It is a strange fact, for which I do not expect ever satisfactorily to account, and which will receive little credence even among those who know that I am not given to romancing - it is a strange fact, I say, that the substance of the following pages has evolved itself during a period of six months, more or less, between the hours of midnight and four o'clock in the morning, proceeding directly from a type-writing machine standing in the corner of my library, manipulated by unseen hands.