"Very well, I'll accept your offer," Irving announced. "But I doubt if Mr. Herrmann will allow it. You are a very valuable man in the office, and the operation would surely make it necessary for you to lay off a few days. He'll probably insist that an office boy or clerk or stenographer make the patriotic sacrifice in your stead."

"That'll suit me-just so there is no delay in finding someone who's willing," Strauss replied.

Irving proved to be correct in his prophecy of the probable attitude of the superintendent toward the proposition. Mr. Herrmann objected strenuously for the reason suggested by the spy and he took it on himself to find a person who would supply the skin to be grafted. Two days later he reported success and preparations for the operation were begun.

But everybody connected with these preliminaries had an important lesson to learn regarding the proper method for a layman to approach a matter of science. None of them, of course, knew anything, except in a very general way, about skin grafting. Irving had assumed that it was a simple process, and, as a matter of fact, it is, if we accept the principle of the simplicity of all things. But what startled him most was the simplicity of the error he had fallen into.

Mr. Herrmann gave Irving a note to the superintendent of one of the city hospitals and directed him to go there and make arrangements for the operation. He was authorized to state that a young soldier who had lost one of his legs in the first battle of the Marne had promised to furnish the needed four-by-two inches of skin to replace the tattooed integument on his arm.

The spy did as instructed and was turned over to a member of the surgical staff. The latter listened to the boy's story and his suggestions and then inquired:

"At what college of physicians and surgeons did you get your degree?"

Irving no doubt flushed like a schoolboy. He realized that the member of the hospital staff was laughing at him, and this confused him more than a veiled suspicion that he was a Canadian spy would have done.

"The college I graduated from was that of mother's home remedies," he replied.

"I thought so," nodded the surgeon with a smile. "Let me see-you are in the intelligence department, are you not?"

"Yes, sir."

"Doing important work, aren't you?'

"I believe so."

"Work that requires sharp wit?"

"Supposedly."

"Well, sharp wits never assume anything without some information to back them up. Your ideas of skin grafting are a good deal like a child's. In the first place we shan't need anybody to supply any skin. Sorry to disappoint the young patriot with really commendable spirit of loyalty."

Irving looked his surprise.

"You'll supply all the skin we need," the surgeon continued.

"But it is important that there be no scars," Irving insisted.

"There won't be any, or so slight that they'll be hardly noticeable," was the surgeon's reassuring reply. "Let me explain the process to an unscientific keen wit of the government's intelligence department."

The surgeon lifted the spy's bared arm with his left hand and began his explanation, indicating with one finger now and then the various moves necessary as he described the process.

"With a razor," he said, "we will cut an outline around this hideous art of yours. Then we'll peel off the atrocity and cremate it over an alcohol flame. Next we'll peel a strip of the same length and three-fourths of an inch wide just below here, leaving the upper end of the strip attached and twisting it around so that it will lie midway between the edges of the raw space where the tattooing was. Then we'll cut under the skin along both sides to loosen it an inch or more back and draw the loosened skin to the piece in the center and make a hair suture. The reason we must run a strip of skin over the middle of the raw area is because this area will be too wide for stretching the skin at the sides over it. Skin that is stretched too tight will die. The narrow raw place produced by the peeling of the strip down over the wrist can be covered by pulling together the edges of the skin on both sides after running the razor back under it a short distance. Quite different from the process you imagined, isn't it?"

"Yes, it is," Irving admitted.

"I bet you thought all that was necessary was to peel off a piece of skin and lay it on the raw place after this cubist art picture had been removed. Isn't that true?"

"Maybe-something of the kind. I hadn't thought it out in detail," Irving replied.

"Of course, you hadn't. You'd have been too scientific for a secret service operative, wouldn't you?"

"Can't secret service people be scientific?" Irving inquired.

"What do you think about it?" asked the surgeon. "You ought to know more about it than I do. But I'll tell you what my frank and unscientific opinion in the matter is."

"What is it?"

"That government secret service is 1 per cent information and 99 per cent bluff."

"That's a little strong on the side of the bluff," said the spy, smiling.

"But there's something to it?"

"Yes."

"Now you need this much science to prevent your bluff from getting you into trouble. When you attempt to bluff a scientific man be sure not to bluff along the line of his knowledge and the line of your ignorance. By the way, when do you want that operation performed?

"The sooner the better," Irving replied.

"How about now?"

This almost took the boy's breath away, but after a few minutes he answered:

"That's all right, I suppose, but I'd better call up my office first and tell the boss what's doing."

"Very well; here's a telephone. Call him up."

Irving did so and in a few minutes had authority to "go ahead and have it over as soon as possible."

