"what's the matter?" cried mrs. baggert, the housekeeper, hurrying in from the kitchen, where she was washing the dishes. "have you seen some of those scoundrels who robbed you, mr. swift? if you have, the police down here ought to—"

"no, it's nothing like that," explained mr. swift. "tom has merely discovered in the paper an account of a sunken treasure ship, and he wants us to go after it, down under the ocean."

"oh, dear! some more of captain kidd's hidden hoard, i suppose?" ventured the housekeeper. "don't you bother with it, mr. swift. i had a cousin once, and he got set in the notion that he knew where that pirate's treasure was. he spent all the money he had and all he could borrow digging for it, and he never found a penny. don't waste your time on such foolishness. it's bad enough to be building airships and submarines without going after treasure." mrs. baggert spoke with the freedom of an old friend rather than a hired housekeeper, but she had been in the family ever since tom's mother died, when he was a baby, and she had many privileges.

"oh, this isn't any of kidd's treasure," tom assured her. "if we get it, mrs. baggert, i'll buy you a diamond ring."

"humph!" she exclaimed, as tom began to hug her in boyish fashion. "i guess i'll have to buy all the diamond rings i want, if i have to depend on your treasure for them," and she went back to the kitchen.

"well," went on mr. swift after a pause, "if we are going into the treasure-hunting business, tom, we'll have to get right to work. in the first place, we must find out more about this ship, and just where it was sunk."

"i can do that part," said mr. sharp. "i know some sea captains, and they can put me on the track of locating the exact spot. in fact, it might not be a bad idea to take an expert navigator with us. i can manage in the air all right, but i confess that working out a location under water is beyond me."

"yes, an old sea captain wouldn't be a bad idea, by any means," conceded mr. swift. "well, if you'll attend to that detail, mr. sharp, tom, mr. jackson and i will finish the submarine. most of the work is done, however, and it only remains to install the engine and motors. now, in regard to the negative and positive electric plates, i'd like your opinion, tom."

for tom swift was an inventor, second in ability only to his father, and his advice was often sought by his parent on matters of electrical construction, for the lad had made a specialty of that branch of science.

while father and son were deep in a discussion of the apparatus of the submarine, there will be an opportunity to make the reader a little better acquainted with them. those of you who have read the previous volumes of this series do not need to be told who tom swift is. others, however, may be glad to have a proper introduction to him.

tom swift lived with his father, barton swift, in the village of shopton, new york. the swift home was on the outskirts of the town, and the large house was surrounded by a number of machine shops, in which father and son, aided by garret jackson, the engineer, did their experimental and constructive work. their house was not far from lake carlopa, a fairly large body of water, on which tom often speeded his motor-boat.

in the first volume of this series, entitled "tom swift and his motor-cycle," it was told how he became acquainted with mr. wakefield damon, who suffered an accident while riding one of the speedy machines. the accident disgusted mr. damon with motor-cycles, and tom secured it for a low price. he had many adventures on it, chief among which was being knocked senseless and robbed of a valuable patent model belonging to his father, which he was taking to albany. the attack was committed by a gang known as the happy harry gang, who were acting at the instigation of a syndicate of rich men, who wanted to secure control of a certain patent turbine engine which mr. swift had invented.

tom set out in pursuit of the thieves, after recovering from their attack, and had a strenuous time before he located them.

in the second volume, entitled "tom swift and his motor-boat," there was related our hero's adventures in a fine craft which was recovered from the thieves and sold at auction. there was a mystery connected with the boat, and for a long time tom could not solve it. he was aided, however, by his chum, ned newton, who worked in the shopton bank, and also by mr. damon and eradicate sampson, an aged colored whitewasher, who formed quite an attachment for tom.

in his motor-boat tom had more than one race with andy foger, a rich lad of shopton, who was a sort of bully. he had red hair and squinty eyes, and was as mean in character as he was in looks. he and his cronies, sam snedecker and pete bailey, made trouble for tom, chiefly because tom managed to beat andy twice in boat races.

it was while in his motor-boat, arrow, that tom formed the acquaintance of john sharp, a veteran balloonist. while coming down lake carlopa on the way to the swift home, which had been entered by thieves, tom, his father and ned newton, saw a balloon on fire over the lake. hanging from a trapeze on it was mr. sharp, who had made an ascension from a fair ground. by hard work on the part of tom and his friends the aeronaut was saved, and took up his residence with the swifts.

his advent was most auspicious, for tom and his father were then engaged in perfecting an airship, and mr. sharp was able to lend them his skill, so that the craft was soon constructed.

in the third volume, called "tom swift and his airship," there was set down the doings of the young inventor, mr. sharp and mr. damon on a trip above the clouds. they undertook it merely for pleasure, but they encountered considerable danger, before they completed it, for they nearly fell into a blazing forest once, and were later fired at by a crowd of excited people. this last act was to effect their capture, for they were taken for a gang of bank robbers, and this was due directly to andy foger.

