Taking into account the necessary air required by lamps or gas burners, the inlet flue should have at least ten square inches area for each person, so that the ordinary single register should provide the necessary amount of air for a living room. When, as happens in houses where a studied effort is made to preserve the health of the inhabitants, an outlet is cut into the wall and a flue carried up through the roof, the flue should be preferably near the floor and on the side of the room opposite the window or inlet. With such an arrangement (see Fig. 20) the air entering rises at first, but sinks at once because of the temperature, so that the direction of the air currents are diagonally across the room from the ceiling to the floor, thus renewing and changing all the air particles except those directly over the outlet. Where the air is introduced mechanically, that is, forced into the room, it is better to have the inlet and outlet on the same side, so that the entering air is shot in at the top, flowing across the room, then sinking and coming back, just below the point where it entered. [Illustration: FIG. 20.--Outlets into the walls.] _Ventilation of stables._ All that has been said on the subject of ventilation in houses applies equally well to the ventilation of stables, and a little book by Professor King of the University of Wisconsin, entitled "Ventilation," deals most thoroughly with the principles and practices of ventilation, not merely for dwellings but also for stables. Professor King proves by his experiments that the condition of cattle is much improved and that the milk-giving qualities are increased by a proper supply of fresh air, and in the book referred to, he gives a number of examples of the proper construction to provide adequate ventilation. It is most convincing to see how unscientific is the old-fashioned underground stable, the sole idea of which was to conserve the animal heat by crowding together the cows and by absolutely excluding the outside air. For further details of his work, its principles and practices, the reader is referred to the book, which may be obtained from the author at Madison, Wisconsin. _Cost of ventilation._