In general, two stories is a sufficient height for library buildings, except in those of the largest class, and the upper floors may be amply lighted by sky-lights. The side-lights can hardly be too numerous: yet I have seen library buildings running back from a street fifty to seventy-five feet, without a single window in either of the side walls. The result was to throw all the books on shelves into a gloomy shade for many hours of each day. The interior construction should be so managed as to effect the finding and delivery of books to readers with the greatest possible economy of time and space. No shelves should be placed higher than can be reached by hand without mounting upon any steps or ladders; _i. e._, seven to seven and a half feet. The system of shelving should all be constructed of iron or steel, instead of surrounding the books on three sides with combustible wood, as is done in most libraries. Shelves of oxidized metal will be found smooth enough to prevent any abrasion of bindings. Shelves should be easily adjustable to any height, to accommodate the various sizes of books. In calculating shelf capacity, one and a half inches thickness a volume is a fair average, so that each hundred volumes would require about thirteen feet of linear shelf measurement. The space between uprights, that is, the length of each shelf, should not exceed two and a half feet. All spaces between shelves should be 101/2 or 11 inches high, to accommodate large octavos indiscriminately with smaller sizes; and a base shelf for quartos and folios, at a proper height from the floor, will restrict the number of shelves to six in each tier. In the arrangement of the cases or book-stacks, the most economical method is to place book-cases of double face, not less than three feet apart, approached by aisles on either side, so as to afford free passage for two persons meeting or passing one another. The cases may be about ten feet each in length. There should be electric lights between all cases, to be turned on only when books are sought. The cases should be set at right angles to the wall, two or three feet from it, with the light from abundant windows coming in between them. The width of shelves may be from 16 to 18 inches in these double cases, thus giving about eight to nine inches depth to each side. No partition is required between the two sides. It should be stated that the light obtained from windows, when thrown more than twenty feet, among cases of books on shelves, becomes too feeble for effective use in finding books. This fact should be considered in advance, while plans of construction, lighting, and interior arrangement are being made. All experience has shown that too much light cannot be had in any public library. Railings and stair-cases for the second or upper floors should be of perforated iron.