of choice marble at the top. In 1876, John Danner of Canton, Ohio, invented a revolving bookcase with a patented "pivot and post" design. The ingenuity of his work resided in the economy of space it provided. Thirty-two volumes of the American Cyclopedia could be stored in a compact space, and readily available for perusal at the touch of a finger. Danner's bookcase appeared in the 1894 Montgomery Ward catalog. In 1878 he exhibited his bookcases at the Paris International Exhibition and won a gold medal. The John Danner Manufacturing Company was known for honorable workmanship and affordability. The woods were oak, black walnut, western ash, and Philippine mahogany. Viewed as a progressive businessman, Danner was credited with drawing a large trade and business to the city of Canton. Library shelving In the great public libraries of the twentieth century, multilevel stacks often served as both structure and shelving, of iron, as in the British Museum where the shelves are covered with cowhide; or steel, as in the Library of Congress in Washington, D.C.; or of slate, as in the Fitzwilliam Library at Cambridge. Systems of arrangement There are three common ways of arranging stationary bookcases: flat against the wall; in stacks or ranges parallel to each other with merely enough space between to allow for the passage of a librarian; or in bays or alcoves, where cases jut out into the room at right angles to the wall-cases. The stack system is suitable only for public libraries where economy of space is essential; the bay system is not only handsome but utilizes the space to great advantage. The library of the City of London at the Guildhall is a peculiarly effective example of the bay arrangement. For libraries where space is extremely tight there is yet another system, usually called mobile aisle shelving. In such systems rows of bookcases are mounted on wheels and packed tightly together with only one or more aisles between them. It is possible then to visit only two bookcase sides at a time, all the others being pressed close together. A gearing mechanism allows users to move the bookcases and open the aisle in the desired location. Because of the danger of tripping on the floor-mounted rails or being crushed between bookcases, these systems may have electronic sensors and/or recessed track, or may be reserved for closed stacks where access is restricted. Barrister's bookcase