This is very necessary in order to intelligently supplement the work of the schools. A child comes wanting information on some subject upon which his ideas are exceedingly vague; for instance, he wants something about the mayor--what, he cannot tell you, but he was sent by his teacher to look up something about the mayor. You ask him what grade he is in, and he tells you the fourth. Your familiarity with the course of study should give you the clue at once, for the fourth grade topics in conduct and government include lessons on the city government, with its principal departments and officers, so you will look up, if you have not already done so, an outline of municipal government describing the position and duties of the mayor, which will be within the comprehension of the child. It should not happen that a dozen children ask for Little white lily, and be turned away without it, before it is discovered to be a poem by George MacDonald which the third grade children are given to read. This course of study the children's librarian should--not eat and sleep with exactly, but verily live and work with; it is one of her most valuable tools, and she should keep it not only within reach, at her finger's end, but as much as possible at her tongue's end, keeping pace with the assignment of work in the different grades and studies from month to month, and from week to week. She should know beforehand when a certain subject will be taken up by a certain grade, and have all available material looked up and ready, and new books bought if they will be needed and can be had--not wait until several hundred children come upon her for some subject on which a frantic search discloses the fact that the library contains not a thing suitable for their use, and then ask that books be bought, which, of course, come in after the demand is over, and stand idle upon the shelves for a whole year, taking the place of just so many more new books on subjects which will be needed later. The course of study, too, will furnish more useful hints for bulletins, exhibitions, reading-lists, and other forms of advertising, than can come from any other source; and not only in supplementing the school work, but also in directing the children in their general reading, is an intimate knowledge of the course of study an invaluable aid, as it gives you the unit of measurement for any child which enables you to correlate his reading along certain lines to that which has gone before, and to that which is to follow. (5) A knowledge of the principles of psychology and of education.