CHAPTER XXXV
THE TAPPING ON THE WINDOW

Irving slept under an anæsthetic during the operation. He objected at first to the administration of ether, but the surgeon insisted.

"I don't want you to make any trouble," he insisted. "Remember you're not a scientific youth and might do something ridiculous. If I'm going to perform this operation you must take orders and obey them."

That settled it; Irving acquiesced. When he recovered consciousness he found himself in a hospital bed with his left arm bandaged and feeling a good deal like a limb of a tree, or anything else with a like degree of life. He remained in bed until the next morning, when his arm was put in a sling and he was permitted to move about as he pleased, although directed to remain in the hospital. Two days later he was allowed to leave the institution, but was instructed to return daily for examination and redressing of the graft.

He returned at once to the intelligence office and reported the success of the operation. The chief surgeon had informed him that his arm might be taken out of the sling in about a week.

During this period Irving was in the office much of the time, although he was able to be of little service with the use of only one arm. Still, he found it possible to add a good deal to his knowledge of the system of which the government was planning to make him an important agent, and this was, on the whole, quite satisfactory to him.

The youthful spy's plans for carrying out his mission for the British government had been developing rapidly since he became a member of the staff in the German intelligence office. And not a little of this development had been quite unforeseen by him. His original plans, therefore, underwent considerable change as time and experience advanced.

For instance, he decided not to attempt to make a list of names of leading enemy agents in the United States and Canada to take back with him. This had been his original purpose. He now regarded it as unwise, unsafe. He would depend on his memory to retain a store of information of this kind. So he watched and examined and probed and memorized, going over the information he had accumulated many times in his leisure hours in order to keep it fixed and unmistakable in his mind.

"I think I could go back to school and memorize history dates as I never did before," he told himself one evening about a week after the skin-grafting operation. "Gee! I never realized I had such a memory. I can run off a string of dope as long as the tune the old cow died on, just like saying the ABC's."

Irving had forgotten the "tune the old cow died on," but the expression stuck in his mind as a relic of nursery days.

One of the divisions of service in the intelligence department that interested the spy particularly was the telegraphic division. It came as an intermediate grade in his course of instruction, and he was required to learn to read the ticking of the telegraph instrument. Fortunately, a few years before, he had learned the alphabet while amusing himself with an amateur wireless outfit, and it now required comparatively little time for him to develop a fair degree of proficiency as a key-listener.

"You can never tell when it'll be greatly to your advantage to be able to read the telegraph instrument," Mr. Herrmann explained. "In fact, that may be one of your most important occupations in America-tapping wires, for instance."

Indeed, the spy caught a number of messages of incalculable importance while pursuing his studies in this division and made careful note of them in his mental repository.

About a week later he had a novel "telegraphic" experience, which, in turn, was to have an important bearing on his fortunes as a spy in the enemy's country. The affair took place in the rooming house where he was living. While he endeavored to get out in the evening, as a rule, and mingle with citizens of all sorts and descriptions, in order to absorb as much general information as possible, still he retired almost every night in good season, and not infrequently went early to his room to study, rehearse, memorize and plan. In this manner he endeavored to improve every opportunity to make his excursion a success.

He had just finished one of these solitary sessions in which several leading newspapers and magazines played an important part, and was about to lay them aside and prepare for bed, when his attention was attracted by a faint tapping sound. At first he gave little heed to it, presuming, in a semi-conscious way, that it was occasioned by a continuous breath of air and a tiny, loose pendant of some sort in the exterior construction work of the house. But it continued in a strangely familiar way and seemed to grow a little louder very gradually.

Suddenly, Irving sat up straight and listened rigidly. Anyone observing him in this attitude could not have failed to be impressed with the feeling that an alarm of some character was thrilling his every nerve center.

"My goodness!" was the exclamation that smothered itself within him. "What in the world can that mean? Yes, no, yes-somebody is trying to communicate with me. He's using the telegraphic signal. He's asking me to answer, to indicate in some way that I am getting his message. He says he's a friend. He knows I'm a British spy. But maybe it's a trap to catch me. What shall I do? If he's a friend he surely ought to know better than to expect me to make such an admission. But he says he has important information. What-what in the world shall I do? I may be in very great danger. Here is certainly the test of my life."

CHAPTER XXXVI
A REVELATION

"I have an important message for you. I am a French spy. I must get this message to you. Answer me in some way. Heave a big yawn or clear your throat and I'll know you hear me and get what I'm saying. I merely want to make sure you are what I think you are. I don't dare reveal myself to you for fear that I may be mistaken and you'd turn me over to the government."

