The decks themselves are of white marble, two and a half feet wide in the aisles and five and a half in the corridors, set in an iron frame. This leaves a five-inch slit on either side, between it and the range. The space is too narrow and too close to the range for anyone to step through, and in order that any small article may not roll off, the deck is protected by a raised edge. It would, of course, be possible, though difficult, to drop a book down the slit, in which case, however, it would be very sure to lodge long before it struck the basement floor. If found necessary, any such accident could be prevented by protecting the opening with a wire netting. The advantages of an open space are many, however: attendants may speak to one another from deck to deck without the trouble of going to the stairways; light is diffused through it; and it keeps the books on the lower shelves from damage, either by being carelessly struck by the foot or one of the wheeled trucks used to carry books from the shelves to the elevator well. =Ventilation and Heating.=--Especially, by allowing a free circulation of air, these desk-slits help to heat and ventilate the stacks. Ventilation is especially important. Books require pure air almost as much as human beings do; if they do not get it they grow “musty,” and gradually decay. As will have been seen, the whole structure of the stack is open; nothing is closed, even the partitions in the ranges being made in the form of gratings. The system of ventilation and heating is one and the same, and both require the freest circulation of air. Air is taken into the cellar through the windows looking out into the court-yards, first, however, passing through filters of cotton cloth to exclude all dust; after being warmed it ascends through gratings to the roof, where it passes out through ventilating flues. In this way the temperature is everywhere kept very nearly even. Electric fans are ready for use in case of any sluggishness in the circulation, and in summer are also used for sending cooled air into the stack.