The climb up Porcupine Hill was not made as soon as most of the boys had hoped. Several weeks elapsed and the program that had been mapped out by the doctor was too full for any additions. Meanwhile all the absent boys of the school returned, and the trip to Grand CaÑon was taken.

Only one-fourth of the boys took this trip this year, it always being reserved for the fourth-year, or senior, pupils. Hal Kenyon was one of the eleven boys who visited these wonders of the Colorado River on this occasion. And on his return he was so full of the delights of the scenery that Mummy CaÑon and Flathead were for a time of minor interest.

But in time their old fascination returned. The cave behind the waterfall at no time ceased to be an object of much interest to him, and he was continually wishing that something would put it into Dr. Byrd’s head to make a thorough inspection of the cavern. And if this were done, Hal naturally hoped that he would be one of the inspectors.

Meanwhile the broken bones of Aviator Miles mended rapidly. As soon as it was deemed safe, he was permitted to leave his bed and hobble around on crutches, his leg still in a cast, however. From the time of his first exit from the Hospital, he was an object of much interest to the boys. They gathered around him at every opportunity and begged for stories of his experiences, and he usually had something of absorbing interest to tell.

He told them that he had been among the Rockies from Yellowstone National Park to the Grand CaÑon for two months before his accident, and he exhibited before their eager eyes his collection of stones and quartz that Hal had discovered in the waterfall cave.

“But they’re not all here,” he remarked as he poured them out on a newspaper that he had spread on the lawn in front of Dr. Byrd’s home. He was seated on the grass while a score of boys stood around in eager attention.

“Where are the others?” inquired Fes.

“I don’t know,” replied the aviator slowly. “There were six pretty fair sized gold nuggets in the bag when I fell; or they were there a few hours before, and I don’t see how they could have disappeared.”

“I didn’t see any when I opened the bag in the cave.”

Hal volunteered this information, but the sentence was finished with a different tone of voice from the tone at the beginning. In the midst of his statement he suddenly realized the importance to him of the disappearance of the nuggets, and a lump arose in his throat, so that he could hardly finish what he started to say.

Everybody noticed the change in Kenyon’s voice, and all looked at him as if for an explanation. Conscious of his seeming self-betrayal and of the inquiring glances directed at him, he blushed with confusion. The aviator suspected at once that these were signs of guilt.

But Hal knew better and flashed back a look of scorn and indignation at his silent accusers. Recovering his natural tone, he said in a cool, measured voice:

“I don’t know what became of those nuggets. They certainly were not in the bag when I opened it.”

Most of the boys believed in Hal and were convinced by this sturdy statement. Mr. Miles, however, was not convinced, although he did not like to hold any suspicion against a boy who had impressed him so well. But he saw nothing to be gained by embarrassing Kenyon at present.

“Well,” he said; “this isn’t the only mystery connected with the affair. I’m just as curious to know how the bag ever got into that cave.”

“You think somebody put it there?” Hal inquired.

“Being an invalid and unable to get around very conveniently, I haven’t been able to inspect the place yet. But from all descriptions received, I’m in need of more evidence to convince me that it bounced in there by accident. In the first place, I’m dead certain it fell to the ground with the aeroplane.”

“Maybe the strut it was tied to was what broke and made you fall,” suggested Hal.

“It wasn’t a strut at all that broke. It was a couple of stay wires. The struts couldn’t break under any but the most extraordinary circumstances.”

“Are you goin’ to fix up your aeroplane again, or get a new one?” asked Hugh Messinger.

“Oh, nothing can be done with that pile of junk. You boys might as well burn the wood and tote the steel framework into your blacksmith shop.”

“Are you going to quit flying?” asked Byron Bowler.

Mr. Miles looked with keen amusement at the last questioner and replied with a wink:

“Do I look like a quitter?”

He surely didn’t, although forced to stop for several weeks with some broken bones. Miles was a sturdy, determined-looking man, with firm-set jaw and clear bright eye that gave no hint of hesitation.

“What you going to do? Buy another airship?” Byron persisted.

“I’ve bought one already.”

“You have!” eagerly exclaimed several of the boys. “Where is it?” one of them continued.

“It’s on its way out here.”

“Out here!”

This exclamation also came in chorus from half a dozen astonished Boy Scouts.

“Yes,” answered the crippled aviator; “it’s on its way out here. But it isn’t put together ready to fly. It’s in the knock-down. I’m going to give you boys the job of putting it together.”

“Oh!—when will it be here?” asked one enthusiastic youngster.

