"_Sapristi!_" hissed the Mexican. "Did you see?" and he bent forward to whisper hurriedly to his companion.

Stewart recovered himself instantly, but the damage had been done.

"h.e.l.lo, Dawson," he answered in English, trusting that the men at the next table had not noted his slip. "Sit down and have something? Rotten weather, isn't it? And not a lead in sight. These Mexicans seem to be afraid to enter into any contract that ties them up more than a year--and eighteen revolutions can happen in that time."

As Dawson seated himself, Stewart gave him a hasty sign to be careful.

Watching the Mexican and his companion out of the corner of his eye, he steered the conversation into harmless channels, but a moment later the pair at the next table called the waiter, gave some whispered instructions, and left.

"What's the matter?" asked Dawson.

"Nothing--except that I involuntarily registered a knowledge of Spanish when you spoke to me just now, and I've spent several months building up a reputation for knowing less about the language than anyone in Mexico City. As luck would have it, there was a couple seated at the next table who were giving me what sounded like the first real dope I've had since I got here. I'll tell you about it later. The question now is to get back to the hotel before that precious pair get in their dirty work. A code message to Washington is all I ask--but, if I'm not mistaken, we are going to have our work cut out for us on the way back."

"Scott! Serious as that, is it?" muttered Dawson. "Well, there are two of us and I'd like to see their whole dam' army try to stop us. Let's go!"

"Wait a minute," counseled Stewart. "There's no real hurry, for they wouldn't dare try to start anything in the open. In case we get separated or--if anything should happen--wire the Department in code that a vessel with a Spanish name--something that begins with 'Eep'--has cleared Hamburg, loaded with guns and ammunition. Expected at Vera Cruz about the twenty-first. Germany's behind the whole plot. Now I'll settle up and we'll move."

But as he reached for his pocketbook a Mexican swaggering along the sidewalk deliberately stumbled against his chair and sent him sprawling.

Dawson was on his feet in an instant, his fists clenched and ready for action.

But Stewart had noted that the Mexican had three companions and that one of the men who had occupied the adjoining table was watching the affair from a vantage point half a block away.

With a leap that was catlike in its agility, Stewart seized the swaggering native by the legs in a football tackle, and upset him against his a.s.sistants.

"Quick, this way!" he called to Dawson, starting up the street away from the watcher at the far corner. As he ran, his hand slipped into his coat pocket where the small, but extremely efficient, automatic with which all government agents are supplied usually rested. But the gun wasn't there! Apparently it had slipped out in the scuffle a moment before.

Hardly had he realized that he was unarmed before he and Dawson were confronted by five other natives coming from the opposite direction. The meager lighting system of the Mexican capital, however, was rather a help than a detriment, for in the struggle which followed it was practically impossible to tell friend from foe. The two Americans, standing shoulder to shoulder, had the added advantage of teamwork--something which the natives had never learned.

"Don't use your gun if you can help it," Stewart warned. "We don't want the police in on this!"

As he spoke his fist shot out and the leader of the attacking party sprawled in the street. No sound came from Dawson, beyond a grunt, as he landed on the man he had singled out of the bunch. The ten seconds that followed were jammed with action, punctuated with the shrill cries for reinforcements from the Mexicans, and brightened here and there by the dull light from down the street which glinted off the long knives--the favorite weapon of the Latin-American fighter.

Stewart and Dawson realized that they must not only fight, but fight fast. Every second brought closer the arrival of help from the rear, but Dawson waited until he could hear the reinforcements almost upon them before he gave the word to break through. Then--

"Come on, Jack!" he called. "Let's go!"

Heads down, fists moving with piston-like precision, the two Americans plowed their way through. Dawson swore later that he felt at least one rib give under the impact of the blows and he knew that he nursed a sore wrist for days, but Stewart claimed that his energies were concentrated solely on the sc.r.a.p and that he didn't have time to receive any impression of what was going on. He knew that he had to fight his way out--that it was essential for one of them to reach the telegraph office or the emba.s.sy with the news they carried.

It was a case of fight like the devil and trust to luck and the darkness for aid.

Almost before they knew it, they had broken through the trio in front of them and had turned down the Calles Ancha, running in a form that would have done credit to a college track team. Behind them they heard the m.u.f.fled oaths of their pursuers as they fell over the party they had just left.

"They don't want to attract the police any more than we do," gasped Dawson. "They don't dare shoot!"

But as he spoke there came the z-z-i-pp of a bullet, accompanied by the sharp crack of a revolver somewhere behind them.

"Careful," warned Stewart. "We've got to skirt that street light ahead.

Duck and--"

But with that he crumpled up, a bullet through his hip.

Without an instant's hesitation Dawson stooped, swung his companion over his shoulder, and staggered on, his right hand groping for his automatic. Once out of the glare of the arc light, he felt that he would be safe, at least for a moment.

