"who were those fellows?" asked the balloonist, of his companion.

"oh, some chaps who think we'll never build our airship, mr. sharp. andy foger, and his crowd."

"well, we'll show them whether we will or not," rejoined the man. "i've just thought of one point where we made a mistake. your father suggested it to me. we need a needle valve in the gas tank. then we can control the flow of vapor better."

"of course!" cried tom. "why didn't i think of that? let's try it." and the pair hurried into the machine shop, eager to make another test, which they hoped would be more successful.

the young inventor, for tom swift was entitled to that title, having patented several machines, lived with his father, barton swift, on the outskirts of the small town of shopton, in new york state. mr. swift was quite wealthy, having amassed a considerable fortune from several of his patents, as he was also an inventor. tom's mother had been dead since he was a small child, and mrs. baggert kept house for the widower and his son. there was also, in their household, an aged engineer, named garret jackson, who attended to the engine and boilers that operated machinery and apparatus in several small shops that surrounded the swift homestead; for mr. swift did most of his work at home.

as related in the first volume of this series, entitled "tom swift and his motor-cycle," the lad had passed through some strenuous adventures. a syndicate of rich men, disappointed in a turbine motor they had acquired from a certain inventor, hired a gang of scoundrels to get possession of a turbine mr. swift had invented. just before they made the attempt, however, tom became possessed of a motor-cycle. it had belonged to a wealthy man, mr. wakefield damon, of waterford, near lake carlopa, which body of water adjoined the town of shopton; but mr. damon had two accidents with the machine, and sold it to tom cheap. tom was riding his motorcycle to albany, to deliver his father's model of the turbine motor to a lawyer, in order to get a patent on it, when he was attacked by the gang of bad men. these included ferguson appleson, anson morse, wilson featherton, alias simpson, jake burke, alias happy harry, who sometimes masqueraded as a tramp, and tod boreck, alias murdock. these men knocked tom unconscious, stole the valuable model and some papers, and carried the youth away in their automobile.

later the young inventor, following a clue given him by eradicate sampson, an aged colored man, who, with his mule, boomerang, went about the country doing odd jobs, got on the trail of the thieves in a deserted mansion in the woods at the upper end of the lake. our hero, with the aid of mr. damon, and some friends of the latter, raided the old house, but the men escaped.

in the second book of the series, called "tom swift and his motor-boat," there was related the doings of the lad, his father and his chum, ned newton, on lake carlopa. tom bought at auction, a motor-boat the thieves had stolen and damaged, and, fixing it up, made a speedy craft of it so speedy, in fact that it beat the racing-boat red streak—owned by andy foger. but tom did more than race in his boat. he took his father on a tour for his health, and, during mr. swift's absence from home, the gang of bad men stole some of the inventor's machinery. tom set out after them in his motor boat, but the scoundrels even managed to steal that, hoping to get possession of a peculiar and mysterious treasure in it, and tom had considerable trouble.

among other things he did when he had his craft, was to aid a miss mary nestor, who, in her cousin's small boat, the dot, was having trouble with the engine, and you shall hear more of miss nestor presently, for she and tom became quite friendly. events so shaped themselves that andy foger was glad to loan tom the red streak in which to search for the stolen arrow, and it was in the later craft that tom, his father and ned newton had a most thrilling adventure.

they were on their way down the lake when, in the air overhead they saw a balloon on fire, with a man clinging to the trapeze. they managed to save the fellow's life, after a strenuous endeavor. the balloonist, john sharp, was destined to play quite a part in tom's life.

mr. sharp was more than an aeronaut—he was the inventor of an airship—that is, he had plans drawn for the more important parts, but he had struck a "snag of clouds," as he expressed it, and could not make the machine work. his falling in with mr. swift and his son seemed providential, for tom and his father were at once interested in the project for navigating the upper air. they began a study of mr. sharp's plans, and the balloonist was now in a fair way to have the difficulty solved.

his airship was, primarily an aeroplane, but with a sustaining aluminum container, shaped like a cigar, and filled with a secret gas, made partly of hydrogen, being very light and powerful. it was testing the effect of this gas on a small model of the aluminum container that the explosion, told of in the first chapter, occurred. in fact it was only one of several explosions, but, as tom said, all the while they were eliminating certain difficulties, until now the airship seemed almost a finished thing. but a few more details remained to be worked out, and mr. swift and his son felt that they could master these.

so it was with a feeling of no little elation, that the young inventor followed mr. sharp into the shop. the balloonist, it may be explained, had been invited to live with the swifts pending the completion of the airship.

"do you think we'll get on the right track if we put the needle valve in?" asked tom, as he noted with satisfaction that the damage from the explosion was not great.

"i'm sure we will," answered the aeronaut. "now let's make another model container, and try the gas again."

they set to work, with mr. swift helping them occasionally, and garret jackson, the engineer, lending a hand whenever he was needed. all that afternoon work on the airship progressed. the joint inventors of it wanted to be sure that the sustaining gas bag, or aluminum container, would do its work properly, as this would hold them in the air, and prevent accidents, in case of a stoppage of the engine or propellers.

the aeroplane part of the airship was all but finished, and the motor, a powerful machine, of new design, built by mr. swift, was ready to be installed.

all that afternoon tom, his father and mr. sharp labored in the shop. as it grew dusk there sounded from the house the ringing of a bell.

"supper time," remarked tom, laying aside a wrench. "i wish mrs. baggert would wait about an hour. i'd have this valve nearly done, then."

but the housekeeper was evidently not going to wait, for her voice supplemented the bell.

