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