It is better, but not absolutely essential, that the opening into the wooden tube be near the floor. The carbonic acid thrown out by the lungs rises, with the warm breath, and the perspirable matter from the skin, with the warm, invisible vapour, to the top of the room. There both soon cool,

and sink towards the floor; and both carbonic air and the vapour bearing the perspirable matter are pretty rapidly and equally diffused through every part of the room.* It matters not, therefore, from what part of the room the outlet is made, as from every part, probably, an equal amount of foul matter will be thrown out. If it be from a point near the floor, it will be accompanied with less heat, and it will, at the same time, increase the tendency of the warm air above to diffuse itself through the space below.

The best possible ventilator is an open fireplace. Many schoolrooms were originally constructed with a fireplace, which, from the superior economy of a stove, has been closed up, and the smoke-pipe has been made to enter into the upper part of the chimney. Where this is the case, a most efficient ventilator may be secured by partially opening the fireplace near the hearth, and commanding the orifice by a slide of wood or metal. The opening of the venWhere no warm air is admitted, an opening made for the purpose of letting out foul air is just as likely to let air in; or, if the opening is a single one, two currents will be established in it, one outward, the other inward, and neither of them active. A ventilator opening into an attic is often quite inefficient.

* This diffusion, from the mutual penetration of gases, is often lost sight of Turner says, "One gas acts as a vacuum with respect to another; and, therefore, if a vessel full of carbonic acid gas be made to communicate with another of hydrogen, the particles of each gas insinuate themselves between the particles of the other, till they are equally diffused through both vessels. . . . . The ultimate effect. . . is the same as if the vessel of hydrogen had been a vacuum."-See Turner's Chemistry, 4th Am. Edition, p. 162. See, also, Manchester's Memoirs, vol. v., for Dalton's original investigations on this subject.

tilator should in any case be not less than 12 inches square, and, in the case in question, it should be near the master's seat, nor far from the floor, two feet long and eight inches high, and open into a box in the wall of these dimensions, or at least 24 inches by six, extending to the ceiling, where it should communicate with the tin box enclosing the smoke-pipe. If the building have two stories, the ventilator tubes must be carried from the lower, upward, within the wall, and communicate in the upper ceiling with the tin box. The supply of fresh air for the upper room should then be brought in from the side of the house between two joists in the floor, and open beneath the stove or behind the fireplace.

This mode of ventilation will be found much more economical, as well as more certain, than a usual mode of making openings into an attic which has windows into the atmosphere. In the latter, you have a flight of stairs to the attic, an attic floor and two windows; in the other, a wooden tube 12 feet long, and a tin one four or five feet long the attic being left unfinished, or, what would be better, having the ceiling of the schoolroom arched, to embrace a part of the space of the attic.

The details of construction will be given in the explanation of the plates.

[Scale 8 feet to the inch

D. Entrance door. E. Entry. F. Fireplace. C. Wood closet. T. Teacher's platform. a. Appara tus shelves. t. Air tube beneath the floor. d. Doors. g. Globes. 7. Library shelves. ble and seat. p. Passages. r. Recitation seats. s. Scholars' desks and seats. rooms in the attic. v. Ventilator. w. Windows. b. Movable blackboard. fireplace.

m. Master's ta rs. Stairs to recitation a s. Air space behind the

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