"supper! sup-per!" she called. "come now, mr. swift; tom, mr. sharp! i can't wait any longer! the meat and potatoes will be spoiled!"

"i s'pose we'd better go in," remarked mr. sharp, with something of a sigh. "we can finish to-morrow."

the shop, where certain parts of the airship were being made, was doubly locked, and jackson, the engineer, who was also a sort of watchman, was bidden to keep good guard, for the fear of the gang of unscrupulous men, who had escaped from jail during a great storm, was still in the minds of mr. swift and his son.

"and give an occasional look in the shed, where the aeroplane is," advised mr. sharp. "it wouldn't take much to damage that, now."

"i'll pay particular attention to it," promised the engineer. "don't worry, mr. sharp."

after supper the three gathered around the table on which were spread out sheets of paper, covered with intricate figures and calculations, which mr. swift and the balloonist went over with care. tom was examining some blue prints, which gave a sectional view of the proposed ship, and was making some measurements when the bell rang, and mrs. baggert ushered in ned newton, the most particular chum of the young inventor.

"hello, ned!" exclaimed tom. "i was wondering what had become of you. haven't seen you in a dog's age."

"that's right," admitted ned. "we've been working late nights at the bank. getting ready for the regular visit of the examiner, who usually comes along about this time. well, how are things going; and how is the airship?" for, of course, ned had heard of that.

"oh, pretty good. had another explosion to-day, i s'pose you heard."

"no, i hadn't."

"i thought everyone in town had, for andy foger and his two cronies were on hand, and they usually tell all they know."

"oh, andy foger! he makes me sick! he was scooting up the street in his auto just as i was coming in, 'honking-honking' his horn to beat the band! you'd think no one ever had an auto but him. he certainly was going fast."

"wait until i get in our airship," predicted tom. "then i'll show you what speed is!"

"do you really think it will go fast?"

"of course it will! fast enough to catch anson morse and his crowd of scoundrels if we could get on their track."

"why, i thought they were in jail," replied ned, in some surprise. "weren't they arrested after they stole your boat?"

"yes, and put in jail, but they managed to get out, and now they're free to make trouble for us again."

"are you sure they're out of jail?" asked ned, and tom noted that his chum's face wore an odd look.

"sure? of course i am. but why do you ask?"

ned did not answer for a moment. he glanced at tom's father, and the young inventor understood. mr. swift was getting rather along in age, and his long years of brain work had made him nervous. he had a great fear of morse and his gang, for they had made much trouble for him in the past. tom appreciated his chum's hesitancy, and guessed that ned had something to say that he did not want mr. swift to hear.

"come on up to my room, ned. i've got something i want to show you," exclaimed tom, after a pause.

the two lads left the room, tom glancing apprehensively at his father. but mr. swift was so engrossed, together with the aeronaut, in making some calculations regarding wind pressure, that it is doubtful if either of the men were aware that the boys had gone.

"now what is it, ned?" demanded our hero, when they were safe in his apartment. "something's up. i can tell by your manner. what is it?"

"maybe it's nothing at all," went on his chum. "if i had known, though that those men had gotten out of jail, i would have paid more attention to what i saw to-night, as i was leaving the bank to come here."

"what did you see?" demanded tom, and his manner, which had been calm, became somewhat excited.

"well, you know i've been helping the paying-teller straighten up his books," went on the young bank employee, "and when i came out to-night, after working for several hours, i was glad enough to hurry away from the 'slave-den,' as i call it. i almost ran up the street, not looking where i was going, when, just as i turned the corner, i bumped into a man."

"nothing suspicious or wonderful in that," commented tom. "i've often run into people."

"wait," advised ned. "to save myself from falling i grabbed the man's arm. he did the same to me, and there we stood, for a moment, right under a gas lamp. i looked down at his hands, and i saw that on the little finger of the left one there was tattooed a blue ring, and—"

"happy harry—the tramp!" exclaimed tom, now much excited. "that's where he wears a tattooed ring!"

"that's what i thought you had told me," resumed ned, "but i didn't pay any attention to it at the time, as i had no idea that the men were out of jail."

"well, what else happened?" inquired tom

"not much more. i apologized to the man, and he to me, and we let go of each other."

"are you sure about the ring on his finger?"

"positive. his hand was right in the light. but wait, that isn't all. i hurried on, not thinking much about it, when, i saw another man step out of the dark shadows of peterby's grocery, just beyond the bank. the man must have mistaken me for some one else, for he spoke to me."

"what did he say?"

"he asked me a question. it was: 'is there any chance to-night?'"

"what did you tell him?"

"well, i was so surprised that i didn't know what to say, and, before i could get my wits together the man had seen his mistake and hurried on. he joined the man i had collided with, and the two skipped off in the darkness. but not before a third man had come across the street, from in front of the bank, and hurried off with them."

"well?" asked tom, as his chum paused.

"i don't know what to think," resumed ned. "these men were certainly acting suspiciously, and, now that you tell me the anson morse gang is not locked up—well, it makes me feel that these must be some of their crowd."

"of course they are!" declared tom positively. "that blue ring proves it!"

"i wouldn't go so far as to say that," declared ned. "the man certainly had a blue ring tattooed on his finger—the same finger where you say happy harry had his. but what would the men be doing in this neighborhood? they certainly have had a lesson not to meddle with any of your things."

"no, i don't believe they are after any of dad's inventions this time. but i tell you what i do believe."

"what?"

"those men are planning to rob the shopton bank, ned! and i advise you to notify the officers. that morse gang is one of the worst in the country," and tom, much excited, began to pace the room, while ned, who had not dreamed of such an outcome to his narrative, looked startled.