"Finally, the fire seeming nearly out along one side of the ridge, I sent the men to fighting it on the other side, where it had been left to rage uncontrolled. No sooner had the men scattered for the danger point than the brooding fire broke out again and it was necessary to recall half the men.

"It was a long fight and a hard one, but with the aid of the blessed old wireless, we finally won out. As a matter of fact, the wireless-equipped airplane has become as necessary to the Forest Service as ships are to the navy.

"In the old days," he went on, seeing that the boys were still deeply interested, "when they depended upon the ordinary telephone to convey warnings of fires they were surely leaning upon a broken reed.

"Often, just when they needed the means of communication most, the fire would sweep through the woods, destroying trees to which the telephone wires were fastened, and melting the wires themselves. So the eyes of the Forest Service were put out and they were forced to work in the dark."

"But I should think," protested Bob, "that there would be times when even wireless would be put out of the job. Suppose the fire were to reach one of the stations equipped with wireless. What then?"

Mr. Bentley laughed as though amused at something.

"I can tell you an interesting incident connected with that," he said.

"And one that shows the pluck and common sense of radio operators in general--don't think that I'm throwing bouquets at myself, now, for first and last, I am a pilot, even if sometimes I find it necessary to employ radio.

"Well, anyway, this operator that I am speaking of, found himself in a perilous position. A fire had been raging for days, and now it was so close to his station that the station itself was threatened.

"One morning when he got up the smoke from the burning forest was swirling about the open s.p.a.ce in front of the station and he knew that before long he would be seeing flame instead of smoke. The fire fighters had been working ceaselessly, fighting gallantly, but the elements were against them. The air was almost as dry and brittle as the wood which the flames lapped up and there was a steady wind that drove the fire on and on.

"If only there might come a fog or the wind change its direction! But the radio man had no intention of waiting on the elements. I don't believe he gave more than a pa.s.sing thought to his own safety--his chief interest was for the safety of his beloved apparatus.

"He decided to dismantle the set, build a raft and set himself and the apparatus adrift upon the water in the attempt to save it.

"And so he worked feverishly, while the fire came closer and he could hear the men who were fighting the fire shouting to each other. Finally he succeeded in dismantling the set and got it down to the water's edge.

"Here he built a rough raft, piled the apparatus upon it, jumped after it, and drifted out into the middle of the lake."

"Did the station burn down?" asked Jimmy excitedly.

"No, fortunately. The wind died down in the nick of time, giving the men a chance to control the blaze. When it was evident the danger was past, the operator set up his apparatus again and prepared to continue his duties, as though nothing had happened.

"There you have the tremendous advantage of radio. There were no wires to be destroyed. Only a radio set which could be dismantled and taken to safety while the fire raged."

"That operator sure had his nerve with him, all right," said Bob admiringly.

"More nerve than common sense perhaps," chuckled Mr. Bentley. "But you certainly can't help admiring him. He was right there when it came to grit."

After a while they began to discuss technicalities, and the boys learned a great many things they had never known before. The pilot happening to mention that there were sometimes a number of airplanes equipped with radio operating within a restricted district, Joe wanted to know if they did not have a good deal of trouble with interference.

"No. There was at first some interference by amateurs, but these soon learned to refrain from using their instruments during patrol periods.

"You see," he explained, "we use a special type of transmitting outfit aboard our fire-detection craft. It's called the SCR-Seventy-three. The equipment obtains its power from a self-excited inductor type alternator. This is propelled by a fixed wooden-blade air fan. In the steam-line casing of the alternator the rotary spark gap, alternator, potential transformer, condenser and oscillation transformer are self-contained. Usually the alternator is mounted on the underside of the fuselage where the propeller spends its force in the form of an air stream. The telegraph sending keys, field and battery switch, dry battery, variometer and antenna reel are the only units included inside the fuselage.

"The type of transmitter is a simple rotary gap, indirectly excited spark and provided with nine taps on the inductance coil of the closed oscillating circuit. Five varying toothed discs for the rotary spark gap yield five different signal tones and nine different wave lengths are possible.

"So," he finished, looking around at their absorbed faces, "you see it is quite possible to press into service a number of airplanes without being bothered by interference."

"It sounds complete," said Bob. "I'd like a chance to see one of those sets at close range sometime."

The time pa.s.sed so quickly that finally the visitor rose with an apology for staying so late. The radio boys were sorry to see him go. They could have sat for hours more, listening to him.

"That fellow sure has had some experiences!" said Joe, as, a little later, the boys mounted the stairs to Bob's room. "It was mighty lucky we happened along while he was here."

"You bet your life," said Herb. "I wouldn't have missed meeting him for a lot."

"Say, fellows," Jimmy announced from the head of the stairs, "I know now what I'm going to do when I'm through school. It's me for the tall timber. I'm going to pilot an airplane in the service of my country."

"Ain't he n.o.ble?" demanded Herb, grinning, as the boys crowded into Bob's room.

CHAPTER VIII

NEAR DISASTER

Several days later while the radio boys were experimenting with their big set and talking over their interesting meeting with the Forest Service ranger, Herb displayed an immense horseshoe magnet.

"Look what he's got for luck," chortled Jimmy. "The superst.i.tious nut!"

"Superst.i.tious nothing!" snorted Herb. "If I'd wanted it for luck I wouldn't have got a magnet, would I? Any old common horseshoe would have done for luck."

"Well, what's the big idea?" asked Bob, looking up from the audion tube he was experimenting with. "Or is there any?" he added, with a grin.

"You bet your life there is!" returned Herb. "It's got to do with that very audion tube you're fussing with."

"Ah, go on," jeered Joe, good-naturedly. "What's a magnet got to do with an audion tube, I'd like to know!"

"Poor old Herb," added Jimmy, with a commiserating shake of the head.

"Say, look here, all you fellows! Don't you go wasting any pity on me,"

cried Herb hotly. "If you don't look out, I won't show you my experiment at all."

"Go on, Herb," said Bob consolingly. "I'm listening."

"Well, I'm glad there's one sensible member of this bunch!" cried Herb, and from then on addressed himself solely to Bob. "Look here," he said.

"You can make the audion tube ever so much more sensitive to vibration if you put this magnet near it."

"Who says so?" asked Bob, with interest.

"I do. Here, put on the headphones and listen. I'll prove it to you."

Bob obeyed and tuned in to the nearest broadcasting station where a concert was scheduled. As soon as he signified by a nod of his head that the connection was satisfactory Herb placed the big horseshoe magnet in such a position that the poles of the magnet were on each side of the tube.

Sure enough, Bob was amazed at the almost magical improvement in the sound. It was clearer, more distinct, altogether more satisfactory. He listened in for another moment then wonderingly took off the headphones while Herb grinned at him in triumph.

"Well, what do you think?" asked the latter while Joe and Jimmy looked at them curiously.