These words were tapped off, alphabetically, with a small instrument, probably a pencil, on the window overlooking a court inclosed by the building on three sides. After a pause of half a minute, following the appeal just recorded, the dot-and-dash tapping continued thus:

"I am looking through the shade of your window and can see that you are listening attentively; so you need not reply. Just continue to listen, and I shall know everything is all right.

"When you leave for America you will be supplied with a message in cipher, prepared by me, for a certain agent of the kaiser. That message will bear the appearance of having been written by a friend of yours to you, but it will contain information in invisible ink for your benefit as a loyal agent of the Allies. This information will be of great value to the Allies, supplying them with material for undermining the Teutonic spy system in England, France, and America, which recently declared war.

"This is all. I merely wished to advise you of what you will find written with invisible ink on the paper that will be placed in your possession when you set out on your return to America."

The tapping ceased. Irving remained like a statue in his chair for several minutes. Then he arose, went to the window and pulled the shade aside. The court was dark, save for a solitary dim light out at the entrance. He could just faintly discern the steel structure of the fire escape near the window.

"That's the way he got up," he half muttered. "He stood there on that landing while he tapped his message. I wonder who he is and how he spotted me. He must be a very clever fellow. I really believe he's what he represented himself to be; and yet, it may possibly be a trap to catch me. However, I don't see what I can do except await developments."

He went to bed and slept better than might have been expected under the circumstances. But he had become so used to critical situations by this time that he felt almost capable of sleeping peacefully on the "edge of the earth" with a torpedo for a pillow.

Next day the mystery of the window-telegraph spy bothered him a good deal, even more than it did immediately after the fellow had "dotted and dashed" his message on the pane of glass.

"I wonder who he was?" he repeated many times. "I wonder if he's somebody I'm in close touch with every day?"

The suggestion caused him to watch narrowly every person in the office with whom he did business for the German government. But the more he watched, the more unsatisfactory the situation became. He continued his furtive outlook several days, but finally admitted to himself that the prospect of his efforts solving the mystery was anything but bright.

Meanwhile the spy's preparations for a new excursion out into a broad field of international espionage were rapidly drawing to a close. The surgeon at the hospital who had performed the skin-grafting operation on his arm pronounced it sufficiently well healed, first, to warrant taking the limb out of the sling, and then, a week later, for the removal of the bandage. There were a few slightly rough places here and there. around the edges of the patch, and one small scar at the lower end of the middle strip of skin where it had been twisted to cause it to lie "right side out" through the middle of the larger patch and make the latter complete by meeting the outer edges that had been undercut and drawn to it.

All things considered, Irving was well pleased with the course of events during his sojourn in the German capital. Although a number of situations had developed with rather dangerous aspect, he had pulled through all of them with apparent success. While he was still reporting daily at the hospital for the dressing of his arm his lieutenancy commission was acted upon in the war office and was delivered to him through Mr. Herrmann.

At last the day arrived for a windup of the young spy's affairs in the intelligence offices, and he was summoned into the presence of "the baron" and Superintendent Herrmann. A third man also was present to receive the young espionage student. He wore a navy uniform and was introduced as Capt. Bartholf of the submarine service.

"You will go with Capt. Bartholf on board his boat," "the baron" announced, addressing "Lieut. Hessenburg." "He will land you on the coast of Spain and from there you will go to a German consul and devise a method for getting you to Mexico and from there into the United States.

"By the way," the high intelligence official remarked, suddenly interrupting himself and addressing Superintendent Herrmann; "how about that letter that was being prepared for Lieut. Hessenburg to take along?"

"I'll see," replied Herrmann, as he started for the door.

"Bring Strauss in with you," "the baron" called after him. "I may want to ask him some questions."

"Strauss!"

The name echoed in Irving's brain with a succession of significant thrills. What did Strauss have to do with the preparation of the letter he was to take with him? Was it possible-?

He did not finish the sentence in words, but the idea was there and remained uppermost in his mind during the remainder of the session in "the baron's" office. Presently Herrmann returned, accompanied by the card-catalog expert, who carried an envelope of ordinary business-correspondence size in one hand. This envelope he laid on the desk in front of the intelligence chief.

The latter picked it up, looked keenly at Strauss and asked with like sharpness of voice:

"This paper was prepared entirely by you, was it?"

"Yes," the cataloger answered.

"And it has been in no other person's hands at any time since you began work on it?"

"No."

"And you vouch for the accuracy and thoroughness of its preparation?"

"Yes."

"That's all. You may retire."

Strauss left the room. "The baron" turned to Irving, handed him the letter, and said:

"This innocent looking missive is of vast importance. It is addressed in cipher to a very important person in America who is high in the confidence of the United States government. You have learned how to read this cipher and will work it out for yourself. That is all. Good-by. I wish you a continuation of the success that has been yours in a remarkable degree heretofore."

