=119. Library Planning.=--In subsequent chapters are set out in detail some of the chief requirements of the different departments of a public library, and here may be noted a few general principles, illustrated with plans. It is impossible to fix any data which will apply to all sizes and shapes of sites, on account of differences introduced by difficulties of lighting, approaches and varying local requirements. The data given above (Section 118) can be applied in most cases, as dimensions of this class seldom vary, but any additional data are certain to be modified by local conditions. The chief principle to be emphasized is the one already stated, that public libraries should be constructed and stocked with the view to constant revision, and that their size should be limited by the number of _live_ books likely to be wanted at any period. It is difficult to say what the number of actually living books will be at any given period, but judging by the selections which have been made in histories of literature and in such books as Sonnenschein’s _Best Books_, it may be roughly estimated that there are about 50,000 works of perennial interest which are worth storing in a modern workshop library. Even this number could be reduced by one-half and still be made fairly representative of every literature, period and subject of human interest. In the largest municipal libraries a very considerable proportion of the stock is composed of duplicates of popular books in central and branch libraries, while practically one-half of the stock of such libraries consists of literature which is rarely used. The provision of book-storage should, therefore, be limited in the case of municipal libraries, not so much by the size of building which can be afforded by the income, but by the actual living books which are likely to be required. [Illustration: FIG. 9.--Sketch Plan for a Small Town Library (Section 119).] [Illustration: FIG. 10.--North Islington Library, with Reading Room on Ground Floor (Section 120).] [Illustration: FIG. 11.--North Islington Library, with Lending Department on First Floor (Section 120).]