The Britannica is a work of reference, and much more: it is a collection of all histories, all biographies, all arts, all litterateurs, and all scientific, professional, and mechanical knowledge; but on account of its comprehensiveness, extending as it does through so many large volumes, it presents such an "embarrassment of riches" that those who consult it fail sometimes to discover all that is suited to their individual needs. It is evident, therefore, that if each reader and patron of this great library can have a guide to point out to him, according to his vocation, the parts that are the most helpful to him, he will be able to systematize his reading or his investigations; and thus, while economizing both time and labor, reach the highest results. The present volume has been prepared for that purpose; and it is believed that, recognizing its helpfulness, the many thousand owners of the Britannica will welcome it as an invaluable addition to their libraries. The plan has been to direct each individual how to draw from this great storehouse of knowledge that which will cover with all desirable completeness the line of work in wh...