ancient fieldpiece located on the front lawn of the school.

“Too dangerous,” interposed his cousin Jack. “That old cannon is too rusty, and it would fly into a million pieces.”

“Yes, but we might——”

Boom!

It was a loud explosion coming from a considerable distance. The cadets, as well as all the others gathered on the ball field, looked at each other in surprise.

“What could that have been?” questioned Fred Rover.

“Sounds like a big cannon going off,” answered Walt Baxter.

Boom! Boom!

Two more explosions rent the air, both much louder than the first. The very ground seemed to be shaken by the concussion.

“Say, that sounds like a warship!”

“No warships around here,” was the answer.

“Maybe it’s a German Zeppelin!”

“Gee! do you suppose the Germans have come over here to bombard us?”

Boom! Boom! Boom!

Several more explosions came now close upon the others, each explosion heavier than those which had gone before. The ground all around seemed to tremble, and those who were still in the grandstand cried out in alarm.

“The grandstand is going down! Everybody jump for his life!”

“Look! Look!” was the sudden cry from Jack Rover, and he pointed to a place on the opposite shore of Clearwater Lake. A dense volume of smoke was rolling skyward. Then came another tremendous explosion, and a mass of wreckage could be seen to be lifted skyward.

“It’s the Hasley ammunition factory going up!” cried Fred Rover. “What an awful thing to happen!”

“That factory is right across the lake from our school!” cried Martha Rover. “I wonder if it will damage that place any?”

“I shouldn’t be surprised,” answered her cousin Andy. And then he added quickly: “I hope Mary will be safe.”

“Oh, oh! do you think Mary is in danger?” cried Ruth Stevenson, who had just joined the others. Mary was Fred Rover’s sister, who had been left behind at the girls’ boarding school because she had been suffering that day with a severe headache, and had said she preferred resting to attending the ball game, even though she loved to be with the others.

“There goes another building!” yelled Andy Rover, as another report rent the air. Then those who were looking down the river and across the lake saw some strange objects being hurled through the sky in the direction of Clearwater Hall.

“If that whole ammunition factory starts to go up, it will certainly mean damage to the boarding school,” declared Jack. “I guess the best we can do is to get down there and see if Mary is safe.”

“That’s just what I say!” declared Fred. “I’m going to get down there just as fast as I can.” And he ran off, to board one of the automobiles headed in that direction.

Now, I know it will not be at all necessary to introduce the Rover boys or their friends to my old readers, but for the benefit of those who have not perused any of my former stories a few words concerning these characters will be necessary. In the first volume, entitled “The Rover Boys at School,” I told how three brothers, Dick, Tom, and Sam Rover, were sent off to Putnam Hall Military Academy, where they made a great number of friends, including a youth named Lawrence Colby. From Putnam Hall the lads went to Brill College, and on leaving that institution of learning went into business in New York City with offices on Wall Street. They organized The Rover Company, of which Dick was now president, Tom secretary and general manager, and Sam treasurer.

While at Putnam Hall the three Rovers had become acquainted with three very charming girls, Dora Stanhope and her two cousins, Nellie and Grace Laning, and when Dick went into business he made Dora Stanhope his lifelong partner. A short time later Tom married Nellie Laning and Sam married Grace.

The three brothers purchased a fine plot of ground on Riverside Drive overlooking the noble Hudson River, and there they built three connecting houses, Dick and his family living in the middle house, with Tom on one side and Sam on the other.

About a year after their marriage Dick and his wife became the proud parents of a little son, who was named John after Mr. Laning. This son was followed by a daughter, called Martha after her great-aunt Martha of Valley Brook Farm, where the older Rovers had spent many of their younger days. Little Jack, as he was commonly called, was a manly lad with many of the qualities which had made his father so well liked and so successful.

It was about this time that Tom and Nellie Rover came to the front with a surprise for all of the others. This was in the shape of a pair of very lively twins, one of whom was named Anderson, after his grandfather, and the other, Randolph, after his great-uncle Randolph of Valley Brook Farm. Andy and Randy, as they were always called, were very active lads, in that particular being a second edition of their father.

About the time Tom’s twins were born Sam and Grace Rover came along with a beautiful little girl, whom they named Mary after Mrs. Laning. Then, a year later, the girl was followed by a sturdy boy, christened Fred after Sam Rover’s old and well-known school chum, Fred Garrison.

Residing so close together, the younger generation of Rover boys, as well as their sisters, were brought up very much like one family. They spent their winters usually in New York City, and during the summer often went out to Valley Brook Farm, where their grandfather, Anderson Rover, still resided with Uncle Randolph and Aunt Martha.

At first the boys and girls had been sent to private schools in the Metropolis; but soon the lads, led by Andy and Randy, showed such a propensity for “cutting loose” that their parents were compelled to hold a consultation.

“We’ll have to do as Uncle Randolph did with us,” said Dick Rover. “We’ll have to send them to some strict boarding school—some military academy.” And to this the others had agreed.

Some time previous their old school chum, Lawrence Colby, who had since become a colonel in the state militia, had opened a military academy, called Colby Hall.

“We’ll send them to that place,” was the decision of the older Rovers. “Lawrence Colby is just the fellow to make them behave themselves, and as we are such good friends he will be sure to give them extra attention.”

So the boys were sent off to this school, as related in detail in the first volume of my second series, entitled “The Rover Boys at Colby Hall.” This military school was located about half a mile from the town of Haven Point on Clearwater Lake, a beautiful sheet of water about two miles long and nearly half a mile wide. At the head of the lake was the Rick Rack River, running down from the hills and woods beyond.

The school consisted of a large stone building, facing the river at a point not far from where the stream emptied into the lake. It was a three-storied structure, and contained the classrooms and a mess hall and also the dormitories and private rooms for the scholars. Close by was a smaller brick building, occupied by Colonel Colby and his family and some of the professors.

On the opposite side was an up-to-date gymnasium, while at the water’s edge were a number of small buildings used as boathouses and bathing pavilions. Behind the hall were a stable and barn, and also a garage, and further back were a large garden and several farm fields and a great athletic field where the boys played baseball in the spring and football in the