Suddenly, from the liner’s whistle, a great cascade of white steam spouted.

“Wough-h-h-h-h-h-h-h-h-h!”

It was the vessel’s siren blowing a greeting to the young adventurers of the air.  At the same instant a deep-throated roar, a cheer from cabin and steerage passengers alike, winged its way upward.  Roy acknowledged it by a graceful wave of his cap.  Then the cheering broke forth afresh.

The passengers of the newest ocean giant, the Ruritania, realized that they were seeing a spectacle that would remain in their memories all their lives.  Having conquered old ocean with leviathan vessels, man was now seeking to subdue the air to his utility.

CHAPTER X.

AN AERIAL POST OFFICE.

Peggy addressed half a dozen cards.  Two, of course, went to Jess and Jimsy, another to Aunt Sallie Prescott; one to the captain of the Ruritania, and one other, which bore the address, “Eugene Mortlake, Esq.”

It was a mischievous freak that made Peggy write this last missive, which read: 

    TO MR. EUGENE MORTLAKE,

        Per Steamer Ruritania—­in Mid-air: 
        Greetings from aeroplane Golden Butterfly.

R. & M. PRESCOTT.

That was all, but Peggy knew that it would serve its prankish purpose.

All this time the Silver Cobweb had been out at sea, but now, apparently detecting the maneuvers of the Golden Butterfly, she headed about, and came racing back.  Peggy deftly attached weights—­spare bolts from the tool locker—­to each of the cards, and then, snatching up a megaphone, she hailed the uniformed figures on the bridge of the great vessel below them.

“Will you be good enough to mail some letters for us?”

“With pleasure!” came the reply in a big, bellowing British voice, from one of the stalwart figures beneath.

“All right; Roy, come down as low as you dare,” cried Peggy, catching her bundle of “mail.”

Roy threw over a couple of levers and turned a valve.  Instantly the Golden Butterfly began to drop in long, beautiful arc.  She shot by above the liner’s bridge at a height of not more than fifteen feet.  At the correct moment Peggy dropped the weighted bundle overboard, and had the satisfaction of seeing one of the officers catch it.  The gallant officers, now realizing for the first time that a girl—­and a pretty one—­was one of the passengers of the big aeroplane, waved their hats and bowed profoundly.

And Peggy—­what would Aunt Sallie have said!—­Peggy blew them a kiss.  But then, as she told Jess later: 

“I was in an aeroplane, my dear—­a sort of an unattainable possibility, in fact.”

In the meantime, Mortlake, in the Silver Cobweb, had been duly mystified as to what the Golden Butterfly was about when she swooped downward on the steamer.  For one instant the thought flashed across him that they were disabled.  An unholy glee filled him at the thought.  If only the Golden Butterfly were to come to grief right under Lieut.  Bradbury’s eyes, it would be a great feather in the cap of the Mortlake-Harding machine.