Then, clattering toward them, he heard a sound that spelled safety--one of the open nighthawk cabs that prowl around the streets of the Mexican capital.

Shifting Stewart so that his feet rested on the ground, he wheeled and raked the street behind him with a fusillade from his automatic. There was only a dull ma.s.s of whitish clothing some fifty yards away at which to aim, but he knew that the counter-attack would probably gain a few precious seconds of time--time sufficient to stop the cab and to put his plan into operation.

The moment the cab came into the circle of light from the street lamp Dawson dragged his companion toward it, seized the horse's bridle with his free hand and ordered the driver to halt.

Before the cabby had recovered his wits the two Americans were in the vehicle and Dawson had his revolver pressed none too gently into the small of the driver's back. The weapon was empty, but the Mexican didn't know that, and he responded instantly to Dawson's order to turn around and drive "as if seventy devils of Hades were after him!"

Outside of a few stray shots that followed as they disappeared up the street, the drive to the Emba.s.sy was uneventful, and, once under the shelter of the American flag, the rest was easy.

Stewart, it developed, had sustained only a flesh wound through the muscles of his hip--painful, but not dangerous. Within ten minutes after he had reached O'Shaughnessy's office he was dictating a code wire to Washington--a cable which stated that a vessel with a Spanish name, commencing with something that sounded like "Eep," had cleared Hamburg on the seventh, loaded with arms and ammunition destined to advance the interests of Mexican revolutionists and to hamper the efforts of the United States to preserve order south of the border.

The wire reached Washington at noon of the following day and was instantly transmitted to Berlin, with instructions to Amba.s.sador Gerard to look into the matter and report immediately.

Vessel in question is probably the _Ypiranga_ [stated a code the following morning]. Cleared Hamburg on date mentioned, presumably loaded with grain. Rumors here of large shipment of arms to some Latin American republic. Practically certain that Wilhelmstra.s.se is behind the move, but impossible to obtain confirmation. Motive unknown.

Ten minutes after this message had been decoded the newspaper correspondents at the White House noted that a special Cabinet meeting had been called, but no announcement was made of its purpose or of the business transacted, beyond the admission that "the insult to the flag at Tampico had been considered."

Promptly at noon the great wireless station at Arlington flashed a message to Admiral Mayo, in command of the squadron off the Mexican coast. In effect, it read:

Proceed immediately to Vera Cruz. Await arrival of steamer _Ypiranga_, loaded with arms. Prevent landing at any cost.

Blockade upon pretext of recent insult to flag. Atlantic Fleet ordered to your support.

"The rest of the story," concluded Quinn, "is a matter of history. How the fleet bottled up the harbor at Vera Cruz, how it was forced to send a landing party ash.o.r.e under fire, and how seventeen American sailors lost their lives during the guerrilla attack which followed. All that was spread across the front pages of American papers in big black type--but the fact that a steamer named the _Ypiranga_ had been held up by the American fleet and forced to anchor at a safe distance offsh.o.r.e, under the guns of the flagship, was given little s.p.a.ce. Apparently it was a minor incident--but in reality it was the crux of the whole situation, an indication of Germany's rancor, which was to burst its bounds before four months had pa.s.sed, another case in which the arm of Uncle Sam had been long enough to stretch halfway across a continent and nip impending disaster."

"But," I inquired, as he paused, "what became of Dawson and Stewart?"

"That I don't know," replied Quinn. "The last time I heard of Jack he had a captain's commission in France and was following up his feud with the Hun that started in Mexico City four months before the rest of the world dreamed of war. Dawson, I believe, is still in the Department, and rendered valuable a.s.sistance in combating German propaganda in Chile and Peru. He'll probably be rewarded with a consular job in some out-of-the-way hole, for, now that the war is over, the organization to which he belongs will gradually dwindle to its previous small proportions.

"Strange, wasn't it, how that pair stumbled across one of the first tentacles of the World War in front of a cafe in Mexico City? That's one beauty of government detective work--you never know when the monotony is going to be blown wide open by the biggest thing you ever happened upon.

"There was little Mary McNilless, who turned up the clue which prevented an explosion, compared to which the Black Tom affair would have been a Sunday-school party. She never dreamed that she would prevent the loss of millions of dollars' worth of property and at least a score of lives, but she did--without moving from her desk."

"How?" I asked.

But Quinn yawned, looked at his watch, and said: "That's entirely too long a story to spin right now. It's past my bedtime, and Mrs. Quinn's likely to be fussy if I'm not home by twelve at least. She says that now I have an office job she can at least count on my being round to guard the house--something that she never could do before. So let's leave Mary for another time. Goodnight"--and he was off.

IV

THE CLUE ON SHELF 45