"what will you do, if, after you have your little monoplane all constructed, and get ready to race, you find that some one else has one exactly like it at the meet?" asked ned newton one day, when he and tom were out in the big workshop, talking things over. "what will you do, tom?"

"i don't see that there is anything i can do. i'll go on to the meet, of course, and trust to some improvements i have since brought out, and to what i know about aeroplanes, to help me win the race. i'll know, too, who stole my plans."

"but it will be too late, then."

"yes, too late, perhaps, to stop them from using the drawings, but not too late to punish them for the theft. it's a great mystery, and i'll be on the anxious seat all the while. but it can't be helped."

"when are you going to start work on the sky racer?"

"pretty soon, now. i've got another set of plans made, and i've fixed them so that if they are stolen it won't do any one any good."

"how's that?"

"i've put in a whole lot of wrong figures and measurements, and scores of lines and curves that mean nothing. i have marked the right figures and lines by a secret mark, and when i work on them i'll use only the proper ones. but any one else wouldn't know this. oh, i'll fool 'em this time!"

"i hope you do. well, when you get the machine done i'd like to ride in it. will it carry two, as your butterfly does?"

"yes, only it will be much different; and, of course, it will go much faster. i'll give you a ride, all right, ned. well, now i must get busy and see what material i need for what i hope will prove to be the speediest aeroplane in the world."

"that's going some! i must be leaving now. don't forget your promise. i saw mary nestor on my way over here. she was asking for you. she said you must be very busy, for she hadn't seen you in some time."

"um!" was all tom answered, but by the blush that mounted to his face it was evident that he was more interested in mary nestor than his mere exclamation indicated.

when ned had gone tom got out pencil and paper, and was busily engaged in making some intricate calculations. he drew odd little sketches on the margin of the sheet, and then wrote out a list of the things he would need to construct the new aeroplane.

this finished, he went to mr. jackson, the engineer, and asked him to get the various things together, and to have them put in the special shop where tom did most of his work.

"i want to get the machine together as soon as i can," he remarked to the engineer, "for it will need to be given a good tryout before i enter in the race, and i may find that i'll have to make several changes in it."

mr. jackson promised to attend to the matter right away, and then tom went in to talk to his father about the motor that was to whirl the propeller of the new air craft.

mr. swift had improved very much in the past few days, and though dr. gladby said he was far from being well, the physician declared there was no reason why he should not do some inventive work.

he and tom were deep in an argument of gasoline motors, discussing the best manner of attaching the fins to the cylinders to make them air-cooled, when a voice sounded outside, the voice of eradicate:

"heah! whar yo' goin'?" demanded the colored man. "whar yo' goin'?"

"somebody's out in the garden!" exclaimed tom, jumping up suddenly.

"perhaps it's the same person who took the plans!" suggested mr. swift.

"hold on, dere!" yelled eradicate again.

then a voice replied:

"bless my insurance policy! what's the matter? have there been burglars around? why all these precautions? bless my steam heater! don't you know me?"

"mr. damon!" cried tom, a look of pleasure coming over his face. "mr. damon is coming!"

"so i should judge," responded mr. swift, with a smile. "i wonder why eradicate didn't recognize him?"

they learned why a moment later, for on looking from the library window, tom saw the colored man coming up the walk behind a well-dressed gentleman.

"why, mah goodness! it's mr. damon!" exclaimed eradicate. "i didn't know yo', sah, wif dem whiskers on! i didn't, fo' a fac'!"

"bless my razor! i suppose it does make a difference," said the eccentric man. "yes, my wife thought i'd look better, and more sedate, with a beard, so i grew one to please her. but i don't like it. a beard is too warm this kind of weather; eh, tom?" and mr. damon waved his hand to the young inventor and his father, who stood in the low windows of the library. "entirely too warm, bless my finger-nails, yes!"

"i agree with you!" exclaimed tom. "come in! we're glad to see you!"

"i called to see if you aren't going on another trip to the north pole, or somewhere in the arctic regions," went on mr. damon.

"why?" inquired tom.

"why, then this heavy beard of mine would come in handy. it would keep my throat and chin warm." and mr. damon ran his hands through his luxuriant whiskers.

"no more northern trips right away," said tom. "i'm about to build a speedy monoplane, to take part in the big meet at eagle park."

"oh, yes, i heard about the meet," said mr. damon. "i'd like to be in that."

"well, i'm building a machine that will carry two," went on tom, "and if you think you can stand a speed of a hundred miles an hour, or better, i'll let you come with me. there are some races where a passenger is allowed."

