looked upward in momentary expectation of a battle in the air. Then the two flying machines passed north along the line of Broadway, crossed over Bronx park, and came to the vicinity of Pelham bay, in Westchester county.

Here the aviator who had shown such pugnacity in his dashes and swirls at the other, and who had been repulsed only by the finest skill and tact, wheeled straight to the west and was soon lost to sight in the gathering darkness.

For a moment it seemed that the aviator who had thus far acted only on the defensive was about to become the aggressor and follow in the wake of his persecutor. In fact, he was about to swing away in pursuit when the ringing of a bell at a hangar below attracted his attention. Then, with a frown showing on a boyish face, he swung to the north a short distance and volplaned to a level space in front of the hangar.

Descending from his seat, the aviator was greeted, rather anxiously it seemed, by two boys not far from his own age. Very little was said until the flying machine had been run into the great shed, and then the three turned away to a rather elaborate office building which stood in a grove of trees at the entrance to the grounds.

A chill wind was blowing off Long Island sound, and the boys found a grate fire burning brightly in a private room at the rear of the structure. They seated themselves before the leaping flames and looked expectantly into each other’s faces for a moment before speaking.

Those who have read the opening volume of this series will need little introduction to James Stuart, Ben Whitcomb and Carl Nichols. Street boys of sixteen, they had, some months before, met Louis Havens, the famous millionaire aviator, and accompanied him on a trip to Mexico which had brought both fame and fortune to every member of the party.

On their return to New York from the “Burning Mountain” the boys had planned a course in college, but, at the request of Mr. Havens, they had promised to undertake a daring commission from the New York chief of police. A short time before their return to the city the night-watchman of the Buyers’ Bank had been murdered, the monster safe dynamited, and thousands of dollars in currency and securities taken.

It was believed by the chief of police that the burglars—two of the craftiest and most desperate criminals on the continent—were in hiding in the wild and mountainous region south of Monterey bay, on the Pacific coast.

On the theory that the Flying Machine Boys would be able to visit every nook and corner of the region where the criminals were supposed to be, with comparative ease, in their new and up-to-date machines, and, also, that the appearance of the lads in that section would not be apt to arouse the suspicions of the hunted men, the chief of police had proposed the journey to Havens, and he had induced the boys to accept the almost princely offer made by the official.

On account of the hazardous nature of the proposed trip, and because of the long distances to be traveled, special attention had been given to the Louise and the Bertha, the two aeroplanes ordered made by the boys immediately upon their arrival at New York. These machines had been completed the previous day, and the trip over New York bay made by Jimmie Stuart that afternoon had been the first tryout for the Louise, a very strong aeroplane, capable of carrying, when necessity required, two passengers and at least a hundred pounds of camp equipage and provisions.

“Who’s your friend?” asked Carl Nichols, short, fat, blue of eyes and pink of skin, as the three boys sat before the open grate fire in the private room in the office building at Havens’ hangar.

“He’s no friend of mine!” Jimmie Stuart,