These sections are still further subdivided until the requisite degree of minuteness is reached. The system has various mnemonic features which are helpful. Every figure has a meaning. An alphabetical list of all these meanings is appended to the classification. Thus after the word Hydraulics is 532, showing where to look in the classification for this subject. All books on hydraulics receive the number 532 and are together on the shelves. This fact illustrates one great advantage of the Dewey system, that as the library grows the new books can be placed with the corresponding old ones without re-marking the old ones, while in the fixed location system the books are marked to certain localities, and when moved by reason of growth of the library have to be re-marked. This re-marking includes not only the books but also the cards referring to the books. The re-marking is very costly and very unsatisfactory. In Dewey’s system the books in any one class are arranged according to some method. In most classes an alphabetical arrangement by the names of the authors is simplest and best. In scientific classes some librarians prefer the chronological arrangement. In any case it should be clear and simple. =Relative Location.=—“With a movable location all new books fall at once into their proper places like the cards which are added to a card catalog, and the new-comers push the other books along on the shelf, just as new cards push the others along in the drawer. The consequence is that a book which is here to-day may be on the next shelf in a month or in the next alcove in a year; and the local memory, which is a great help in finding books quickly is disturbed. The only remedy that I can see for this is to substitute a subject memory for a local memory, to get a habit of thinking of a book as belonging to a certain class instead of as on a certain shelf (a much more rational memory, by the way), and then to make it very easy to find the classes. This last is not hard to accomplish. A class memory can be cultivated and may be assisted by local memory which will find books by their position relative to other books, instead of by their position relative to alcoves and shelves, or doors and windows.”—C. A. Cutter. CHAPTER V. =CATALOGING.= =Definition.=—The catalog is a directory of the library. A library without a catalog is described by Thomas Carlyle as “a Polyphemus without any eye in its head, and you must front the difficulties, whatever they may be, of making proper catalogs.” A good catalog must be an accurate and easily used index of the resources of the library, answering the questions of the readers in the simplest and most direct way possible. Such questions fall into the following groups: 1. Has the library a certain book by a certain author? 2. What books by a certain author has the library?