Readers of the first volume of this series, "The Girl Aviators and The Phantom Airship," will recall Mr. Harding. They will also be likely to recollect his son, Fanning, who made so much trouble for Peggy Prescott and her brother, culminating in a daring attempt to "bluff" them out of entering a competition for a big aerial prize by constructing a phantom aeroplane. Fanning's part in the mystery of the stolen jewels of Mrs. Bancroft, the mother of Jess and Jimsy, will likewise be probably held in memory by those who perused that volume. The elder Harding's part in the attempt to coerce the young Prescotts into parting with their aerial secrets, consisted in trying to foreclose a mortgage he held on the Prescott home, with the alternative of Roy turning over to him the blue prints and descriptions of his devices left the lad by his dead father. How the elder Harding was routed and how the Girl Aviator, Peggy Prescott, came into her own, was all told in this volume. Since that time Mr. Harding's revengeful nature had brooded over what he chose to fancy were his wrongs. What the fruit of his moody and mean meditations was to be, the Mortlake plant, which he had financed, was, in part, the answer.

In the volume referred to, it was also related how Peter Bell, an old hermit, had been discovered by means of the Prescott aeroplane, and restored to his brother, a wealthy mining magnate.

In the second volume of the Girl Aviators, we saw what came of the meeting between James Bell, the westerner, and the young flying folk. By the agency of the aeroplane, a mine—otherwise inaccessible—had been opened up by Mr. Bell in a remote part of the desert hills of Nevada. The aeroplane and Peggy Prescott played an important part in their adventures and perils. Notably so, when in a neck-to-neck dash with an express train, the aeroplane won out in a race to file the location papers of the mine at Monument Rocks. The rescue of a desert wanderer from a terrible death on the alkali, and the routing of a gang of rascally outlaws were also set forth in full in that book, which was called "The Girl Aviators on Golden Wings."

The present story commences soon after the return of the party from the Far West, when they were much surprised—as has been said—to observe the mushroom-like rise of the Mortlake factory. But of what the new plant was to mean to them, and how intimately they were to be brought in contact with it, none of them guessed.

"Well, Mortlake," observed Mr. Harding, in his harsh, squeaky voice—not unlike the complaint of a long unused door, "well, Mortlake, we are getting ahead, I see."

The two men had, by this time, passed within the big sliding doors of the freshly-painted shed, and now stood in a maze of machinery and strange looking bits of apparatus. From skylights in the roof—there were no side windows to gratify the inquisitive—the sunlight streamed down on three or four partially completed aircraft. With their yellow wings of vulcanized cloth, and their slender bodies, like long tails, they resembled so many dragon-flies, or "devil's darning needles," assembled in conclave upon the level floor. At the farther end of the shed was a small blast furnace, shooting upward a livid, blue spout of flame, which roared savagely. Actively engaged at their various tasks at lathes and work-benches, were a dozen or more overalled mechanics, the most skillful in their line that could be gathered. Here and there were the motors, the driving power of the "dragon flies." The engines glistened with new paint and bright brass and copper parts. Behind them were ranged big propellers of laminated, or joined wood, in stripes of brown and yellow timber. Altogether, the Mortlake plant was as complete a one for the manufacture of aerial machines as could have been found in the country.

"Yes, we are getting along, Mr. Harding," returned Mortlake, "and it's time, too. By the way, Lieut. Bradbury is due here at noon. I want to have everything as far advanced as possible in time for his visit. You won't mind accompanying me then, while I oversee the workmen?"

Followed by Mr. Harding, he made an active, nervous tour of the work-benches, dropping a reproof here and a nod of commendation or advice there.

When he saw a chance, Mr. Harding spoke.

"So the government really means to give us an opportunity to show the worth of our machines?" he grated out, rubbing his hands as if washing them in some sort of invisible soap.

"Yes, so it seems. At any rate, they notified me that this officer would be here to-day to inspect the place. It means a great deal for us if the government consents to adopt our form of machine for the naval experiments."

"To us! To you, you mean," echoed Mr. Harding, with an unpleasant laugh. "I've put enough capital into this thing now, Mortlake. I'm not the man to throw good money after bad. If we are defeated by any other make of machine at the tests I mean to sell the whole thing and at least realize what I've put into it."

Mortlake turned a little pale under his swarthy skin. He rubbed his blue chin nervously.

"Why, you wouldn't chuck us over now, Mr. Harding," he said deprecatingly. "It was at your solicitation that the plant was put up here, and I had relied on you for unlimited support. Why did you go into the manufacture of aerial machines, if you didn't mean to stick it out?"

"I had two reasons," was the rejoinder, in tones as cold as a frigid blast of wind, "one was that I thought it was certain we should capture the government contract, and the other was—well, I had a little grudge I wished to satisfy."

"But we will capture the government business. I am not afraid. There is no machine to touch the Mortlake that I know of——"

"Yes, there is," interrupted Mr. Harding; "a machine that may be able to discount it in every way."

"Nonsense! Where is such an aeroplane?" "Within a quarter of a mile from here. To be accurate, young Prescott's—you know whom I mean?"

The other nodded abstractedly.

"Well, that youth has a monoplane that has already caused me a lot of trouble." The old man's yellow skin darkened with anger, and his blue pinpoints of eyes grew flinty. "It was partly out of revenge that I decided to start up an opposition business to his. He was in the West till a few days ago, and I never dreamed that he would return till I had secured the government contract. But I am now informed—oh, I have ears everywhere in Sandy Beach—that this boy and his sister, who is in a kind of partnership with him have had the audacity to offer their machine for the government tests also."

"Audacity," muttered Mortlake under his breath, but Harding's keen ears caught the remark.

"It is audacity," agreed the leathern-faced old financier; "and it's audacity that we must find some way to checkmate. I've never had a business rival yet that I haven't broken into submission or crushed, and a boy and a girl are not going to outwit me now. They did it once, I admit, but this time I shall arrange things differently."

"You mean——"

"That I intend to cinch that government business."

"But what if, as you fear, the Prescotts have a superior aeroplane?"

"My dear Mortlake," the pin-point eyes almost closed, and the thin, bloodless lips drew together in a tight line, "if they have a superior machine, we must arrange so that nobody but ourselves is ever aware of the fact."

With a throaty gurgle, that might, or might not, have been meant for a chuckle, the old man glided through the doors, which, by this time, he had reached, and sliding rather than stepping into his machine, gave the chauffeur some orders. Mortlake, a peculiar expression on his face, looked after the car as it chugged off and then turned and re-entered the shop. His head was bent, and he seemed to be lost in deep thought.