"have you got a razor?" asked mr. damon suddenly.

"what for?" inquired mr. swift, wondering what the eccentric man was going to do.

"why, bless my shaving soap! i'm going to cut off my beard. if i go in a monoplane at a hundred miles an hour i don't want to make any more resistance to the wind than possible, and my whiskers would certainly hold back tom's machine. where's a razor? i'm going to shave at once. my wife won't mind when i tell her what it's for. lend me a razor, please, tom."

"oh, there's plenty of time," explained the lad, with a laugh. "the race doesn't take place for over two months. but when it does, i think you would be better off without a beard."

"i know it," said mr. damon simply. "i'll shave before we enter the contest, tom. but now tell me all about it."

tom did so, relating the story of the theft of the plans. mr. damon was for having andy arrested at once, but mr. swift and his son pointed out that they had no evidence against him.

"all we can do," said the young inventor, "is to keep watch on him, and see if he is building another aeroplane. he has all the facilities, and he may attempt to get ahead of me. if he enters a sky craft at the meet i'll be pretty sure that he has made it from my stolen plans."

"bless my wing tips!" cried mr. damon. "but can't we do anything to stop him?"

"i'm afraid not," answered tom; and then he showed mr. damon his re-drawn plans, and told in detail of how he intended to construct the new aeroplane.

the eccentric man remained as the guest of the swift family that night, departing for his home the next day, and promising to be on hand as soon as tom was ready to test his new craft, which would be in about a month.

as the days passed, tom, with the help of his father, whose health was slightly better, and with the aid of mr. jackson, began work on the speedy little sky racer.

as you boys are all more or less familiar with aeroplanes, we will not devote much space to the description of the new one tom swift made. we can describe it in general terms, but there were some features of it which tom kept a secret from all save his father.

suffice it to say that tom had decided to build a small air craft of the single-wing type, known as the monoplane. it was to be a cross between the bleriot and the antoinette, with the general features of both, but with many changes or improvements.

the wings were shaped somewhat like those of a humming-bird, which, as is well known, can, at times, vibrate its wings with such velocity that the most rapid camera lens cannot quite catch.

and when it is known that a bullet in flight has been successfully photographed, the speed of the wings of the humming-bird can be better appreciated.

the writer has seen a friend, with a very rapid camera, which was used to snap automobiles in flight, attempt to take a picture of a humming-bird. he got the picture, all right, but the plate was blurred, showing that the wings had moved faster than the lens could throw them on the sensitive plate.

not that tom intended the wings of his monoplane to vibrate, but he adopted that style as being the best adapted to allow of rapid flight through the air; and the young inventor had determined that he would clip many minutes from the best record yet made.

the body of his craft, between the forward wings and the rear ones, where the rudders were located, was shaped like a cigar, with side wings somewhat like the fin keels of the ocean liner to prevent a rolling motion. in addition, tom had an ingenious device to automatically adapt his monoplane to sudden currents of air that might overturn it, and this device was one of the points which he kept secret.

the motor, which was air-cooled, was located forward, and was just above the heads of the operator and the passenger who sat beside him. the single propeller, which was ten feet in diameter, gave a minimum thrust of one thousand pounds at two thousand revolutions per minute.

this was one feature wherein tom's craft differed from others. the usual aeroplane propeller is eight feet in diameter, and gives from four to five hundred pounds thrust at about one thousand revolutions per minute, so it can be readily seen wherein tom had an advantage.

"but i'm building this for speed," he said to mr. jackson, "and i'm going to get it! we'll make a hundred miles an hour without trouble."

"i believe you," replied the engineer. "the motor you and your father have made is a wonder for lightness and power."

in fact, the whole monoplane was so light and frail as to give one the idea of a rather large model, instead of a real craft, intended for service. but a careful inspection showed the great strength it had, for it was braced and guyed in a new way, and was as rigid as a steel-trussed bridge.

"what are you going to call her?" asked mr. jackson, about two weeks after they had started work on the craft, and when it had begun to assume shape and form.

"i'm going to name her the humming-bird," replied tom. "she's little, but oh, my!"

"and i guess she'll bring home the prize," added the engineer.

and as the days went by, and tom, his father and mr. jackson continued to work on the speedy craft, this hope grew in the heart of the young inventor. but he could not rid himself of worry as to the fate of the plans that had disappeared. who had them? was some one making a machine like his own from them? tom wished he knew.