Tom rushed from his private office, and when he reached the outer doorhe heard with more distinctness the sounds that had alarmed him. Theyseemed to come from a small building given over to electricalapparatus, and which, at the time, was not supposed to be in use. Ithad been Tom's workroom, so to speak, when he was developing hiselectric runabout and rifle, but of late he had not spent much time init.
"Somebody's in there!" reflected the young inventor, as he heard yellscoming from the open door of the place. "And if it isn't Koku andEradicate I miss my guess! Wonder what they can be doing there."
He crossed the yard between his private office and the electrical shopin a few rapid strides, and, as he entered the latter place, he wasgreeted with a series of wild yells.
"Good volume of sound here, at all events," mused Tom. "Almost as muchas my motor made when I was trying to talk to Mary. Hello there! What'sgoing on? Is any one hurt? What's the matter?" he cried, for, at first,he could see no one in the dim light of the place. The interior was amaze of electrical apparatus.
"Who's here?" demanded Tom, as he advanced.
"Oh, Master! Come quick! Koku 'most dead an' no can let go!" was thecry.
"Yo' jest bet yo' cain't let go!" chimed in the voice of Eradicate. "Idone knowed yo would git into trouble ef yo' come heah, an' I'se gladob it! So I is!"
"What is it, Rad? What has happened to Koku?" cried Tom, runningforward, for though no very powerful current could be turned on in theelectrical shop at this period of unuse, there was enough to be verypainful. "What is it, Rad?"
"Oh, dat big foolish giant, Koku, done got his se'f into trouble!"chuckled the colored man. "He done got holt ob one ob dem aircontraptions, Massa Tom, an' he cain't let go! Ha! Ha! Golly! Look athim squirm!" and Rad laughed shrilly, which accounted for some of thesounds Tom had heard.
Then came yells of rage and pain from the giant, and they were so loudand vigorous, mingling with Eradicate's as they did, that it was nowonder Tom was startled. The sounds were heard in the other shops, andmen came running out. But before then Tom had put an end to the trouble.
One look showed him what had happened. Just how or why Koku andEradicate had entered the electrical shop Tom did not then stop toinquire. But he saw that the giant had grasped the handles of one ofthe electric machines, designed for charging Leyden jars used in Tom'sexperiments, and the powerful, though not dangerous, current had soparalyzed, temporarily, the muscles of the giant's hands and arms thathe could not let go, and there he was, squirming, and not knowing howto turn off the current, and unable to ease himself, while Eradicatestood and laughed at him, fairly howling with delight.
"Ha! Guess yo' won't do no mo' spadin' in' Massa Tom's garden rightaway, big man!" taunted Eradicate.
"Be quiet, Rad!" ordered Tom, as he reached up and pulled out theswitch, thus shutting off the current. "This isn't anything to laughat."
"But he done look so funny, Massa Tom!" pleaded the colored man. "Hedone squirm laik--"
But Eradicate did not finish what he intended to say. Once free fromthe powerful current, the giant looked at his numb hands, and then,seeming to think that Eradicate was the cause of it all, he sprang atthe colored man with a yell. But Eradicate did not stay to see whatwould happen. With a howl of terror, he raced out of the door, and, oldand rheumatic as he was, he managed to gain the stable of his mule,Boomerang, over which he had his humble but comfortable quarters.
"Well, I guess he's safe for a while!" laughed Tom, as he saw the giantturn away, shaking his fist at the closed door, for Koku, big as hewas, stood in mortal terror of the mule's heels.
Tom locked the door of the electrical shop and went back to hisinterrupted problem. From Jackson he learned that Koku and Eradicatehad merely happened to stroll into the forbidden place, which had beenleft open by accident. There, it appeared, Koku had handled some of themachinery, ending by switching on the current of the machine thehandles of which he later unsuspectingly picked up. Then he received ashock he long remembered, and for many days he believed Eradicate hadbeen responsible for it, and there was more than the usual hostilefeeling between the two. But Eradicate was innocent of that trick, atall events.
"Though," said Tom, telling his father about it later, "Rad would haveturned on the current if he had known he could make trouble for Koku byit. I never saw their like for having disagreements!"