Irving took "the baron's" offered hand and then left the office accompanied by Capt. Bartholf. As he went the name of Strauss continued to ring in his head, together with this startling conviction:

The catalog expert was the French spy who had tapped the "telegraph message" on his window at the rooming house!

CHAPTER XXXVII
THE SUBMARINES

Lieut. Ellis of the Canadian army, alias Lieut. Hessenburg of the German army, had quite enough to think about as he left the office of "the baron" in company with the submarine commander. Out in the reception room the latter took leave of him, saying, "Meet me at the Kaiserhof at 9 o'clock tomorrow morning"; then the youthful spy, with a counter-spying commission from the enemy, went to his desk and began to make arrangements for his departure.

Mr. Herrmann selected from the office force a former soldier who had lost one arm, and to him Irving made a brief statement of the work he had been doing so that his successor might continue where he had left off. For a short time he debated in his mind whether to go to those of his fellow workmen with whom he had been more intimately associated and bid them farewell, but he decided that this would not be in harmony with the "community conduct" of the officials and employes of the bureau. In fact, he had observed little in the association of the office that had suggested real community life. Everybody connected with the intelligence bureau seemed either to have been born with a cold furtiveness of manner or to have developed an espionage attitude of this sort in the atmosphere of the greatest spy system the world had ever known.

However, he disliked very much to leave the place for the last time without passing at least an "aufwiedersehen" to the one person there who he felt certain was a friend of the great cause of human liberty for which the allied nations were fighting. But Strauss seemed disposed to ignore him if possible. He passed several times near the expert's desk, but the latter pored more diligently than ever over his work. Once Irving caught his eye and attempted to pass him a look of intelligent meaning, but Strauss turned away quickly, and Irving left the building without saying good-by to one of the occupants.

"A very cold-blooded business," he told himself. "My! I'm glad to be out of there. I'm afraid I'm not built along cold enough lines for a spy even in behalf of a great and meritorious cause. That fellow Strauss is an ideal spy. He must be the best any nation ever produced. He certainly has worked himself into a powerful position of confidence with the enemy. But that was some chance he took when he tapped that message on my window. I wonder if he expected me to discover who he was after he told me he was the fellow that prepared the letter that was to be given to me. And when he assured the baron that nobody else had had the letter in his possession, nobody else remained for me to suspect. Well, he must know now that I spotted him; but he surely exhibited extremely wise caution when he refused to recognize even a significant look from me. Good-by, Mr. Strauss, or whatever your name is. You were too shrewd to let me shake your hand, and cold judgment tells me you were right. I hope after the war is over I may take a trip to Europe and look you up. But, judging from the way you looked at me, or avoided looking at me, I'm afraid you'd take advantage of the opportunity to give me a calling down such as few people have ever received. I'd probably feel the knives of your sarcasm making ridiculous mince meat out of me."

Next morning, promptly at the appointed hour, Irving was at the information desk of the Kaiserhof, asking for Capt. Bartholf. The latter was in his room waiting for the young intelligence officer. Two hours later, arrangements having been made for the transfer of baggage, the captain and the lieutenant were on board a train and headed for one of the principal submarine ports of the German coast.

The trip was uneventful, except that it afforded Irving an opportunity to make a study of the character of an official representative of the policy of ruthlessness of the military government of Germany. Capt. Bartholf was a fit exponent of this policy and exceedingly efficient because of the intelligence with which he could disguise the barbarous nature of his ideas. Hours before they reached the port of their destination, the spy was convinced that an enemy who fell into the clutches of this sub-sea commander might as well toss hope to the fishes.

"I don't believe he'd take a prisoner if he could help it," Irving mused as a climax to his conclusions. "I'd never surrender to a man like him if I knew in advance what kind of fellow he was. It'd be a finish fight even though there were no hope in it for me."

They arrived at the seaport in the evening and took rooms at a hotel. Two days they remained in this city. The captain explained the delay by saying that he was awaiting orders to start on a raiding cruise. Finally the orders arrived, and he announced that they would go on board at once.

Half an hour later they were at the docks, where a dozen U-boats were lined up, some of them taking in provisions and oil, or undergoing inspection and minor repairs. Irving's eyes were busy with new objects of interest at this submarine harbor, for he had never before seen an undersea craft. Eagerly he took in the scene, regarding the various objects with more than the calculating interest of an international spy; and while in the act of boarding the vessel in which he was about to take his first submarine trip, he almost forgot, as the romantic thrill of the experience went through him, that he was surrounded by enemies in whose hands his life would be worth only a volley of rifle balls if his real identity were revealed to them.