time and those below, discovering just about where they meant to land, had started on the run, apparently eager to be on hand in order to obtain a fresh close-up of the mysterious chums who had been hanging around the airport for such a length of time.

Never had a boat dropped down more lightly than did their craft—Jack could not help giving his mate a look of overpowering joy at the slight impact, which was returned in full measure by the proud Perk who anticipated wonderful things to come when they got going for fair up among the clouds or dodging through the canyons of the mighty Rockies, wherever the hand of Fate, and orders from Headquarters, took them.

So the landing was made and the wonder ship safely housed in the hangar they had hired which could be securely locked to keep curious minded or unscrupulous people from trying to get a line on its several novel features.

A short but serious-looking chap came up to have a few words with Jack—this was the party who had been hired especially to keep watch and ward over their highly prized aerial steed. Cal Stevens had been recommended as a man to be trusted and although he had no positive knowledge of their identity, he did know they were clean sportsmen and men of their word. Consequently Jack felt the precious ship given into their charge by the Government would be carefully guarded throughout each night.

They left the field with several figures trailing after them for the mystery hovering over their movements had piqued the curiosity of a number of men. All manner of queer stories, resting on insecure foundations, had been rumored so that people pointed them out in the street and some wise-acres even gained considerable notoriety by pretending to know it all, though under a pledge to keep their secret inviolate.

It became even necessary to resort to expedients in order to shake these snoopers as the indignant Perk called them and usually a vehicle of some sort offered them an easy way to beat out the clan. On this particular evening, however, a big car occupied by several men whom they did not remember having noticed before, kept after their own vehicle up to the very door of the modest house in which they had a room.

“I say it’s a danged shame,” stormed the angry Perk as the two of them started to strip and get into ordinary citizen’s clothing so they would not attract unpleasant attention while eating their supper and attending the movies later on—“that pesky car kept on our tail right up to the door an’ chances are it’s parked somewhere out there right now, awaitin’ for us to hike over to the Emporium restaurant. Riles me for fair, partner, an’ for two cents I’d like to stand them hoboes on their heads, on’y I guess that’d be fool’s play for me.”

“It certainly would, Perk,” his chum assured him as they dressed. “Men in the detective line never want to draw attention to themselves for once it’s known what calling they’re engaged in and a lot of their value to their employers is lost. That’s just why the detectives in big cities like New York wear masks when suspects are lined up each morning for inspection. You know that, of course, Perk, but I’m just reminding you because if you get all ‘het up’ you might say or do something that would spill the beans for us.”

“I’ll cool down right away, Jack old hoss,” the other assured him contritely. “That’s my greatest weakness you know, an’ I’m countin’ on my best pal to keep a finger on my pulse so’s to check me up when I threaten to run loose with my too ready tongue. Wait a minute, Jack, till I get a paper so I c’n read up on the dope as I munch my feed. I’m wanting to learn whether anything’s been heard from our mutual friend, Buddy Warner, the best air mail pilot on the job today.”

“I certainly hope he’s turned up since we jumped off this morning,” said Jack