“In a week. Dr. Byrd and I had several talks about the matter, and he’s decided to let you boys have the job. I won’t be strong enough to do much on it myself, but I’ll be on hand and boss.”

“What kind is it going to be?” asked Pickles. “Like the one you fell in?”

“Not exactly. It’ll be a biplane, but a much better one than the Ozone.”

“What’s the Ozone?” inquired Ferdinand.

“That’s the name of the biplane I fell in. The new one will carry two passengers besides the operator.”

“Oh, ain’t that fine!” cried Glen Juza. “It’s just swell. And can we all have a ride?”

“Oh-ho,” laughed Miles. “I thought it would come to that. But it really isn’t up to me to decide. I might say yes, and Dr. Byrd might say no. He probably would.”

Disappointed looks and expressions followed this prophecy. The doctor’s pupils could just as well have predicted such an outlook without the assistance of older heads, but they were naturally optimistic.

“But don’t be discouraged, boys,” added the aviator. “Your time will come sooner or later. Maybe you’ll be afraid to go up with me when you see the airship all finished. It—”

“Yes we will!” “I wouldn’t!” “You don’t know me!” were some of the brave interruptions.

“It won’t look very safe,” was the aviator’s warning. “Pretty thin and flimsy.”

“I don’t care; I’ll go up in anything you will,” cried Frank Bowler, who had listened to the conversation in silence up to this time. He was gradually regaining his former nerve and bluster, but his voice did not yet have a natural ring.

“What will the new aeroplane look like?” asked Hal Kenyon.

Most of the boys by this time had spent their enthusiasm and settled down to quieter attention. Seated on the grass, they waited eagerly for the answer to Kenyon’s question. By this time a good crowd of boys had joined the audience.

“I may as well give you your first lesson in aeroplane building right now,” began the aviator, shifting slightly to ease his crippled limb. “First, do you all know what a biplane is?”

“It’s an airship,” said one.

“No, it’s an aeroplane,” corrected another.

“What’s the difference between a biplane and a monoplane?” interposed Mr. Miles.

This was a puzzler for most of the boys. After several had answered and flunked, Hal Kenyon spoke up:

“A monoplane has one plane, and a biplane has two planes.”

“What is a plane?”

“A flat surface.”

“Good,” complimented Mr. Miles. “You know a little geometry. The planes are the wings of an aeroplane.

“Now, the aeroplane will be built on this plan: The part that will interest you boys most will be the cabin. As I said, it will carry two passengers comfortably besides the operator. And it is to be so arranged with an automatically shifting weight that these passengers can move about without disturbing the balance of the ship.

“This will really be an airship. The ordinary aeroplane is not entitled to such a name, for it is merely a skeleton without any body. This vessel will have a real body, made mostly of aluminum, except the glass windows and ports. The front, or prow, will be blunt in accord with the latest ideas of air friction. The front and rear of this cabin will be supplied with flexible slides that may be slipped around to the sides, leaving the front and rear open. This will remove practically all resistance, except for perpendicular rods six inches apart, giving the cabin something of the appearance of a cage.

“In cold weather, or in high altitudes, these slides can be closed and the cabin warmed with a small alcohol stove. Otherwise there will be little remarkable about the ship. You will all be interested in such details as the motor and the steering and weather apparatus. You will learn all about the altitude barometer and the anemometer, or speed measurer. In other words, you will absorb a lot of information on air navigation while putting this airship together.”

“How about that weight?” Hal questioned as the instructor in aeronautics paused. “You say it shifts automatically. Can you explain that so we can understand how it keeps the ship from turning over?”

“I’ll try. The floor is of a flexible material. As one walks here and there, it is pressed downward and by means of a delicate mechanical device, shifts a weight on a rod. The shifting of this weight alters the angle of the ailerons at the ends of the wings and prevents the machine from tipping out of balance. Understand?”

Blank looks on every face before him advised Mr. Miles that he had been too technical for the boys, so he added:

“You’ll understand easily as you advance in the construction of the vessel. But possibly this may give you a hint of what I am driving at: Changing the angles of the ailerons has the same effect in an up-and-down direction, as turning a boat’s rudder has from side to side.”

Still few of the boys understood what he meant, although Hal Kenyon and one or two others believed they did. Later, when he found an opportunity, Hal, with pencil and paper, made some drawings and studied over them and altered them until he was certain that he had a clear idea of the plan. Then he took his last drawing to Mr. Miles and explained it to him, and the aviator told him he was right.