The root purpose of a book-stack, of course, is to make it capable of holding the greatest number of volumes in the smallest possible space--always, however, bearing in mind that every book must be perfectly accessible and so placed that it can be easily and quickly handled. The space being limited and the number of volumes large, the old way of arranging cases along the walls, even when the wall space is materially increased by dividing a room into alcoves, has to be abandoned in favor of a more compact system. The modern substitute is to erect the cases in stories, or tiers, with corridors and passages only large enough to give convenient access to the books. Throughout, the aim of the builder is to dispose of every inch of space as economically as possible. Of the three stacks in the Library of Congress, those to the north and south are, as the visitor has seen, the largest, each having a length of one hundred and twelve feet against thirty for the East Stack. All three are of the same width, however--forty-five feet--and the same height--sixty-three feet. The method of construction is the same throughout, and each is absolutely fireproof, the only materials used being steel, iron, brick, glass, and marble. Few things which can be destroyed by fire at all are more difficult to burn than books, and a fire in the stacks, even if carefully nursed by an incendiary, could hardly do more than a trifling injury. =Arrangement and Construction.=--The stacks are divided into nine tiers, each tier being seven feet high, and into an equal number of stories the same distance apart. This distance was adopted in order that the books on the highest shelf of a tier might not be beyond the convenient reach of a man of average height, or so far away that he could not easily read their titles. By the present arrangement every book can be handled or its title read without effort.