Donald sprang forward and tested it with his weight and it held.

Then, without another word, he braced his feet against the wall and in almost less time than it takes to write it, he was at the top.

"Do you see any one?" asked the officer from below.

"No, sir; but there is the sound of pacing footsteps on the walk that runs along the side of the house."

"Good! Now make fast the grapnel and we will see if there are a dozen men here who can climb to the top."

The dozen were quickly found and they were soon at the top of the wall. The officer finally decided to add to their armament one of the rapid-firers.

"If there is any resistance," he said, "we'll knock the side of the house in."

"What shall I do?" asked Donald.

"You'd better come back into the boat. You can help us to enter the house in front."

Donald slid down the rope and the expedition quickly made a landing. Silently, so as not to alarm those within the house, the men took their way to the front of the mansion, which was at once recognized as one of the finest in Vera Cruz.

"Do you know who lives here?" asked the officer.

"No, sir," replied Donald.

"I was told it was the residence of one of Huerta's generals,"

said a soldier. "It was pointed out to me the first day we landed."

"Good!" from the officer. "I shall now have no compunction about entering the place."

With Donald and a couple of soldiers, the officer approached the door and gave a vigorous knock.

There was no response and he knocked again.

"Who is there?" finally came a voice, evidently a mozo.

"An American officer. Open the door in the name of the law."

There was a still further delay and then another voice asked: "How do we know it is an officer?"

"Open and find out, before I force the door."

There was a still further delay.

"It's a good thing we guarded the rear," said Donald. "They are evidently trying to gain time to spirit Lieut. Grimes away, thinking it is Gen. Funston."

"True," said the officer. "We'll wait no longer. Corporal, force the door!"

The corporal placed a small stick of dynamite under the door and fixed a fuse.

"Step back a little," he said.

All obeyed, when at the instant the door was thrown open and a hand, in which was an automatic revolver, was thrust out, directly in the face of the officer.

But quick as was the action, Donald was not taken off his guard.

With a single swift blow he struck the weapon into the air.

At the same instant a fusillade from the rear of the house gave evidence that the men stationed upon the wall had done their work.

CHAPTER XXVIII.

BILLIE MAKES A DISCOVERY.

Having released Lieut. Grimes and taken the plotters into custody, the next task was to locate Strong, the mountebank, and to solve the mystery of the box.

Outside the one exclamation which Donald had uttered when he recognized Strong as the third of the plotters, he had not betrayed his secret to any one, and when Lieut. Grimes told his story to Gen. Funston and described the American, Donald vouchsafed no information which might help to apprehend him.

"The plotter told the Mexicans that he could be found at the Hidalgo Hotel whenever he was wanted," explained Lieut. Grimes.

"While he may have lied about it, I think we should send a force and try to locate him."

The advice was acted upon and a corporal and four soldiers sent to the hotel, but to no purpose. No such man was known there.

"What was the use of my saying anything about it?" asked Donald of the other two boys when he returned to Santiago's house and narrated to them the story of his evening's adventure. "I was sure he was not there and I have no idea where he is; but we'll find him and the ten thousand dollars given him by the Mexicans for his treachery."

"That's right, Don," was Billie's comment. "If you can get that money and I can get my ten thousand pounds, this trip won't be so bad."

"In the meantime," said Adrian, "I propose that some of us go to bed. It's nearly two o'clock and there is mighty little chance that any one will try to steal the box again."

"I think you are right," said Billie. "We might as well all go.

The doctor is still with Santiago and will stay until he is better. It isn't at all likely that any one will try to come into this room while he is here."

Billie voiced the unanimous opinion, and a few minutes later the young people had all sought their beds, leaving instructions with the physician and the servants that they were to be called if any change for the worse occurred in Santiago's condition.

It was probably two hours later, just about daylight, that the entire household was awakened by a terrible shriek and one of the maids rushed out into the patio.

The boys came to their feet with a bound and hurried from their room on the ground floor, while the others appeared at the head of the stairs.

"What is it?" asked Donald as soon as he could quiet the shrieking maid. "What are you yelling about?"

"Oh, senor!" she cried. "I have seen the devil."

"Nonsense," laughed the boy. "You had a nightmare."

"No, senor. It was the devil. He had horns and a tail and he had the little box under his arm. I saw him!"

"The box!" cried Billie. "Quick, Lucia, look and see if the box is gone!"