AFTER completing the "History of the Thirty Years' Peace," which treats of the period between 1815 and 1846, HARRIET MARTINEAU undertook to supplement it by histories of the country during the fifteen preceding and five following years, and thus to make a complete History of the Half Century. On account of ill-health, she was never able to accomplish the latter portion, but the former was written, and was published as an Introduction to the larger work. This Introduction, which for a long time has been scarce and much sought after, is here reproduced, with a modification of the title. It has been reprinted without any alteration, except the necessary correction of a few trifling errors, and the addition of an Index, which before was wanting.