the morning after tom and his friends started on their trip in the air, the shopton bank was found to have been looted of seventy-five thousand dollars. andy foger at once told the police that tom swift had taken the money, and when asked how he knew this, he said he had seen tom hanging around the bank the night before the vault was burst open, and that the young inventor had some burglar tools in his possession. warrants were at once sworn out for tom and mr. damon, who was also accused of being one of the robbers, and a reward of five thousand dollars was offered.

tom, mr. damon and mr. sharp sailed on, all unaware of this, and unable to account for being fired upon, until they accidentally read in the paper an account of their supposed misdeeds. they lost no time in starting back home, and on the way got on the track of the real bank robbers, who were members of the happy harry gang.

how the robbers were captured in an exciting raid, how tom recovered most of the stolen money, and how he gave andy foger a deserved thrashing for giving a false clue was told of, and there was an account of a race in which the red cloud (as the airship was called) took part, as well as details of how tom and his friends secured the reward, which andy foger hoped to collect.

those of you who care to know how the red cloud was constructed, and how she behaved in the air, even during accidents and when struck by lightning, may learn by reading the third volume, for the airship was one of the most successful ever constructed.

when the craft was finished, and the navigators were ready to start on their first long trip, mr. swift was asked to go with them. he declined, but would not tell why, until tom, pressing him for an answer, learned that his father was planning a submarine boat, which he hoped to enter in some trials for government prizes. mr. swift remained at home to work on this submarine, while his son and mr. sharp were sailing above the clouds.

on their return, however, and after the bank mystery had been cleared up, tom and mr. sharp, aided mr. swift in completing the submarine, until, when the present story opens, it needed but little additional work to make the craft ready for the water.

of course it had to be built near the sea, as it would have been impossible to transport it overland from shopton. so, before the keel was laid, mr. swift rented a large cottage at a seaside place on the new jersey coast and there, after erecting a large shed, the work on the advance, as the under-water ship was called, was begun.

it was soon to be launched in a large creek that extended in from the ocean and had plenty of water at high tide. tom and mr. sharp made several trips back and forth from shopton in their airship, to see that all was safe at home and occasionally to get needed tools and supplies from the shops, for not all the apparatus could be moved from shopton to the coast.

it was when returning from one of these trips that tom brought with him the paper containing an account of the wreck of the boldero and the sinking of the treasure she carried.

until late that night the three fortune-hunters discussed various matters.

"we'll hurry work on the ship," said mr. swift at length. "tom, i wonder if your friend, mr. damon, would care to try how it seems under water? he stood the air trip fairly well."

"i'll write and ask him," answered the lad. "i'm sure he'll go."

securing, a few days later, the assistance of two mechanics, whom he knew he could trust, for as yet the construction of the advance was a secret, mr. swift prepared to rush work on the submarine, and for the next three weeks there were busy times in the shed next to the seaside cottage. so busy, in fact, were tom and mr. sharp, that they only found opportunity for one trip in the airship, and that was to get some supplies from the shops at home.

"well," remarked mr. swift one night, at the close of a hard day's work, "another week will see our craft completed. then we will put it in the water and see how it floats, and whether it submerges as i hope it does. but come on, tom. i want to lock up. i'm very tired to-night."

"all right, dad," answered the young inventor coming from the darkened rear of the shop. "i just want to—"

he paused suddenly, and appeared to be listening. then he moved softly back to where he had come from.

"what's the matter?" asked his father in a whisper. "what's up, tom?"

the lad did not answer mr. swift, with a worried look on his face, followed his son. mr. sharp stood in the door of the shop.

"i thought i heard some one moving around back here," went on tom quietly.

"some one in this shop!" exclaimed the aged inventor excitedly. "some one trying to steal my ideas again! mr. sharp, come here! bring that rifle! we'll teach these scoundrels a lesson!"

tom quickly darted back to the extreme rear of the building. there was a scuffle, and the next minute tom cried out:

"what are you doing here?"

"ha! i beg your pardon," replied a voice. "i am looking for mr. barton swift."

"my father," remarked tom. "but that's a queer place to look for him. he's up front. father, here's a man who wishes to see you," he called.

"yes, i strolled in, and seeing no one about i went to the rear of the place," the voice went on. "i hope i haven't transgressed."

"we were busy on the other side of the shop, i guess," replied tom, and he looked suspiciously at the man who emerged from the darkness into the light from a window. "i beg your pardon for grabbing you the way i did," went on the lad, "but i thought you were one of a gang of men we've been having trouble with."

"oh, that's all right," continued the man easily. "i know mr. swift, and i think he will remember me. ah, mr. swift, how do you do?" he added quickly, catching sight of tom's father, who, with mr. sharp, was coming to meet the lad.

"addison berg!" exclaimed the aged inventor as he saw the man's face more plainly. "what are you doing here?"

"i came to see you," replied the man. "may i have a talk with you privately?"

"i—i suppose so," assented mr. swift nervously. "come into the house."

mr. berg left tom's side and advanced to where mr. swift was standing. together the two emerged from the now fast darkening shop and went toward the house.

"who is he?" asked mr. sharp of the young inventor in a whisper.

"i don't know," replied the lad; "but, whoever he is, dad seems afraid of him. i'm going to keep my eyes open."