it must not be admitted at the bottom of a room, but about nine or ten feet from the floor, and be directed, toward the ceiling so that it may pass up and then fall and mix gradually with the air of the room.

Outlet Pipes.-The place for outlet pipes is one of important consideration. If there is no means of heating the air passing through them, they should be placed at the highest point of the room, enclosed as far as possible within walls, so as to prevent the air being cooled. They should be straight, with perfectly smooth internal surfaces so the friction may be reduced to the minimum. They may be round or square. They must be covered above by some apparatus (cowl, hexagon tube, etc.) which may aid the aspirating power of the tube and prevent the passage of rain into the shaft. The causes of downward draught in outlet tubes are, wind which forces down the air, rain gets in, evaporates and cools the air so it becomes heavier than the air in the room, or the air becomes too much cooled by passage through an exposed tube, so it cannot overcome the weight of the superincumbent atmosphere; or another outlet shaft with greater discharge may reverse the current.

Should down draught occur, flanges may be placed a little distant below the tubes so as to throw the air upward before it mixes with the air of the room. If there are several outlet tubes in a room they should all commence at the same distance from the floor, be of the same height (or the discharge will be unequal), and have the same exposure to the sun and wind.

Simple ridge openings may be used in one-story buildings with slanting roofs; they ventilate thoroughly, but snow sometimes drifts in. The discharge of outlets is much more certain and constant if the air be

warmed. The chimney with open fire is an excellent outlet-so good that in dwelling-houses, if there are proper inlets, no other outlet need. be made. Every room and dwelling-house should have an open fireplace. When rooms are large and more crowded, other outlets are needed. The heat of the fire in such rooms may be utilized by surrounding the smoke flues with foul-air shafts.

Plans of Pipes.-Of the various plans proposed for air-tubes and shafts, Dr. Parkes recommends McKinnell's circular tube.*

See Figs. 1 and 2. It consists of two cylinders, one encircling the other, the area of the inner tube and encircling tube being equal. The inner one is the outlet tube; it is so because the case of the other tube maintains the temperature of the air in it; and it is also always made rather higher than the other; above it should be protected by a revolving cowl. The outer cylinder, or ring. is the inlet tube; the air is taken at a lower level than the top of the outlet tube; when it enters the room it is thrown up towards the ceiling, and then to the walls by a flange placed at the bottom of the inner tube; the air then passes from the walls along the floor toward the centre of the room and upward to the outlet shaft. Both tubes can be arranged to close by valves. If there is a fire in the room both tubes may become inlet tubes; to prevent this the outlet tube should be closed. The movement of air by this plan is imperceptible, and it is an admirable plan for square rooms or small churches; for long halls it is less adapted. The tubes may be of any size, from six inches for ordinary living rooms, to seven or eight feet for churches. The two tubes after passing out of the room may be carried in different directions, care being taken that the inner tube is the largest. An advantage would be gained if the two tubes could be kept together some distance, as the hot air of the inner tube would transmit heat to the incoming air, so that it would enter the room at a higher temperature than it otherwise would.

Ventilation by the open fireplace is admirably illustrated by Burn in his little work on

66

Sanitary Science," page 82.

The withdrawal of pure air is illustrated by Fig. 3 copied from Mr. Burn's work. An opening is made at a in the breast of the chimney b of the fireplace c.

*Parkes Hygiene, page 163.

Fig. 4 shows the ordinary kind of ventilation carried on in which there is a fireplace a. The currents are mostly drawn by this along the floor from the door b, as shown by the arrow c. The arrows de indicate the currents from windows in close proximity to the fireplace, but the air lying near the ceiling, as at ff and g, is rarely influenced by the draught of a fireplace a, even when there is an opening in the breast as at a in Fig. 3.

FIG. 5.

Vorleont vectim through chimney.

Probably the best plan of ventilation by open fireplaces is that known as the Galton, a description of which was given by our Secretary in his last report, and which is here reproduced:

These grates are built into the chimney (Fig. 5)—this is a very delicate job, and requires an expert in the building of fireplaces and chimneys-and are of iron, but have a lining of fire-clay. From the throat of this grate an iron pipe rises some ten feet (just above the ceiling ) terminating in a flue, which runs the remainder of the chimney, and should terminate in a cap.

The air admitted from the outside, at the base of structure A, is warmed by contact with the heated sides and back F of the grate; then, after being still further heated by surrounding the iron pipe in its ascent, it is delivered into the ward from a lowered opening or register R, placed just beneath the ceiling C. The opening at A should be carried to the outer air by a pipe or flue.