"Yes, but they are both devoted to you, Tom," said the aged inventor."But what is this you hinted at--a silent motor you called it, Ibelieve? Are you really serious in trying to invent one?"
"Yes, Dad, I am. I think there's a big field for an aeroplane thatcould travel along over the enemy's lines--particularly at night--andnot be heard from below. Think of the scout work that could be done.
"Well, yes, it could be done if you could get a silent motor, orpropellers that made no noise, Tom. But I don't believe it can be done."
"Well, maybe not, Dad. But I'm going to try!" and Tom, after a furthertalk with his father, began work in earnest on the big problem. That itwas a big one Tom was not disposed to deny, and that it would be avaluable invention even his somewhat skeptical father admitted.
"How are you going to start, Tom?" asked Mr. Swift, several days afterthe big idea had come to the young man.
"I'm going to experiment a bit, at first. I've got a lot of old motors,that weren't speedy enough for any of my flying machines, and I'm goingto make them over. If I spoil them the loss won't amount to anything,and if I succeed--well, maybe I can help out Uncle Sam a bit more."
As Tom had said he would do, he began at the very foundation, andstudied the fundamental principles of sound.
"Sound," the young inventor told Ned Newton, in speaking about theproblem, "is a sensation which is peculiar to the ear, though thevibrations caused by sound waves may be felt in many parts of the body.But the ear is the great receiver of sound."
"You aren't going to invent a sort of muffler for the ears, are you,Tom?" asked Ned. "That would be an easy way of solving the problem, butI doubt if you could get the Germans to wear your ear-tabs so theywouldn't hear the sound of the Allied aeroplanes."
"No, I'm not figuring on doing the trick that way," said Tom with alaugh. "I've really got to cut down the sound of the motor and thepropeller blades, so a person, listening with all his ears, won't hearany noise, unless he's within a few feet of the plane."
"Well, I can tell you, right off the reel, how to do it," said the bankemployee.
"How?" asked Tom eagerly.
"Run your engine and propellers in a vacuum," was the prompt reply.
"Hum!" said Tom, musingly. "Yes, that would be a simple way out, andI'll do it, if you'll tell me how to breathe in a vacuum."
"Oh, I didn't agree to do that," laughed Ned.
But he had spoken the truth, as those who have studied physics wellknow. There must be an atmosphere for the transmission of sound, whichis the reason all is cold and silent and still at the moon. There is noatmosphere there. Sound implies vibration. Something, such as liquid,gas, or solid, must be set in motion to produce sound, and for thepurpose of science the air we breathe may be considered a gas, beingcomposed of two.
Not only must the object, either solid, liquid, or gaseous, be inmotion to produce sound, but the air surrounding the vibrating bodymust also be moving in unison with it. And lastly there must be somemedium of receiving the sound waves--the ear or some part of the body.Totally deaf persons may be made aware of sound through the vibrationsreceived through their hands or feet. They receive, of course, only themore intense, or largest, sound waves, and can not hear notes of musicnor spoken words, though they may feel the vibration when a piano isplayed. And, as Ned has said, no sound is produced in a vacuum.
"But," said Tom, "since I can't run my aeroplane in a vacuum, or evenhave the propellers revolve in one, it's up to me to solve the problemsome other way. The propellers don't really make noise enough to worryabout when they're high in the air. It's the exhaust from the motor,and to get rid of
that will be my first attempt."
"Can it be done?" asked Ned.
"I don't know," was Tom's frank answer.
"They do it on an automobile to a great extent," went on Ned. "Some of'em you cant hardly hear."
"Yes, but an aeroplane engine runs many, many times faster than themotor of an auto," said Tom, "and there are more explosions to muffle.I doubt if the muffler of an auto would cut down the sound of an aeroengine to any appreciable extent. But, of course, I'll try along thoselines."
"They have mufflers or silencers for guns and rifles," went on Ned."Couldn't you make a big one of those contraptions and put it on anaeroplane?"
"I doubt it," said Tom, shaking his head. "Of course it's the sameprinciple as that in an auto muffler, or on a motor boat--a series ofbaffle plates arranged within a hollow cylinder. But all such devicescut down power, and I don't want to do that. However, I'm going tosolve the problem or--bust!"
And Tom came near "busting," Ned remarked later, when he and his friendtalked over the progress of the invention.
