This story is complete in itself, but forms the sixth volume in a line issued under the general title of "Putnam Hall Series."

As mentioned several times, this line was started because many young folks wanted to know what happened at Putnam Hall Military School previous to the arrival at that institution of the Rover boys, as already related in my "Rover Boys Series."

To gratify this curiosity I wrote the first volume of this series, called "The Putnam Hall Cadets," showing how Captain Putnam organized his famous school, and how it was Jack Ruddy and Pepper Ditmore came to be among his first pupils.

In the second book, entitled "The Putnam Hall Rivals," I gave the particulars of several contests on the field of sports, and also told about a thrilling balloon ride and of an odd discovery in the woods.

Following the second book came a third, "The Putnam Hall Champions," with more bitterly-contested games, in one of which young Major Ruddy's enemies played him a foul trick.

From the opening of the school there had been dissatisfaction with one of the teachers, and when another was engaged who proved to be a man of peculiar whims, the boys went into open revolt, as related in another volume, called "The Putnam Hall Rebellion." The cadets literally ran away, and did not return to the Hall until Captain Putnam came upon the scene to straighten matters out.

The rebellion was followed by a grand outing, as related in "The Putnam Hall Encampment." The cadets marched far away from the school, to the shore of a beautiful lake, and there our heroes managed to have a good time in spite of the mean work of several of their enemies.

In the present volume are related the particulars of a most puzzling mystery which at one time threatened to bring disaster to the whole school. How the mystery was at last solved I leave for the pages which follow to explain.

Again I thank both young and old for all the nice things they have said about my books. I hope the reading of the volumes affords all both pleasure and profit.