As the reader is told, he was made a prisoner.

He took with him into the terrible prison enclosure not only a brave, vigorous, youthful spirit, but invaluable habits of mind and thought for storing up the incidents and experiences of his prison life.

As a journalist he had acquired the habit of noticing and memorizing every striking or thrilling incident, and the experiences of his prison life were adapted to enstamp themselves indelibly on both feeling and memory.

He speaks from personal experience and from the stand-paint of tender and complete sympathy with those of his comrades who suffered more than he did himself.

Of his qualifications, the writer of these introductory words need not speak.
The sketches themselves testify to his ability with such force that no commendation is required.
This work is needed.