Edward Smith, a Member of the Royal College of Physicians, in his standard work on "Foods," gives the following general directions for ventilation:

1st. Inhabited rooms should if possible have external walls on two sides, so that air may be admitted through both.

2d. The openings should be small and many, rather than few and large, defended by perforated zinc, and placed as distant as possible from those who inhabit the room. Hence the cornice above and the skirting below are convenient protections, but rooms of less than ten feet in height are not easily ventilated without draughts. The connection between the inside and outside of the room should not be direct, but at an angle, so that a direct current may not be produced.

3d. Such ventilations as direct the current to the ceiling are useful in a degree, but the cold air thus admitted will descend before the air is warmed.

4th. Channels which are divided by a perpendicular diaphragm, on the theory that there will be an ascending current in one and a descending current in the other, are for the most part based on a fallacy, and when the heat of the air is very great, there will be an upward and downward current in both.

5th. An air flue by the side of the chimney flue, into which the exit tubes lead, will act in some degree so long as there is fire in the chimney to rarify the air; but if an Archimedean screw can be placed at the top of such a flue, it will produce an upward current when there is no fire, provided there is wind to move it. This apparatus may be attached to any air flue placed in any position.

6th. The use of the chimney flue as an exit for the air is liable to allow the smoke to enter the room through it when there is a down draught, notwithstanding the excellent contrivances which have been devised to prevent the return current.

7th. Whenever it is proposed to renew air, means for supplying a larger quantity of air should be provided, or the attempt at ventilation. will be ineffectual.

8th. It is often desirable to warm as much of the air which is admitted as possible, and for that purpose stoves have been designed with an exit from a special channel into the room.

The idea advanced in the last rule of Dr. Smith may be carried out by placing the stove or heating apparatus in the cellar, or one room of the cellar. By this method the warmed air may be carried through every room in the building to the roof, where it makes its exit through a cowled flue.

Dr. John S. Billings, of the United States Army, says, in his work on "Heating and Ventilation":"For an ordinary room in a dwelling-house which is to be heated by indirect radiation-that is by warm air-it will be necessary, in order to secure satisfactory warming, when the temperature of the external air is below the freezing point and the room has the usual proportion of external wall and window surface, that the amount of air supply shall be about one and one-half times the cubic contents of the room. I would advise that heating surface, foul and fresh air flues and registers be provided for an air supply of one cubic foot per second per head for rooms which are to be occupied constantly."

F. Schumann, civil engineer of the U. S. Treasury, in his "Manual on Heating and Ventilation," says: "Currents in ventilated rooms are either directed upward or downward; in the upward direction the pure air is admitted at or near the floor, the impure air passing off at or near the ceiling; in the downward direction the pure air is admitted at or near the ceiling or through inlets through the wall near the floor, and the impure air passing off through the floor or openings in the walls near the floor. Public places where large crowds assemble should have the upward direction. Smaller rooms, offices, dwellings, etc., may be ventilated downward." The pure air inlets should be distributed equally

around the room, with the outlets for impure air in such position as to cause the currents to sweep the entire room, being careful not to place an outlet directly over an inlet.

"In the upward movement the inlets may be in the floor, in risers of platforms, sides of walls near the floor. In stationary desks, in front of stationary benches, etc., the outlet may be in the cornice or ceiling, or sides of walls near the ceiling. This method requires no change with the seasons, the fresh air entering the same way in summer that it does in winter, when the room is heated.

"In the downward movement, on the other hand, the fresh air in summer may be admitted at or near the floor and passed off near the ceiling. Where windows are available and so placed that currents pass through the room, no other provision need be made for summer ventilation."

Velocity of currents per second.-When entering at or near the ceiling, and not less than twelve feet above the floor, and descending 1.8 feet; when the openings are the same as above and horizontal 4.0 feet; when entirely at or near the floor (maximum) 2.0 feet; in ducts, shafts, etc., 3 to 10.0 feet.

"In cold weather, when the temperature of a room is higher than the external air, the air should be admitted at the bottom and passed off at the top of a room; in warm weather, when the temperature of the room is lower than the external air, the pure air should come in at the top and pass off at the bottom of the room.

"The movement, as shown in Fig. 6, can be reversed by either the vacuum or plenum method (heat or fan), when desirable, but they should coincide with and assist nature (gravity) it being more economical where ventilation is required. The vacuum movement is represented by Fig. 7, which represents a section through a building showing downward and upward movement of air currents."