Our young adventurer had intended to pass the night in the little bed at his hotel, but the atmosphere of Paris proved so hot and disagreeable that he decided it would be more enjoyable to sleep while journeying through the cooler air that lay far above the earth's surface. So just as the clocks were striking the midnight hour Rob mounted skyward and turned the indicator of the traveling machine to the east, intending to make the city of Vienna his next stop.  
He had risen to a considerable distance, where the air was remarkably fresh and exhilarating, and the relief he experienced from the close and muggy streets of Paris was of such a soothing nature that he presently fell fast asleep. His day in the metropolis had been a busy one, for, like all boys, he had forgotten himself in the delight of sight-seeing and had tired his muscles and exhausted his strength to an unusual degree.
 
It was about three o'clock in the morning when Rob, moving restlessly in his sleep, accidently touched with his right hand the indicator of the machine which was fastened to his left wrist, setting it a couple of points to the south of east. He was, of course, unaware of the slight alteration in his course, which was destined to prove of serious importance in the near future. For the boy's fatigue induced him to sleep far beyond daybreak, and during this period of unconsciousness he was passing over the face of European countries and approaching the lawless and dangerous dominions of the Orient.
 
When, at last, he opened his eyes, he was puzzled to determine where he was. Beneath him stretched a vast, sandy plain, and speeding across this he came to a land abounding in luxuriant vegetation.
 
The centrifugal force which propelled him was evidently, for some reason, greatly accelerated, for the scenery of the country he was crossing glided by him at so rapid a rate of speed that it nearly took his breath away.
 
"I wonder if I've passed Vienna in the night," he thought. "It ought not to have taken me more than a few hours to reach there from Paris."
 
Vienna was at that moment fifteen hundred miles behind him; but Rob's geography had always been his stumbling block at school, and he had not learned to gage the speed of the traveling machine; so he was completely mystified as to his whereabouts.
 
Presently a village having many queer spires and minarets whisked by him like a flash. Rob became worried, and resolved to slow up at the next sign of habitation.
 
This was a good resolution, but Turkestan is so thinly settled that before the boy could plan out a course of action he had passed the barren mountain range of Thian-Shan as nimbly as an acrobat leaps a jumping-bar.
 
"This won't do at all!" he exclaimed, earnestly. "The traveling machine seems to be running away with me, and I'm missing no end of sights by scooting along up here in the clouds."
 
He turned the indicator to zero, and was relieved to find it obey with customary quickness. In a few moments he had slowed up and stopped, when he found himself suspended above another stretch of sandy plain. Being too high to see the surface of the plain distinctly he dropped down a few hundred feet to a lower level, where he discovered he was surrounded by billows of sand as far as his eye could reach.
 
"It's a desert, all right," was his comment; "perhaps old Sahara herself."
 
He started the machine again towards the east, and at a more moderate rate of speed skimmed over the surface of the desert. Before long he noticed a dark spot ahead of him which proved to be a large body of fierce looking men, riding upon dromedaries and slender, spirited horses and armed with long rifles and crookedly shaped simitars.
 
"Those fellows seem to be looking for trouble," remarked the boy, as he glided over them, "and it wouldn't be exactly healthy for an enemy to get in their way. But I haven't time to stop, so I'm not likely to get mixed up in any rumpus with them."
 
However, the armed caravan was scarcely out of sight before Rob discovered he was approaching a rich, wooded oasis of the desert, in the midst of which was built the walled city of Yarkand. Not that he had ever heard of the place, or knew its name; for few Europeans and only one American traveler had ever visited it. But he guessed it was a city of some importance from its size and beauty, and resolved to make a stop there.
 
Above the high walls projected many slender, white minarets, indicating that the inhabitants were either Turks or some race of Mohammedans; so Rob decided to make investigations before trusting himself to their company.
 
A cluster of tall trees with leafy tops stood a short distance outside the walls, and here the boy landed and sat down to rest in the refreshing shade.
 
The city seemed as hushed and still as if it were deserted, and before him stretched the vast plain of white, heated sands. He strained his eyes to catch a glimpse of the band of warriors he had passed, but they were moving slowly and had not yet appeared.
 
The trees that sheltered Rob were the only ones without the city, although many low bushes or shrubs grew scattering over the space between him and the walls. An arched gateway broke the enclosure at his left, but the gates were tightly shut.
 
Something in the stillness and the intense heat of the mid-day sun made the boy drowsy. He stretched himself upon the ground beneath the dense foliage of the biggest tree and abandoned himself to the languor that was creeping over him.
 
"I'll wait until that army of the desert arrives," he thought, sleepily. "They either belong in this city or have come to capture it, so I can tell better what to dance when I find out what the band plays."
 
The next moment he was sound asleep, sprawling upon his back in the shade and slumbering as peacefully as an infant.
 
And while he lay motionless three men dropped in quick succession from the top of the city wall and hid among the............