The lessons which compose this volume originally appeared in monthly form meeting a hearty and generous response from the public. The present volume was issued in response to the demand for the lessons in a permanent and durable form. Students have told us that they have found it necessary to read and study each lesson carefully, in order to absorb the varied information contained within its pages. They have also stated that they have found it advisable to re-read the lessons several times, allowing an interval between the readings, and that at each reading they would discover information that had escaped them during the course of the previous study. This has been repeated to us so often that we feel justified in mentioning it, that others may avail themselves of the same plan of study.