Two weeks had passed since the start of his evolution of his new idea,and following the visiting of the representatives of the UniversalFlying Machine Company. Since then neither Gale nor Ware hadcommunicated with Tom.
"But I must be on the watch against them," thought the young inventor."I'm pretty sure Gale heard me mention what I was going to try toinvent, and he may get ahead of me, and put a silent motor on themarket first. Not that I'm afraid of being done out of any profits, butI simply don't want to be beaten."
The details of Tom's invention cannot be gone into, but, roughly, itwas based on the principle of not only a muffler but also of producingless noise when the charges of gasoline exploded in the cylinders. Itis, of course, the explosion of gasoline mixed with air that causes aninternal combustion engine to operate. And it is the expulsion of theburned gases that causes the exhaust and makes the noise that is heard.
Tom was working along the well-known line of the rate of travel ofsound, which progresses at the rate of about 1090 feet a second whenair is at the freezing point. And, roughly, with every degree increasein the atmosphere's temperature the velocity of sound increases by onefoot. Thus at a temperature of 100 degrees Fahrenheit, or 68 degreesabove freezing, there would be added to the 1090 feet the 68 feet,making sound travel at 100 degrees Fahrenheit about 1158 feet a second.
Tom had set up in his shop a powerful, but not very speedy, oldaeroplane engine, and had attached to it the device he hoped would helphim toward solving his problem of cutting down the noise. He had hadsome success with it, and, after days and nights of labor, he invitedhis father and Ned, as well as Mr. Damon, over to see what he hopedwould be a final experiment.
His visitors had assembled in the shop, and Eradicate was setting outsome refreshments which Tom had provided, the colored man being in hiselement now.
"What's all this figuring, Tom?" asked Mr. Damon, as he saw a series ofcalculations on some sheets of paper lying on Tom's desk.
"That's where I worked out how much faster sound traveled in hydrogengas than in the ordinary atmosphere," was the answer. "It goes aboutfour times as fast, or nearly four thousand two hundred feet a second.You remember the rule, I suppose. 'The speed of sonorous vibrationsthrough gases varies inversely as the squares of the weights of equalvolumes of the gases,' or, in other words--"
"Give it to us chiefly in 'other words,' if you please, Tom!" pleadedNed, with a laugh. "Let that go and do some tricks. Start the engineand let's see if we can hear it."
"Oh, you can hear it all right," said Tom, as he approached the motor,which was mounted on a testing block. "The thing isn't perfected yet,but I hope to have it soon. Rad! Where is that black rascal? Oh, thereyou are! Come here, Rad!"
"Yaas sah, Massa Tom! Is I gwine to help yo' all in dish yeah job?"
"Yes. Just take hold of this lever, and when I say so pull it as hardas you can."
"Dat's whut I will, Massa Tom. Golly! ef dat no 'count giant was heahnow he'd see he ain't de only one whut's got muscle. I'll pull goodan' hard, Massa Tom."
"Yes, that's what I want you to. Now I guess we're all ready. Can yousee, Dad--and Ned and Mr. Damon?"
"Yes," they answered. They stood near the side wall of the shop, whileTom and Eradicate were at the testing block, on which the motor, withthe noise-eliminating devices attached, had been temporarily mounted.
"All ready," called the young inventor, as he turned on the gas andthrew over the electrical switch. "All ready! Pull the starting lever,Rad, and when it's been running a little I'll throw on the silencer andyou can see the difference."
The motor began to hum, and there was a deafening roar, just as therealways is when the engine of an aeroplane starts. It was as though halfa dozen automobile engines were being run with the mufflers cut out.
"Now I'll show you the difference!" yelled Tom, though such was thenoise that not a word could be heard. "This shows you what my silencerwill do."
Tom pulled another lever. There was at once a cessation of thedeafening racket, though it was not altogether ended. Then, after amoment or two, there suddenly came a roar as though a blast had beenlet off in the shop.
Tom and Eradicate were tossed backward, head over heels, as though bythe giant hands of Koku himself, and Mr. Damon, Ned, and Tom's fathersaw the motor fly from the testing block and shoot through the roof ofthe building with a rending, crashing, and splintering sound that couldbe heard for a mile.