gazed out upon the rosy dawn across the water.